The paradox of Pompe disease

Until 2006, when a breakthrough therapy first made treatment possible, Pompe disease was a little-known metabolic myopathy fatal to infants. Those with later-onset disease experienced progressive, often severe disability into adulthood.

In this rare autosomal recessive disorder, which occurs in approximately one in 40,000 births worldwide, a deficiency or absence of the enzyme acid alpha-glucosidase causes glycogen to build up in the lysosomes of cells. While many tissues are affected, skeletal and cardiac muscle see the earliest involvement, with muscle hypotonia, cardiomyopathy, and breathing difficulties (mainly due to diaphragm weakness) comprising the hallmark symptoms of the infantile form. Muscle weakness and progressive respiratory failure are prominent in later-onset disease.

The spectrum of severity and age of onset in Pompe disease is linked to combinations of mutations on the GAA gene, some of which destroy the body’s ability to produce acid alpha-glucosidase whereas others merely hamper it. Less enzyme produced in the body generally corresponds with more severe disease activity.

The most severe end of the disease spectrum, or “classic infantile Pompe disease,” presents at birth and is recognized in early infancy. Until treatment with enzyme replacement therapy (ERT) became available, patients usually died of cardiorespiratory failure within their first year of life. With therapy, patients have survived into their 20s and beyond. Late-onset disease is a far broader category in which patients can present at any time from their first year, including into middle age.

The emergence in 2006 of alglucosidase alfa (Lumizyme, Sanofi Genzyme), an ERT used long-term to improve survival and slow progression in children and adults, resulted in a boom of research interest, a push to timelier diagnosis, and – with patients living longer – a more thorough understanding of the natural history of Pompe disease. In addition to the usual clinical picture of progressive muscle weakness, difficulty breathing, and cardiomyopathy, investigators are seeing nervous system involvement in patients with Pompe disease.

A patient with Pompe, especially one with late-onset disease, may be diagnosed and even managed by his or her neurologist. To learn more, Neurology Reviews talked to two global experts in Pompe disease: Priya Kishnani, MD, of Duke University in Durham, N.C., and Antonio Toscano, MD, of the University of Messina, in Messina, Italy. 

 

Diagnosis: Still room to improve

“Most neurologists will encounter a patient with Pompe disease,” said Dr. Kishnani, who has been working with Pompe for her entire career as a pediatrician and medical geneticist, treating patients of all ages and disease phenotypes.

“In newborns, diagnosis is more straightforward, because you’ve got an enlarged heart,” she said. And thanks to efforts of researchers like Dr. Kishnani and Pompe advocacy groups, Pompe disease is now a part of the RUSP (Recommended Uniform Screening Panel) for newborns; currently 28 U.S. states are screening for Pompe disease.

“The challenge really is for the later-onset cases, which are 80% of all cases,” Dr. Kishnani said.

Previously, muscle biopsies were the first step toward diagnosis. Dried blot spot assays to detect enzyme deficiency have since become the standard, along with other biochemical tests. Confirmation of the diagnosis is through gene sequencing panels to detect GAA mutations.

“Now that there is a treatment for Pompe disease and the availability of blood-based testing, many previously undiagnosed patients with limb girdle weakness are evaluated and the diagnostic odyssey ends,” Dr. Kishnani said. “But there is still a diagnostic delay, and many cases remain undiagnosed.”

Routine blood tests for creatine kinase and for liver enzymes can help point to Pompe disease. But elevated liver enzymes are often misinterpreted. “It’s about the ratios,” Dr. Kishnani said. “ALT is usually much more elevated if it is coming from a liver cause, and AST is usually higher than ALT if it is coming from muscle. But patients often end up getting a liver biopsy due to so-called elevated liver enzymes. As the workup continues, it is often later recognized that the source of the elevated enzymes is muscle involvement, and a referral to the geneticist or neurologist is made. Only then is appropriate testing to confirm a diagnosis initiated.”

Dr. Toscano, a neurologist who specializes in Pompe disease and other myopathies and who has published on tools for diagnosing late-onset Pompe disease,¹ said that clinicians should be vigilant when evaluating any patient with limb girdle weakness and elevated creatine kinase (CK) – “especially if the CK is under 2,000,” he said, “because it is very rare that patients with Pompe disease have a more elevated CK than that.”

“Elevated CK, myalgia, and exercise intolerance” should prompt clinicians to suspect Pompe disease in a patient of any age, Dr. Toscano said. “When you come across this, you should be very persistent and get to the end of the story.”

Dr. Toscano noted that the blood spot assay, while an important early step, is not fully diagnostic, “because you can have false positives.” The molecular GAA assay is used to confirm Pompe disease. But detecting pathogenic variants on the GAA gene – of which there are more than 500 – can be more complicated than it sounds. Whereas two mutations are required for Pompe disease, sometimes only one can be detected. Dr. Toscano said he also treated some patients for Pompe with only one known mutation but with unequivocal clinical and biochemical aspects of Pompe disease.

While delays in diagnosis for late-onset Pompe disease remain significant -- between 5 and 6 years on average for older patients, and up to 20 years for those with pediatric onset – both Dr. Kishnani and Dr. Toscano said they perceive them to be improving. With McArdle disease, another inherited glycogen storage disorder that is more common than Pompe disease but for which there is no treatment, “the delay is nearly 12 years,” Dr. Toscano said.
 

 

ERT: The sooner the better

Enzyme replacement therapy is indicated for all patients with Pompe disease. Currently two are commercially available: alglucosidase alfa (Lumizyme, Sanofi Genzyme), indicated for all forms of Pompe disease, and avalglucosidase alfa-ngpt (Nexviazyme, Sanofi Genzyme), approved in 2021 for later-onset Pompe, though its indications have yet to be fully defined.

