Jonhan Ho, MD

To the Editor:

Bullous pemphigoid (BP) is an autoimmune blistering disease that typically affects the elderly, with an incidence of approximately 7 new cases per million.¹ The pathogenesis of BP involves autoantibodies to BP antigens 180 and 230 at the dermoepidermal junction. Bullous pemphigoid has been associated with the use of multiple medications; vaccines; and physical damage to the skin, including trauma, radiation, and surgery.²

Several classes of medications may cause BP; one study described an association of BP with loop diuretics,³ while others found higher incidences of BP in patients taking aldosterone antagonists and neuroleptics.⁴ We describe a case of drug-triggered BP to liraglutide, a glucagonlike peptide 1 (GLP-1) receptor agonist.

A 75-year-old man presented to dermatology for evaluation of a vesicular eruption on the head, neck, trunk, and arms of 6 months’ duration. The eruption developed 2 weeks after starting liraglutide 1.2 mg subcutaneously daily for diabetes mellitus. The patient had a medical history of type 2 diabetes mellitus, hypertension, stroke, and prostate cancer treated with prostatectomy, and he also was taking insulin. Liraglutide was discontinued shortly after the onset of the eruption.

Physical examination revealed annular plaques on the head, neck, trunk, and arms with central hypopigmentation and hyperpigmented borders (Figure 1). Two tense bullae were evident on the left flank (Figure 2). Histopathology revealed a subepidermal blister, mixed perivascular infiltrate with numerous eosinophils, and pigment incontinence (Figure 3). Direct immunofluorescence showed linear deposition of IgG and C3 along the basement membrane zone that was localized to the roof of the blister on salt-split analysis. No microorganisms were identified on periodic acid–Schiff, Grocott-Gomori methenamine-silver, acid-fast bacilli, and Fite stains. The patient initially was treated with clobetasol ointment 0.05%, leading to marginal improvement. He declined treatment with prednisone or dapsone, and he was started on doxycycline. Seven months after stopping liraglutide and starting doxycycline, the patient had no blisters, but residual pigmentary changes remained.

Two types of BP have been described in response to medications: drug-induced BP and drug-triggered BP. Drug-induced BP presents as an acute, self-limited eruption that typically resolves after withdrawal of the offending agent. It tends to involve a younger population and may present with mucosal involvement and target lesions on the palms and soles. Direct immunofluorescence shows linear IgG and C3 deposition at the basement membrane zone. Patients tend to respond quickly to systemic corticosteroids and have low recurrence rates. Drug-triggered BP is a chronic form of BP that is caused by a medication and is not resolved with removal of the offending agent.⁵ Therefore, drug-triggered BP is more difficult to detect, especially in patients taking multiple medications.

Our patient represents a case of drug-triggered BP to liraglutide. Liraglutide is a GLP-1 receptor agonist that is US Food and Drug Administration approved for the treatment of type 2 diabetes mellitus. Glucagonlike peptide 1 is an incretin hormone that is secreted by the intestine during digestion. It binds to the GLP-1 receptor leading to an increase in glucose-dependent insulin secretion and a decrease in glucagon secretion.⁶ Glucagonlike peptide 1 agonists also affect the immune system; liraglutide has been shown to modestly improve psoriasis, reduce the number of dermal gamma delta T cells, and decrease IL-17 expression.⁷ Glucagonlike peptide 1 agonists also produce anti-inflammatory effects on multiple organs including the liver, brain, vasculature, kidney, and skin.⁸

Dipeptidyl peptidase 4 (DPP-4) inhibitors that function to inhibit the degradation of GLP-1 and other peptides also have been reported to cause BP. In several patients, the DPP-4 inhibitors vildagliptin and sitagliptin caused drug-induced BP that resolved with discontinuation of the medication.⁹ Dipeptidyl peptidase 4 is expressed in various organ systems including the skin, and inhibition of DPP-4 enhances eosinophil mobilization in the blood and recruitment to the skin in animal models.¹⁰

Although the pathogenesis of BP involves autoantibodies to BP antigens 180 and 230, these antibodies are not sufficient to cause disease, as antibasement antibodies have been detected in patients without clinically evident BP. These patients, however, may be more susceptible to developing medication-induced BP. Several hypotheses regarding the pathogenesis of medication-induced BP have been proposed, including immune dysregulation, molecular mimicry, and cross-reactivity to a prior sensitizing agent.⁵ Liraglutide and the DPP-4 inhibitors affect the immune system, supporting the hypothesis of immune dysregulation; however, the exact mechanism of how immune modulating medications such as GLP-1 agonists and DPP-4 inhibitors cause BP remains unclear.