The semimonthly infusions represent, to date, the only disease-modifying therapies commercially available. Enzyme replacement therapy can reverse cardiac damage seen in infants and allow them to meet developmental milestones previously unthinkable. In adults, it can slow progression, though many treated patients will still develop chronic disability and require a wheelchair, respiratory support, or both. “The phenotype of the patients we are seeing today is not as involved as it was prior to enzyme therapy,” said Dr. Kishnani, who was part of the research team that developed ERT and launched the first clinical trials. “This is across the disease spectrum.”

But optimal management means more than just getting a patient on therapy fast, Dr. Kishnani said.

“Very often the thinking is if the patient is on ERT, we’ve done right by the patient. Aspects we don’t look at enough include: Are we monitoring these patients well? Are patients being followed by a multidisciplinary team that includes cardiology, physical therapy, and pulmonary medicine? Are we doing appropriate musculoskeletal assessments? They might have sleep hypoventilation. The BiPap settings may not be correct. Or they have not been assessed for antibodies,” she said.

Many infants with severe phenotypes, notably those who produce no enzyme naturally, will develop immune reactions to the exogenous enzyme therapy. High antibody titers also have been seen and are associated with poor therapeutic response. While this is very clear in the infantile setting, late-onset patients also develop antibodies in response to ERT. In one study in 64 patients,² Dr. Toscano and his colleagues saw that antibodies may affect clinical response during the first 3 years of treatment, while a small study³ by Dr. Kishnani’s group saw clinical decline associated with high antibody titers in patients with late-onset disease.

While the relationship of specific titers to therapeutic response remains unclear, it is important to consider antibodies, along with other factors, in the monitoring of patients with Pompe disease. “We need to always ask, if a patient is falling behind, what could be the reason?” Dr. Kishnani said. “These are the things we as clinicians can do to improve or enhance the impact of ERT.”

Dr. Toscano noted that a common misconception about late-onset Pompe disease is that cardiac manifestations are minimal or absent, whereas as many as about 20% of patients will have heart problems and need to be carefully monitored.
 

Neurological manifestations

With patients surviving longer on ERT, researchers have been able to develop a deeper understanding of the natural history of Pompe disease. Increasingly, they are seeing it as a multisystem disease that includes central nervous system involvement.

“Is Pompe an overt neurodegenerative disease? I would say no,” Dr. Kishnani said. “But there is a neurological component that we’ve got to understand and follow more.”

Glycogen accumulation, she noted, has been found in anterior horn cells, motor neurons, and other parts of the brain. “We have been doing MRIs on children with infantile Pompe, and we have seen some white matter hyperintensities. The clinical significance of this finding is still emerging. Sometimes it is present, but the child is cognitively intact. We have had college graduates who have white matter hyperintensities. So putting it in context will be important. But we know that glycogen is ubiquitous, and autopsy studies have shown that it is present in the brain.”

In recent years, Dr. Toscano’s group has investigated neurovascular complications of Pompe in late-onset patients. “This was something that really surprised us because for several years we have investigated mainly heart, muscle, or respiratory manifestations of the disease, but the central nervous system was really neglected,” he said.

“Occasionally we did some brain MRIs and we found in even young patients some ischemic areas. We thought this was related to slowed circulation – that blood vessels in these patients are weak because they are impaired by glycogen accumulation.” Dr. Toscano and his colleagues followed that observation with a study of late-onset patients,⁴ in which they found that more than half had cerebrovascular abnormalities. “Even in, say, patients 30 to 35 years old we saw this – it’s unusual to have a vascular disorder at that age.”

Dr. Toscano and his colleagues also reported cerebral aneurysms⁵ in patients with Pompe disease and have recommended that clinicians conduct MRI or cerebral angiograms on patients as part of routine follow-up. Blood pressure in Pompe patients should be carefully watched and managed with antihypertensive medication as needed, he said.

Part of the problem is that the proteins in ERT are not able to cross the blood-brain barrier, Dr. Toscano noted, adding that researchers are investigating other treatments that can.
 

 

Pompe disease as a research model

The successful development of ERT for Pompe disease marked a boom in research interest into not just Pompe – for which several experimental therapies are currently in the pipeline – but for other myopathies and glycogen storage disorders.

“I think that Pompe has served as a template both as a muscle disease and a lysosomal storage disease, and so some of our learnings from Pompe have been applied across different diseases,” Dr. Kishnani said.

Studies in spinal muscular atrophy, for example, “in some ways mirrored what was done for Pompe – treatment trials were initiated in babies at the most severe end of the disease population with infantile disease, and used similar clinical trial endpoints,” Dr. Kishnani said. “Even for the later-onset end of the spectrum, the endpoints we used in Pompe for muscle strength and function have been relevant to many other neuromuscular disorders.”

Pompe disease research also ushered in a new appreciation of immune responses in protein replacement therapies, Dr. Kishnani noted.

“In the field today, you hear the term cross-reactive immunological material, or CRIM, all the time,” she said. “But when we first started talking about it in the space of Pompe disease, there was a lot of scientific debate about what the significance of CRIM-negative status was in relationship to the risk for development of high and sustained antibody titer and a poor clinical response. To understand this involved a lot of going back to the data and digging into the small subset of nonresponders. One of the powers of rare disease research is that every patient matters, and it’s important to understand what’s going on at the patient level rather than just the group data level.”
 

A robust pipeline

The decade and a half since the advent of ERT has seen what Dr. Toscano described as “an explosion of interest” in Pompe disease.

“We’re seeing an extraordinary number of papers on everything from clinical, biomarkers, genetics, and rehabilitation – this disease is now considered from every point of view, and this is very important for patients,” Dr. Toscano said. Alongside this has come industry interest in this rare disease, with several companies investigating a range of treatment approaches.

The existence of a treatment, “while not perfect,” he said, “has interested the patient associations and doctors to try and improve service to patients. Patients with Pompe disease are well attended, probably more so than patients with degenerative disorders in which there is no therapy.”

Last year the second ERT, avalglucosidase alfa (Nexviazyme, Sanofi Genzyme) was approved by the U.S. Food and Drug Administration to treat late-onset Pompe disease. The drug, currently being investigated in infants as well, was designed to improve the delivery of the therapeutic enzyme to muscles and enhance glycogen clearance, and results from ongoing trials suggest some functional and clinical benefit over standard ERT.

Other drugs in development for Pompe disease include substrate reduction therapies, which aim to reduce the storage of glycogen in cells, and therapies that improve residual function of mutant GAA enzyme in the body. These and other therapies in development have the potential to modify nervous system manifestations of Pompe disease.⁶

Because a single gene is implicated in Pompe disease, it has long been considered a good candidate for gene therapies that prompt the body to make stable enzyme. Seven companies are now investigating gene therapies in Pompe disease.⁷ Some of these deliver to skeletal muscles and others aim for the liver, where proteins are synthesized and secreted and adverse immune responses might be more easily mitigated. Other gene therapies use an ex vivo approach, removing and replacing cells in bone marrow.

Dr. Kishnani’s research group at Duke University is leading a small clinical trial in late-onset patients of a GAA gene transfer to the liver using adeno-associated virus (AAV) vectors.⁸

“We have started AAV gene therapy trials in in adults with Pompe disease and will later evaluate children because ERT is available as a standard of care, and so from a safety perspective this makes the most sense,” Dr. Kishnani said. “We do have challenges in the field of gene therapy, but I think if we are able to overcome the immune responses, and … to treat at a lower dose, there’s a very good pathway forward.”

Dr. Toscano and Dr. Kishnani have received reimbursement from Sanofi and other manufacturers for participation on advisory boards and as speakers.



Jennie Smith is a freelance journalist and editor specializing in medicine and health.

 

References

1. Musumeci O, Toscano A. Diagnostic tools in late onset Pompe disease (LOPD). Ann Transl Med. 2019 Jul;7(13):286. doi: 10.21037/atm.2019.06.60.

2. Filosto M et al. Assessing the role of anti rh-GAA in modulating response to ERT in a late-onset Pompe disease cohort from the Italian GSDII Study Group. Adv Ther. 2019 May;36(5):1177-1189. doi: 10.1007/s12325-019-00926-5.

3. Patel TT et al. The impact of antibodies in late-onset Pompe disease: A case series and literature review. Mol Genet Metab. 2012 Jul;106(3):301-9. doi: 10.1016/j.ymgme.2012.04.027.

4. Montagnese F et al. Intracranial arterial abnormalities in patients with late onset Pompe disease (LOPD). J Inherit Metab Dis. 2016 May;39(3):391-398. doi: 10.1007/s10545-015-9913-x.

5. Musumeci O et al. Central nervous system involvement in late-onset Pompe disease: Clues from neuroimaging and neuropsychological analysis. Eur J Neurol. 2019 Mar;26(3):442-e35. doi: 10.1111/ene.13835.

6. Edelmann MJ, Maegawa GHB. CNS-targeting therapies for lysosomal storage diseases: Current advances and challenges. Front Mol Biosci. 2020 Nov 12;7:559804. doi: 10.3389/fmolb.2020.559804

7. Ronzitti G et al. Progress and challenges of gene therapy for Pompe disease. Ann Transl Med. 2019 Jul;7(13):287. doi: 10.21037/atm.2019.04.67.

8. Kishnani PS, Koeberl DD. Liver depot gene therapy for Pompe disease. Ann Transl Med. 2019 Jul;7(13):288. doi: 10.21037/atm.2019.05.02.

Until 2006, when a breakthrough therapy first made treatment possible, Pompe disease was a little-known metabolic myopathy fatal to infants. Those with later-onset disease experienced progressive, often severe disability into adulthood.

In this rare autosomal recessive disorder, which occurs in approximately one in 40,000 births worldwide, a deficiency or absence of the enzyme acid alpha-glucosidase causes glycogen to build up in the lysosomes of cells. While many tissues are affected, skeletal and cardiac muscle see the earliest involvement, with muscle hypotonia, cardiomyopathy, and breathing difficulties (mainly due to diaphragm weakness) comprising the hallmark symptoms of the infantile form. Muscle weakness and progressive respiratory failure are prominent in later-onset disease.

The spectrum of severity and age of onset in Pompe disease is linked to combinations of mutations on the GAA gene, some of which destroy the body’s ability to produce acid alpha-glucosidase whereas others merely hamper it. Less enzyme produced in the body generally corresponds with more severe disease activity.

The most severe end of the disease spectrum, or “classic infantile Pompe disease,” presents at birth and is recognized in early infancy. Until treatment with enzyme replacement therapy (ERT) became available, patients usually died of cardiorespiratory failure within their first year of life. With therapy, patients have survived into their 20s and beyond. Late-onset disease is a far broader category in which patients can present at any time from their first year, including into middle age.

The emergence in 2006 of alglucosidase alfa (Lumizyme, Sanofi Genzyme), an ERT used long-term to improve survival and slow progression in children and adults, resulted in a boom of research interest, a push to timelier diagnosis, and – with patients living longer – a more thorough understanding of the natural history of Pompe disease. In addition to the usual clinical picture of progressive muscle weakness, difficulty breathing, and cardiomyopathy, investigators are seeing nervous system involvement in patients with Pompe disease.