The effects of liraglutide and the DPP-4 inhibitors on the immune system may play a role in the pathogenesis of drug-triggered BP and drug-induced BP, respectively. Additional studies of the immunomodulatory effects of GLP-1 agonists and DPP-4 inhibitors may help elucidate the pathogenesis of drug-triggered or drug-induced BP.

To the Editor:

Bullous pemphigoid (BP) is an autoimmune blistering disease that typically affects the elderly, with an incidence of approximately 7 new cases per million.¹ The pathogenesis of BP involves autoantibodies to BP antigens 180 and 230 at the dermoepidermal junction. Bullous pemphigoid has been associated with the use of multiple medications; vaccines; and physical damage to the skin, including trauma, radiation, and surgery.²

Several classes of medications may cause BP; one study described an association of BP with loop diuretics,³ while others found higher incidences of BP in patients taking aldosterone antagonists and neuroleptics.⁴ We describe a case of drug-triggered BP to liraglutide, a glucagonlike peptide 1 (GLP-1) receptor agonist.

A 75-year-old man presented to dermatology for evaluation of a vesicular eruption on the head, neck, trunk, and arms of 6 months’ duration. The eruption developed 2 weeks after starting liraglutide 1.2 mg subcutaneously daily for diabetes mellitus. The patient had a medical history of type 2 diabetes mellitus, hypertension, stroke, and prostate cancer treated with prostatectomy, and he also was taking insulin. Liraglutide was discontinued shortly after the onset of the eruption.

Physical examination revealed annular plaques on the head, neck, trunk, and arms with central hypopigmentation and hyperpigmented borders (Figure 1). Two tense bullae were evident on the left flank (Figure 2). Histopathology revealed a subepidermal blister, mixed perivascular infiltrate with numerous eosinophils, and pigment incontinence (Figure 3). Direct immunofluorescence showed linear deposition of IgG and C3 along the basement membrane zone that was localized to the roof of the blister on salt-split analysis. No microorganisms were identified on periodic acid–Schiff, Grocott-Gomori methenamine-silver, acid-fast bacilli, and Fite stains. The patient initially was treated with clobetasol ointment 0.05%, leading to marginal improvement. He declined treatment with prednisone or dapsone, and he was started on doxycycline. Seven months after stopping liraglutide and starting doxycycline, the patient had no blisters, but residual pigmentary changes remained.

Two types of BP have been described in response to medications: drug-induced BP and drug-triggered BP. Drug-induced BP presents as an acute, self-limited eruption that typically resolves after withdrawal of the offending agent. It tends to involve a younger population and may present with mucosal involvement and target lesions on the palms and soles. Direct immunofluorescence shows linear IgG and C3 deposition at the basement membrane zone. Patients tend to respond quickly to systemic corticosteroids and have low recurrence rates. Drug-triggered BP is a chronic form of BP that is caused by a medication and is not resolved with removal of the offending agent.⁵ Therefore, drug-triggered BP is more difficult to detect, especially in patients taking multiple medications.

Our patient represents a case of drug-triggered BP to liraglutide. Liraglutide is a GLP-1 receptor agonist that is US Food and Drug Administration approved for the treatment of type 2 diabetes mellitus. Glucagonlike peptide 1 is an incretin hormone that is secreted by the intestine during digestion. It binds to the GLP-1 receptor leading to an increase in glucose-dependent insulin secretion and a decrease in glucagon secretion.⁶ Glucagonlike peptide 1 agonists also affect the immune system; liraglutide has been shown to modestly improve psoriasis, reduce the number of dermal gamma delta T cells, and decrease IL-17 expression.⁷ Glucagonlike peptide 1 agonists also produce anti-inflammatory effects on multiple organs including the liver, brain, vasculature, kidney, and skin.⁸

Dipeptidyl peptidase 4 (DPP-4) inhibitors that function to inhibit the degradation of GLP-1 and other peptides also have been reported to cause BP. In several patients, the DPP-4 inhibitors vildagliptin and sitagliptin caused drug-induced BP that resolved with discontinuation of the medication.⁹ Dipeptidyl peptidase 4 is expressed in various organ systems including the skin, and inhibition of DPP-4 enhances eosinophil mobilization in the blood and recruitment to the skin in animal models.¹⁰