A patient with Pompe, especially one with late-onset disease, may be diagnosed and even managed by his or her neurologist. To learn more, Neurology Reviews talked to two global experts in Pompe disease: Priya Kishnani, MD, of Duke University in Durham, N.C., and Antonio Toscano, MD, of the University of Messina, in Messina, Italy. 

 

Diagnosis: Still room to improve

“Most neurologists will encounter a patient with Pompe disease,” said Dr. Kishnani, who has been working with Pompe for her entire career as a pediatrician and medical geneticist, treating patients of all ages and disease phenotypes.

“In newborns, diagnosis is more straightforward, because you’ve got an enlarged heart,” she said. And thanks to efforts of researchers like Dr. Kishnani and Pompe advocacy groups, Pompe disease is now a part of the RUSP (Recommended Uniform Screening Panel) for newborns; currently 28 U.S. states are screening for Pompe disease.

“The challenge really is for the later-onset cases, which are 80% of all cases,” Dr. Kishnani said.

Previously, muscle biopsies were the first step toward diagnosis. Dried blot spot assays to detect enzyme deficiency have since become the standard, along with other biochemical tests. Confirmation of the diagnosis is through gene sequencing panels to detect GAA mutations.

“Now that there is a treatment for Pompe disease and the availability of blood-based testing, many previously undiagnosed patients with limb girdle weakness are evaluated and the diagnostic odyssey ends,” Dr. Kishnani said. “But there is still a diagnostic delay, and many cases remain undiagnosed.”

Routine blood tests for creatine kinase and for liver enzymes can help point to Pompe disease. But elevated liver enzymes are often misinterpreted. “It’s about the ratios,” Dr. Kishnani said. “ALT is usually much more elevated if it is coming from a liver cause, and AST is usually higher than ALT if it is coming from muscle. But patients often end up getting a liver biopsy due to so-called elevated liver enzymes. As the workup continues, it is often later recognized that the source of the elevated enzymes is muscle involvement, and a referral to the geneticist or neurologist is made. Only then is appropriate testing to confirm a diagnosis initiated.”

Dr. Toscano, a neurologist who specializes in Pompe disease and other myopathies and who has published on tools for diagnosing late-onset Pompe disease,¹ said that clinicians should be vigilant when evaluating any patient with limb girdle weakness and elevated creatine kinase (CK) – “especially if the CK is under 2,000,” he said, “because it is very rare that patients with Pompe disease have a more elevated CK than that.”

“Elevated CK, myalgia, and exercise intolerance” should prompt clinicians to suspect Pompe disease in a patient of any age, Dr. Toscano said. “When you come across this, you should be very persistent and get to the end of the story.”

Dr. Toscano noted that the blood spot assay, while an important early step, is not fully diagnostic, “because you can have false positives.” The molecular GAA assay is used to confirm Pompe disease. But detecting pathogenic variants on the GAA gene – of which there are more than 500 – can be more complicated than it sounds. Whereas two mutations are required for Pompe disease, sometimes only one can be detected. Dr. Toscano said he also treated some patients for Pompe with only one known mutation but with unequivocal clinical and biochemical aspects of Pompe disease.

While delays in diagnosis for late-onset Pompe disease remain significant -- between 5 and 6 years on average for older patients, and up to 20 years for those with pediatric onset – both Dr. Kishnani and Dr. Toscano said they perceive them to be improving. With McArdle disease, another inherited glycogen storage disorder that is more common than Pompe disease but for which there is no treatment, “the delay is nearly 12 years,” Dr. Toscano said.
 

 

ERT: The sooner the better

Enzyme replacement therapy is indicated for all patients with Pompe disease. Currently two are commercially available: alglucosidase alfa (Lumizyme, Sanofi Genzyme), indicated for all forms of Pompe disease, and avalglucosidase alfa-ngpt (Nexviazyme, Sanofi Genzyme), approved in 2021 for later-onset Pompe, though its indications have yet to be fully defined.

The semimonthly infusions represent, to date, the only disease-modifying therapies commercially available. Enzyme replacement therapy can reverse cardiac damage seen in infants and allow them to meet developmental milestones previously unthinkable. In adults, it can slow progression, though many treated patients will still develop chronic disability and require a wheelchair, respiratory support, or both. “The phenotype of the patients we are seeing today is not as involved as it was prior to enzyme therapy,” said Dr. Kishnani, who was part of the research team that developed ERT and launched the first clinical trials. “This is across the disease spectrum.”

But optimal management means more than just getting a patient on therapy fast, Dr. Kishnani said.

“Very often the thinking is if the patient is on ERT, we’ve done right by the patient. Aspects we don’t look at enough include: Are we monitoring these patients well? Are patients being followed by a multidisciplinary team that includes cardiology, physical therapy, and pulmonary medicine? Are we doing appropriate musculoskeletal assessments? They might have sleep hypoventilation. The BiPap settings may not be correct. Or they have not been assessed for antibodies,” she said.

Many infants with severe phenotypes, notably those who produce no enzyme naturally, will develop immune reactions to the exogenous enzyme therapy. High antibody titers also have been seen and are associated with poor therapeutic response. While this is very clear in the infantile setting, late-onset patients also develop antibodies in response to ERT. In one study in 64 patients,² Dr. Toscano and his colleagues saw that antibodies may affect clinical response during the first 3 years of treatment, while a small study³ by Dr. Kishnani’s group saw clinical decline associated with high antibody titers in patients with late-onset disease.

While the relationship of specific titers to therapeutic response remains unclear, it is important to consider antibodies, along with other factors, in the monitoring of patients with Pompe disease. “We need to always ask, if a patient is falling behind, what could be the reason?” Dr. Kishnani said. “These are the things we as clinicians can do to improve or enhance the impact of ERT.”

Dr. Toscano noted that a common misconception about late-onset Pompe disease is that cardiac manifestations are minimal or absent, whereas as many as about 20% of patients will have heart problems and need to be carefully monitored.
 

Neurological manifestations

With patients surviving longer on ERT, researchers have been able to develop a deeper understanding of the natural history of Pompe disease. Increasingly, they are seeing it as a multisystem disease that includes central nervous system involvement.

“Is Pompe an overt neurodegenerative disease? I would say no,” Dr. Kishnani said. “But there is a neurological component that we’ve got to understand and follow more.”

Glycogen accumulation, she noted, has been found in anterior horn cells, motor neurons, and other parts of the brain. “We have been doing MRIs on children with infantile Pompe, and we have seen some white matter hyperintensities. The clinical significance of this finding is still emerging. Sometimes it is present, but the child is cognitively intact. We have had college graduates who have white matter hyperintensities. So putting it in context will be important. But we know that glycogen is ubiquitous, and autopsy studies have shown that it is present in the brain.”

In recent years, Dr. Toscano’s group has investigated neurovascular complications of Pompe in late-onset patients. “This was something that really surprised us because for several years we have investigated mainly heart, muscle, or respiratory manifestations of the disease, but the central nervous system was really neglected,” he said.

“Occasionally we did some brain MRIs and we found in even young patients some ischemic areas. We thought this was related to slowed circulation – that blood vessels in these patients are weak because they are impaired by glycogen accumulation.” Dr. Toscano and his colleagues followed that observation with a study of late-onset patients,⁴ in which they found that more than half had cerebrovascular abnormalities. “Even in, say, patients 30 to 35 years old we saw this – it’s unusual to have a vascular disorder at that age.”

Dr. Toscano and his colleagues also reported cerebral aneurysms⁵ in patients with Pompe disease and have recommended that clinicians conduct MRI or cerebral angiograms on patients as part of routine follow-up. Blood pressure in Pompe patients should be carefully watched and managed with antihypertensive medication as needed, he said.

Part of the problem is that the proteins in ERT are not able to cross the blood-brain barrier, Dr. Toscano noted, adding that researchers are investigating other treatments that can.
 

 

Pompe disease as a research model

The successful development of ERT for Pompe disease marked a boom in research interest into not just Pompe – for which several experimental therapies are currently in the pipeline – but for other myopathies and glycogen storage disorders.

“I think that Pompe has served as a template both as a muscle disease and a lysosomal storage disease, and so some of our learnings from Pompe have been applied across different diseases,” Dr. Kishnani said.

Studies in spinal muscular atrophy, for example, “in some ways mirrored what was done for Pompe – treatment trials were initiated in babies at the most severe end of the disease population with infantile disease, and used similar clinical trial endpoints,” Dr. Kishnani said. “Even for the later-onset end of the spectrum, the endpoints we used in Pompe for muscle strength and function have been relevant to many other neuromuscular disorders.”

Pompe disease research also ushered in a new appreciation of immune responses in protein replacement therapies, Dr. Kishnani noted.

“In the field today, you hear the term cross-reactive immunological material, or CRIM, all the time,” she said. “But when we first started talking about it in the space of Pompe disease, there was a lot of scientific debate about what the significance of CRIM-negative status was in relationship to the risk for development of high and sustained antibody titer and a poor clinical response. To understand this involved a lot of going back to the data and digging into the small subset of nonresponders. One of the powers of rare disease research is that every patient matters, and it’s important to understand what’s going on at the patient level rather than just the group data level.”
 

A robust pipeline

The decade and a half since the advent of ERT has seen what Dr. Toscano described as “an explosion of interest” in Pompe disease.

“We’re seeing an extraordinary number of papers on everything from clinical, biomarkers, genetics, and rehabilitation – this disease is now considered from every point of view, and this is very important for patients,” Dr. Toscano said. Alongside this has come industry interest in this rare disease, with several companies investigating a range of treatment approaches.

The existence of a treatment, “while not perfect,” he said, “has interested the patient associations and doctors to try and improve service to patients. Patients with Pompe disease are well attended, probably more so than patients with degenerative disorders in which there is no therapy.”

Last year the second ERT, avalglucosidase alfa (Nexviazyme, Sanofi Genzyme) was approved by the U.S. Food and Drug Administration to treat late-onset Pompe disease. The drug, currently being investigated in infants as well, was designed to improve the delivery of the therapeutic enzyme to muscles and enhance glycogen clearance, and results from ongoing trials suggest some functional and clinical benefit over standard ERT.

Other drugs in development for Pompe disease include substrate reduction therapies, which aim to reduce the storage of glycogen in cells, and therapies that improve residual function of mutant GAA enzyme in the body. These and other therapies in development have the potential to modify nervous system manifestations of Pompe disease.⁶

Because a single gene is implicated in Pompe disease, it has long been considered a good candidate for gene therapies that prompt the body to make stable enzyme. Seven companies are now investigating gene therapies in Pompe disease.⁷ Some of these deliver to skeletal muscles and others aim for the liver, where proteins are synthesized and secreted and adverse immune responses might be more easily mitigated. Other gene therapies use an ex vivo approach, removing and replacing cells in bone marrow.

Dr. Kishnani’s research group at Duke University is leading a small clinical trial in late-onset patients of a GAA gene transfer to the liver using adeno-associated virus (AAV) vectors.⁸

“We have started AAV gene therapy trials in in adults with Pompe disease and will later evaluate children because ERT is available as a standard of care, and so from a safety perspective this makes the most sense,” Dr. Kishnani said. “We do have challenges in the field of gene therapy, but I think if we are able to overcome the immune responses, and … to treat at a lower dose, there’s a very good pathway forward.”