Although the pathogenesis of BP involves autoantibodies to BP antigens 180 and 230, these antibodies are not sufficient to cause disease, as antibasement antibodies have been detected in patients without clinically evident BP. These patients, however, may be more susceptible to developing medication-induced BP. Several hypotheses regarding the pathogenesis of medication-induced BP have been proposed, including immune dysregulation, molecular mimicry, and cross-reactivity to a prior sensitizing agent.⁵ Liraglutide and the DPP-4 inhibitors affect the immune system, supporting the hypothesis of immune dysregulation; however, the exact mechanism of how immune modulating medications such as GLP-1 agonists and DPP-4 inhibitors cause BP remains unclear.

The effects of liraglutide and the DPP-4 inhibitors on the immune system may play a role in the pathogenesis of drug-triggered BP and drug-induced BP, respectively. Additional studies of the immunomodulatory effects of GLP-1 agonists and DPP-4 inhibitors may help elucidate the pathogenesis of drug-triggered or drug-induced BP.

To the Editor:

Bullous pemphigoid (BP) is an autoimmune blistering disease that typically affects the elderly, with an incidence of approximately 7 new cases per million.¹ The pathogenesis of BP involves autoantibodies to BP antigens 180 and 230 at the dermoepidermal junction. Bullous pemphigoid has been associated with the use of multiple medications; vaccines; and physical damage to the skin, including trauma, radiation, and surgery.²

Several classes of medications may cause BP; one study described an association of BP with loop diuretics,³ while others found higher incidences of BP in patients taking aldosterone antagonists and neuroleptics.⁴ We describe a case of drug-triggered BP to liraglutide, a glucagonlike peptide 1 (GLP-1) receptor agonist.

A 75-year-old man presented to dermatology for evaluation of a vesicular eruption on the head, neck, trunk, and arms of 6 months’ duration. The eruption developed 2 weeks after starting liraglutide 1.2 mg subcutaneously daily for diabetes mellitus. The patient had a medical history of type 2 diabetes mellitus, hypertension, stroke, and prostate cancer treated with prostatectomy, and he also was taking insulin. Liraglutide was discontinued shortly after the onset of the eruption.

Physical examination revealed annular plaques on the head, neck, trunk, and arms with central hypopigmentation and hyperpigmented borders (Figure 1). Two tense bullae were evident on the left flank (Figure 2). Histopathology revealed a subepidermal blister, mixed perivascular infiltrate with numerous eosinophils, and pigment incontinence (Figure 3). Direct immunofluorescence showed linear deposition of IgG and C3 along the basement membrane zone that was localized to the roof of the blister on salt-split analysis. No microorganisms were identified on periodic acid–Schiff, Grocott-Gomori methenamine-silver, acid-fast bacilli, and Fite stains. The patient initially was treated with clobetasol ointment 0.05%, leading to marginal improvement. He declined treatment with prednisone or dapsone, and he was started on doxycycline. Seven months after stopping liraglutide and starting doxycycline, the patient had no blisters, but residual pigmentary changes remained.

Two types of BP have been described in response to medications: drug-induced BP and drug-triggered BP. Drug-induced BP presents as an acute, self-limited eruption that typically resolves after withdrawal of the offending agent. It tends to involve a younger population and may present with mucosal involvement and target lesions on the palms and soles. Direct immunofluorescence shows linear IgG and C3 deposition at the basement membrane zone. Patients tend to respond quickly to systemic corticosteroids and have low recurrence rates. Drug-triggered BP is a chronic form of BP that is caused by a medication and is not resolved with removal of the offending agent.⁵ Therefore, drug-triggered BP is more difficult to detect, especially in patients taking multiple medications.

Our patient represents a case of drug-triggered BP to liraglutide. Liraglutide is a GLP-1 receptor agonist that is US Food and Drug Administration approved for the treatment of type 2 diabetes mellitus. Glucagonlike peptide 1 is an incretin hormone that is secreted by the intestine during digestion. It binds to the GLP-1 receptor leading to an increase in glucose-dependent insulin secretion and a decrease in glucagon secretion.⁶ Glucagonlike peptide 1 agonists also affect the immune system; liraglutide has been shown to modestly improve psoriasis, reduce the number of dermal gamma delta T cells, and decrease IL-17 expression.⁷ Glucagonlike peptide 1 agonists also produce anti-inflammatory effects on multiple organs including the liver, brain, vasculature, kidney, and skin.⁸