Dr. Toscano and Dr. Kishnani have received reimbursement from Sanofi and other manufacturers for participation on advisory boards and as speakers.



Jennie Smith is a freelance journalist and editor specializing in medicine and health.

 

References

1. Musumeci O, Toscano A. Diagnostic tools in late onset Pompe disease (LOPD). Ann Transl Med. 2019 Jul;7(13):286. doi: 10.21037/atm.2019.06.60.

2. Filosto M et al. Assessing the role of anti rh-GAA in modulating response to ERT in a late-onset Pompe disease cohort from the Italian GSDII Study Group. Adv Ther. 2019 May;36(5):1177-1189. doi: 10.1007/s12325-019-00926-5.

3. Patel TT et al. The impact of antibodies in late-onset Pompe disease: A case series and literature review. Mol Genet Metab. 2012 Jul;106(3):301-9. doi: 10.1016/j.ymgme.2012.04.027.

4. Montagnese F et al. Intracranial arterial abnormalities in patients with late onset Pompe disease (LOPD). J Inherit Metab Dis. 2016 May;39(3):391-398. doi: 10.1007/s10545-015-9913-x.

5. Musumeci O et al. Central nervous system involvement in late-onset Pompe disease: Clues from neuroimaging and neuropsychological analysis. Eur J Neurol. 2019 Mar;26(3):442-e35. doi: 10.1111/ene.13835.

6. Edelmann MJ, Maegawa GHB. CNS-targeting therapies for lysosomal storage diseases: Current advances and challenges. Front Mol Biosci. 2020 Nov 12;7:559804. doi: 10.3389/fmolb.2020.559804

7. Ronzitti G et al. Progress and challenges of gene therapy for Pompe disease. Ann Transl Med. 2019 Jul;7(13):287. doi: 10.21037/atm.2019.04.67.

8. Kishnani PS, Koeberl DD. Liver depot gene therapy for Pompe disease. Ann Transl Med. 2019 Jul;7(13):288. doi: 10.21037/atm.2019.05.02.

Until 2006, when a breakthrough therapy first made treatment possible, Pompe disease was a little-known metabolic myopathy fatal to infants. Those with later-onset disease experienced progressive, often severe disability into adulthood.

In this rare autosomal recessive disorder, which occurs in approximately one in 40,000 births worldwide, a deficiency or absence of the enzyme acid alpha-glucosidase causes glycogen to build up in the lysosomes of cells. While many tissues are affected, skeletal and cardiac muscle see the earliest involvement, with muscle hypotonia, cardiomyopathy, and breathing difficulties (mainly due to diaphragm weakness) comprising the hallmark symptoms of the infantile form. Muscle weakness and progressive respiratory failure are prominent in later-onset disease.

The spectrum of severity and age of onset in Pompe disease is linked to combinations of mutations on the GAA gene, some of which destroy the body’s ability to produce acid alpha-glucosidase whereas others merely hamper it. Less enzyme produced in the body generally corresponds with more severe disease activity.

The most severe end of the disease spectrum, or “classic infantile Pompe disease,” presents at birth and is recognized in early infancy. Until treatment with enzyme replacement therapy (ERT) became available, patients usually died of cardiorespiratory failure within their first year of life. With therapy, patients have survived into their 20s and beyond. Late-onset disease is a far broader category in which patients can present at any time from their first year, including into middle age.

The emergence in 2006 of alglucosidase alfa (Lumizyme, Sanofi Genzyme), an ERT used long-term to improve survival and slow progression in children and adults, resulted in a boom of research interest, a push to timelier diagnosis, and – with patients living longer – a more thorough understanding of the natural history of Pompe disease. In addition to the usual clinical picture of progressive muscle weakness, difficulty breathing, and cardiomyopathy, investigators are seeing nervous system involvement in patients with Pompe disease.

A patient with Pompe, especially one with late-onset disease, may be diagnosed and even managed by his or her neurologist. To learn more, Neurology Reviews talked to two global experts in Pompe disease: Priya Kishnani, MD, of Duke University in Durham, N.C., and Antonio Toscano, MD, of the University of Messina, in Messina, Italy. 

 

Diagnosis: Still room to improve

“Most neurologists will encounter a patient with Pompe disease,” said Dr. Kishnani, who has been working with Pompe for her entire career as a pediatrician and medical geneticist, treating patients of all ages and disease phenotypes.

“In newborns, diagnosis is more straightforward, because you’ve got an enlarged heart,” she said. And thanks to efforts of researchers like Dr. Kishnani and Pompe advocacy groups, Pompe disease is now a part of the RUSP (Recommended Uniform Screening Panel) for newborns; currently 28 U.S. states are screening for Pompe disease.

“The challenge really is for the later-onset cases, which are 80% of all cases,” Dr. Kishnani said.

Previously, muscle biopsies were the first step toward diagnosis. Dried blot spot assays to detect enzyme deficiency have since become the standard, along with other biochemical tests. Confirmation of the diagnosis is through gene sequencing panels to detect GAA mutations.

“Now that there is a treatment for Pompe disease and the availability of blood-based testing, many previously undiagnosed patients with limb girdle weakness are evaluated and the diagnostic odyssey ends,” Dr. Kishnani said. “But there is still a diagnostic delay, and many cases remain undiagnosed.”