Dipeptidyl peptidase 4 (DPP-4) inhibitors that function to inhibit the degradation of GLP-1 and other peptides also have been reported to cause BP. In several patients, the DPP-4 inhibitors vildagliptin and sitagliptin caused drug-induced BP that resolved with discontinuation of the medication.⁹ Dipeptidyl peptidase 4 is expressed in various organ systems including the skin, and inhibition of DPP-4 enhances eosinophil mobilization in the blood and recruitment to the skin in animal models.¹⁰

Although the pathogenesis of BP involves autoantibodies to BP antigens 180 and 230, these antibodies are not sufficient to cause disease, as antibasement antibodies have been detected in patients without clinically evident BP. These patients, however, may be more susceptible to developing medication-induced BP. Several hypotheses regarding the pathogenesis of medication-induced BP have been proposed, including immune dysregulation, molecular mimicry, and cross-reactivity to a prior sensitizing agent.⁵ Liraglutide and the DPP-4 inhibitors affect the immune system, supporting the hypothesis of immune dysregulation; however, the exact mechanism of how immune modulating medications such as GLP-1 agonists and DPP-4 inhibitors cause BP remains unclear.

The effects of liraglutide and the DPP-4 inhibitors on the immune system may play a role in the pathogenesis of drug-triggered BP and drug-induced BP, respectively. Additional studies of the immunomodulatory effects of GLP-1 agonists and DPP-4 inhibitors may help elucidate the pathogenesis of drug-triggered or drug-induced BP.

Lesions of porokeratosis are thought to arise from disordered keratinization, though the exact pathogenesis remains uncertain. At least 5 clinical subtypes of porokeratosis have been identified: porokeratosis of Mibelli, disseminated superficial porokeratosis and disseminated superficial actinic porokeratosis (DSAP), linear porokeratosis, punctuate porokeratosis, and porokeratosis palmaris et plantaris disseminata (PPPD).^1,2 Linear porokeratosis is a rare subtype with a clinical differential diagnosis that includes lichen striatus, linear lichen planus, linear verrucous epidermal nevus, segmental Darier disease, and incontinentia pigmenti.³ Definitive diagnosis of linear porokeratosis is made by histopathologic examination demonstrating a cornoid lamella, defined as a column of parakeratotic cells that lies at 45°to the surface of the epidermis and contains pyknotic basophilic nuclei.⁴ Patients with linear porokeratosis typically develop lesions along the lines of Blaschko in infancy or childhood.^5,6 Among the different subtypes of porokeratosis, linear porokeratosis demonstrates the highest rate of malignant transformation, therefore requiring close clinical observation.⁷

Case Report

An 83-year-old woman presented to the outpatient clinic with a large linear plaque on the right leg that had been present since birth. Ten years prior to presentation, a portion of the lesion started to bleed; biopsy of the area was performed by an outside provider demonstrating squamous cell carcinoma (SCC), which was treated with wide local excision. One year prior to presentation, a separate portion of the plaque was biopsied by an outside provider and another diagnosis of SCC was made.

On examination performed during the initial presentation to our clinic, there was a well-demarcated tan to violaceous linear plaque present at the lower buttock and extending along the posterior leg to the skin overlying the Achilles tendon and dorsal aspect of the right foot. Within the plaque, there were areas of atrophy and areas of inflammation, induration, and hyperkeratosis (Figures 1 and 2). Two punch biopsies were performed: one from the edge of the plaque and one from a hyperkeratotic region within the plaque. Histology from the edge of the plaque demonstrated a cornoid lamella, consistent with a porokeratosis (Figure 3), whereas the histology from the hyperkeratotic region demonstrated a lichenoid infiltrate (Figure 4).

Several treatment options directed at the entire lesion were offered to the patient, but she declined these therapies and opted to address only those areas with clinical features of SCC, such as hyperkeratosis, bleeding, and rapid growth. Although biopsies performed by an outside provider were consistent with SCC, it had not been detected on biopsy performed during her initial visit to our clinic.

The patient was educated on the risk associated with her condition and instructed to follow up every 6 months to monitor for the development of SCC.