Routine blood tests for creatine kinase and for liver enzymes can help point to Pompe disease. But elevated liver enzymes are often misinterpreted. “It’s about the ratios,” Dr. Kishnani said. “ALT is usually much more elevated if it is coming from a liver cause, and AST is usually higher than ALT if it is coming from muscle. But patients often end up getting a liver biopsy due to so-called elevated liver enzymes. As the workup continues, it is often later recognized that the source of the elevated enzymes is muscle involvement, and a referral to the geneticist or neurologist is made. Only then is appropriate testing to confirm a diagnosis initiated.”

Dr. Toscano, a neurologist who specializes in Pompe disease and other myopathies and who has published on tools for diagnosing late-onset Pompe disease,¹ said that clinicians should be vigilant when evaluating any patient with limb girdle weakness and elevated creatine kinase (CK) – “especially if the CK is under 2,000,” he said, “because it is very rare that patients with Pompe disease have a more elevated CK than that.”

“Elevated CK, myalgia, and exercise intolerance” should prompt clinicians to suspect Pompe disease in a patient of any age, Dr. Toscano said. “When you come across this, you should be very persistent and get to the end of the story.”

Dr. Toscano noted that the blood spot assay, while an important early step, is not fully diagnostic, “because you can have false positives.” The molecular GAA assay is used to confirm Pompe disease. But detecting pathogenic variants on the GAA gene – of which there are more than 500 – can be more complicated than it sounds. Whereas two mutations are required for Pompe disease, sometimes only one can be detected. Dr. Toscano said he also treated some patients for Pompe with only one known mutation but with unequivocal clinical and biochemical aspects of Pompe disease.

While delays in diagnosis for late-onset Pompe disease remain significant -- between 5 and 6 years on average for older patients, and up to 20 years for those with pediatric onset – both Dr. Kishnani and Dr. Toscano said they perceive them to be improving. With McArdle disease, another inherited glycogen storage disorder that is more common than Pompe disease but for which there is no treatment, “the delay is nearly 12 years,” Dr. Toscano said.
 

 

ERT: The sooner the better

Enzyme replacement therapy is indicated for all patients with Pompe disease. Currently two are commercially available: alglucosidase alfa (Lumizyme, Sanofi Genzyme), indicated for all forms of Pompe disease, and avalglucosidase alfa-ngpt (Nexviazyme, Sanofi Genzyme), approved in 2021 for later-onset Pompe, though its indications have yet to be fully defined.

The semimonthly infusions represent, to date, the only disease-modifying therapies commercially available. Enzyme replacement therapy can reverse cardiac damage seen in infants and allow them to meet developmental milestones previously unthinkable. In adults, it can slow progression, though many treated patients will still develop chronic disability and require a wheelchair, respiratory support, or both. “The phenotype of the patients we are seeing today is not as involved as it was prior to enzyme therapy,” said Dr. Kishnani, who was part of the research team that developed ERT and launched the first clinical trials. “This is across the disease spectrum.”

But optimal management means more than just getting a patient on therapy fast, Dr. Kishnani said.

“Very often the thinking is if the patient is on ERT, we’ve done right by the patient. Aspects we don’t look at enough include: Are we monitoring these patients well? Are patients being followed by a multidisciplinary team that includes cardiology, physical therapy, and pulmonary medicine? Are we doing appropriate musculoskeletal assessments? They might have sleep hypoventilation. The BiPap settings may not be correct. Or they have not been assessed for antibodies,” she said.

Many infants with severe phenotypes, notably those who produce no enzyme naturally, will develop immune reactions to the exogenous enzyme therapy. High antibody titers also have been seen and are associated with poor therapeutic response. While this is very clear in the infantile setting, late-onset patients also develop antibodies in response to ERT. In one study in 64 patients,² Dr. Toscano and his colleagues saw that antibodies may affect clinical response during the first 3 years of treatment, while a small study³ by Dr. Kishnani’s group saw clinical decline associated with high antibody titers in patients with late-onset disease.

While the relationship of specific titers to therapeutic response remains unclear, it is important to consider antibodies, along with other factors, in the monitoring of patients with Pompe disease. “We need to always ask, if a patient is falling behind, what could be the reason?” Dr. Kishnani said. “These are the things we as clinicians can do to improve or enhance the impact of ERT.”

Dr. Toscano noted that a common misconception about late-onset Pompe disease is that cardiac manifestations are minimal or absent, whereas as many as about 20% of patients will have heart problems and need to be carefully monitored.
 

Neurological manifestations

With patients surviving longer on ERT, researchers have been able to develop a deeper understanding of the natural history of Pompe disease. Increasingly, they are seeing it as a multisystem disease that includes central nervous system involvement.

“Is Pompe an overt neurodegenerative disease? I would say no,” Dr. Kishnani said. “But there is a neurological component that we’ve got to understand and follow more.”

Glycogen accumulation, she noted, has been found in anterior horn cells, motor neurons, and other parts of the brain. “We have been doing MRIs on children with infantile Pompe, and we have seen some white matter hyperintensities. The clinical significance of this finding is still emerging. Sometimes it is present, but the child is cognitively intact. We have had college graduates who have white matter hyperintensities. So putting it in context will be important. But we know that glycogen is ubiquitous, and autopsy studies have shown that it is present in the brain.”

In recent years, Dr. Toscano’s group has investigated neurovascular complications of Pompe in late-onset patients. “This was something that really surprised us because for several years we have investigated mainly heart, muscle, or respiratory manifestations of the disease, but the central nervous system was really neglected,” he said.

“Occasionally we did some brain MRIs and we found in even young patients some ischemic areas. We thought this was related to slowed circulation – that blood vessels in these patients are weak because they are impaired by glycogen accumulation.” Dr. Toscano and his colleagues followed that observation with a study of late-onset patients,⁴ in which they found that more than half had cerebrovascular abnormalities. “Even in, say, patients 30 to 35 years old we saw this – it’s unusual to have a vascular disorder at that age.”