Comment

Porokeratosis is a disorder of keratinization with at least 5 clinical subtypes that share histologic similarities: porokeratosis of Mibelli, disseminated superficial porokeratosis and DSAP, linear porokeratosis, punctate porokeratosis, and PPPD.^1,2 Other less common variants of porokeratosis include porokeratosis ptychotropica (a verrucous variant confined to the perianal area) and congenital unilateral linear porokeratosis.^8,9

Linear porokeratosis appears in infancy or childhood with plaques that follow the lines of Blaschko.^5,6 Most commonly, it presents unilaterally with annular plaques and linear hyperkeratotic papules that preferentially affect the extremities, though it also may present in a more generalized form or appear in a zosteriform pattern.^10,11 Linear porokeratosis affects fewer than 20,000 individuals in the United States and accounts for fewer than 13% of all porokeratosis cases.^12,13

Despite its relatively low prevalence, early identification of linear porokeratosis is important due to its high oncogenic potential, with malignant transformation to basal cell carcinoma or, more commonly, SCC reported in 19% of reported cases.^1,5,7,14 The malignant transformation rate of linear porokeratosis is reported to be higher than rates seen in other porokeratosis subtypes (9.5%, 7.6%, and 3.4% for PPPD, porokeratosis of Mibelli, and DSAP, respectively).⁷ The risk of malignant transformation from porokeratosis increases with exposure to ionizing radiation, duration of the lesion, larger or coalescing lesions, and advanced age.^7,15,16 Histologic studies have provided support for correlation between lesion size and oncogenic potential, with greater numbers of mitotic cells and more abnormal DNA ploidy seen in larger lesions.¹⁷

Histopathology
All subtypes of porokeratosis share certain histopathologic features that aid in the diagnosis of the disorder.¹⁸ Identification of the clinically observed hyperkeratotic ridged border or cornoid lamella is the primary means of definitively diagnosing porokeratosis; however, cornoid lamellae may be observed in other conditions, including verruca vulgaris and actinic keratosis.^4,14

The cornoid lamella appears as a skewed column of densely packed parakeratotic cells with pyknotic basophilic nuclei extending through the stratum corneum from an epidermal invagination.⁴ Directly beneath the cornoid lamella, the granular layer is markedly diminished or absent, and cells of the stratum spinosum may demonstrate vacuolar changes or dyskeratosis.^4,19 The superficial layer of the cornoid lamella may appear to be more centrifugally located and the cornoid lamella may be seen in several locations throughout the lesion.^2,20 The degree of epidermal invagination, which is present under the cornoid lamella, varies by porokeratosis subtype; the central portion of the lesion may contain epidermis that ranges from hyperplastic to atrophic.² Shumack et al²¹ noted that histologic changes under the cornoid lamella may include a lichenoid tissue reaction, papillary dermal lymphocytic infiltrate, vacuolar changes, dyskeratosis, and liquefaction degeneration of the basal layer. Because many of these histologic features also can be identified in lichen planus, a biopsy of the edge of lesions of porokeratosis is essential for making the correct diagnosis.

Heritability
Although linear porokeratosis has no identified pattern of inheritance and appears sporadic in onset, reports have described concomitant occurrence of linear porokeratosis and DSAP as well as linear porokeratosis arising in children of parents who have a diagnosis of DSAP.^5,18,22,23 Based on these findings, it has been hypothesized that linear porokeratosis may represent a mosaic or segmental form of autosomal-dominant inherited subtypes of porokeratosis, such as DSAP.⁵ According to this hypothesis, loss of heterozygosity in patients with a DSAP mutation during early embryogenesis leads to proliferation of cells that are homozygous or hemizygous for the underlying mutation along lines of Blaschko.²⁴ It has been suggested that the allelic loss implicated in the development of linear porokeratosis is the first step in a multistage process of carcinogenesis, which may help to explain the higher rates of malignant transformation that can be seen in linear porokeratosis.²⁴

Management
Several treatment options exist for porokeratosis, including cryotherapy, topical 5-fluorouracil with or without adjunctive retinoid treatment, topical imiquimod, CO₂ laser, shave and linear excision, curettage, dermabrasion, and oral acitretin for widespread lesions.^1,25-29 One case report detailed successful treatment of adult-onset linear porokeratosis with tacrolimus ointment 0.1%.³⁰ Treatments for porokeratosis demonstrate variable degrees of success, with the aim of eradicating the clonal population of mutant keratinocytes.² Additionally, protection from UV radiation should be encouraged, especially in patients who have lesions that occur in areas of high actinic damage.¹

Conclusion

We report of a case of linear porokeratosis with associated multiple SCCs that developed within the lesion. Definitive diagnosis of linear porokeratosis is important due to the higher rate of malignant transformation than the rate seen in other porokeratoses. In larger lesions, appropriate sampling and orientation of the pathology specimen is essential for identifying cornoid lamellae, thus allowing for appropriate follow-up and management. Several treatment options are available, though evidence for the effectiveness of any particular therapy is lacking. Research has shed light on possible genetic and molecular abnormalities in linear porokeratosis, but the exact pathogenesis of the disorder remains unclear.