Dr. Toscano and his colleagues also reported cerebral aneurysms⁵ in patients with Pompe disease and have recommended that clinicians conduct MRI or cerebral angiograms on patients as part of routine follow-up. Blood pressure in Pompe patients should be carefully watched and managed with antihypertensive medication as needed, he said.

Part of the problem is that the proteins in ERT are not able to cross the blood-brain barrier, Dr. Toscano noted, adding that researchers are investigating other treatments that can.
 

 

Pompe disease as a research model

The successful development of ERT for Pompe disease marked a boom in research interest into not just Pompe – for which several experimental therapies are currently in the pipeline – but for other myopathies and glycogen storage disorders.

“I think that Pompe has served as a template both as a muscle disease and a lysosomal storage disease, and so some of our learnings from Pompe have been applied across different diseases,” Dr. Kishnani said.

Studies in spinal muscular atrophy, for example, “in some ways mirrored what was done for Pompe – treatment trials were initiated in babies at the most severe end of the disease population with infantile disease, and used similar clinical trial endpoints,” Dr. Kishnani said. “Even for the later-onset end of the spectrum, the endpoints we used in Pompe for muscle strength and function have been relevant to many other neuromuscular disorders.”

Pompe disease research also ushered in a new appreciation of immune responses in protein replacement therapies, Dr. Kishnani noted.

“In the field today, you hear the term cross-reactive immunological material, or CRIM, all the time,” she said. “But when we first started talking about it in the space of Pompe disease, there was a lot of scientific debate about what the significance of CRIM-negative status was in relationship to the risk for development of high and sustained antibody titer and a poor clinical response. To understand this involved a lot of going back to the data and digging into the small subset of nonresponders. One of the powers of rare disease research is that every patient matters, and it’s important to understand what’s going on at the patient level rather than just the group data level.”
 

A robust pipeline

The decade and a half since the advent of ERT has seen what Dr. Toscano described as “an explosion of interest” in Pompe disease.

“We’re seeing an extraordinary number of papers on everything from clinical, biomarkers, genetics, and rehabilitation – this disease is now considered from every point of view, and this is very important for patients,” Dr. Toscano said. Alongside this has come industry interest in this rare disease, with several companies investigating a range of treatment approaches.

The existence of a treatment, “while not perfect,” he said, “has interested the patient associations and doctors to try and improve service to patients. Patients with Pompe disease are well attended, probably more so than patients with degenerative disorders in which there is no therapy.”

Last year the second ERT, avalglucosidase alfa (Nexviazyme, Sanofi Genzyme) was approved by the U.S. Food and Drug Administration to treat late-onset Pompe disease. The drug, currently being investigated in infants as well, was designed to improve the delivery of the therapeutic enzyme to muscles and enhance glycogen clearance, and results from ongoing trials suggest some functional and clinical benefit over standard ERT.

Other drugs in development for Pompe disease include substrate reduction therapies, which aim to reduce the storage of glycogen in cells, and therapies that improve residual function of mutant GAA enzyme in the body. These and other therapies in development have the potential to modify nervous system manifestations of Pompe disease.⁶

Because a single gene is implicated in Pompe disease, it has long been considered a good candidate for gene therapies that prompt the body to make stable enzyme. Seven companies are now investigating gene therapies in Pompe disease.⁷ Some of these deliver to skeletal muscles and others aim for the liver, where proteins are synthesized and secreted and adverse immune responses might be more easily mitigated. Other gene therapies use an ex vivo approach, removing and replacing cells in bone marrow.

Dr. Kishnani’s research group at Duke University is leading a small clinical trial in late-onset patients of a GAA gene transfer to the liver using adeno-associated virus (AAV) vectors.⁸

“We have started AAV gene therapy trials in in adults with Pompe disease and will later evaluate children because ERT is available as a standard of care, and so from a safety perspective this makes the most sense,” Dr. Kishnani said. “We do have challenges in the field of gene therapy, but I think if we are able to overcome the immune responses, and … to treat at a lower dose, there’s a very good pathway forward.”

Dr. Toscano and Dr. Kishnani have received reimbursement from Sanofi and other manufacturers for participation on advisory boards and as speakers.



Jennie Smith is a freelance journalist and editor specializing in medicine and health.

 

References

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2. Filosto M et al. Assessing the role of anti rh-GAA in modulating response to ERT in a late-onset Pompe disease cohort from the Italian GSDII Study Group. Adv Ther. 2019 May;36(5):1177-1189. doi: 10.1007/s12325-019-00926-5.

3. Patel TT et al. The impact of antibodies in late-onset Pompe disease: A case series and literature review. Mol Genet Metab. 2012 Jul;106(3):301-9. doi: 10.1016/j.ymgme.2012.04.027.

4. Montagnese F et al. Intracranial arterial abnormalities in patients with late onset Pompe disease (LOPD). J Inherit Metab Dis. 2016 May;39(3):391-398. doi: 10.1007/s10545-015-9913-x.

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6. Edelmann MJ, Maegawa GHB. CNS-targeting therapies for lysosomal storage diseases: Current advances and challenges. Front Mol Biosci. 2020 Nov 12;7:559804. doi: 10.3389/fmolb.2020.559804

7. Ronzitti G et al. Progress and challenges of gene therapy for Pompe disease. Ann Transl Med. 2019 Jul;7(13):287. doi: 10.21037/atm.2019.04.67.

8. Kishnani PS, Koeberl DD. Liver depot gene therapy for Pompe disease. Ann Transl Med. 2019 Jul;7(13):288. doi: 10.21037/atm.2019.05.02.