Lesions of porokeratosis are thought to arise from disordered keratinization, though the exact pathogenesis remains uncertain. At least 5 clinical subtypes of porokeratosis have been identified: porokeratosis of Mibelli, disseminated superficial porokeratosis and disseminated superficial actinic porokeratosis (DSAP), linear porokeratosis, punctuate porokeratosis, and porokeratosis palmaris et plantaris disseminata (PPPD).^1,2 Linear porokeratosis is a rare subtype with a clinical differential diagnosis that includes lichen striatus, linear lichen planus, linear verrucous epidermal nevus, segmental Darier disease, and incontinentia pigmenti.³ Definitive diagnosis of linear porokeratosis is made by histopathologic examination demonstrating a cornoid lamella, defined as a column of parakeratotic cells that lies at 45°to the surface of the epidermis and contains pyknotic basophilic nuclei.⁴ Patients with linear porokeratosis typically develop lesions along the lines of Blaschko in infancy or childhood.^5,6 Among the different subtypes of porokeratosis, linear porokeratosis demonstrates the highest rate of malignant transformation, therefore requiring close clinical observation.⁷

Case Report

An 83-year-old woman presented to the outpatient clinic with a large linear plaque on the right leg that had been present since birth. Ten years prior to presentation, a portion of the lesion started to bleed; biopsy of the area was performed by an outside provider demonstrating squamous cell carcinoma (SCC), which was treated with wide local excision. One year prior to presentation, a separate portion of the plaque was biopsied by an outside provider and another diagnosis of SCC was made.

On examination performed during the initial presentation to our clinic, there was a well-demarcated tan to violaceous linear plaque present at the lower buttock and extending along the posterior leg to the skin overlying the Achilles tendon and dorsal aspect of the right foot. Within the plaque, there were areas of atrophy and areas of inflammation, induration, and hyperkeratosis (Figures 1 and 2). Two punch biopsies were performed: one from the edge of the plaque and one from a hyperkeratotic region within the plaque. Histology from the edge of the plaque demonstrated a cornoid lamella, consistent with a porokeratosis (Figure 3), whereas the histology from the hyperkeratotic region demonstrated a lichenoid infiltrate (Figure 4).

Several treatment options directed at the entire lesion were offered to the patient, but she declined these therapies and opted to address only those areas with clinical features of SCC, such as hyperkeratosis, bleeding, and rapid growth. Although biopsies performed by an outside provider were consistent with SCC, it had not been detected on biopsy performed during her initial visit to our clinic.

The patient was educated on the risk associated with her condition and instructed to follow up every 6 months to monitor for the development of SCC.

Comment

Porokeratosis is a disorder of keratinization with at least 5 clinical subtypes that share histologic similarities: porokeratosis of Mibelli, disseminated superficial porokeratosis and DSAP, linear porokeratosis, punctate porokeratosis, and PPPD.^1,2 Other less common variants of porokeratosis include porokeratosis ptychotropica (a verrucous variant confined to the perianal area) and congenital unilateral linear porokeratosis.^8,9

Linear porokeratosis appears in infancy or childhood with plaques that follow the lines of Blaschko.^5,6 Most commonly, it presents unilaterally with annular plaques and linear hyperkeratotic papules that preferentially affect the extremities, though it also may present in a more generalized form or appear in a zosteriform pattern.^10,11 Linear porokeratosis affects fewer than 20,000 individuals in the United States and accounts for fewer than 13% of all porokeratosis cases.^12,13

Despite its relatively low prevalence, early identification of linear porokeratosis is important due to its high oncogenic potential, with malignant transformation to basal cell carcinoma or, more commonly, SCC reported in 19% of reported cases.^1,5,7,14 The malignant transformation rate of linear porokeratosis is reported to be higher than rates seen in other porokeratosis subtypes (9.5%, 7.6%, and 3.4% for PPPD, porokeratosis of Mibelli, and DSAP, respectively).⁷ The risk of malignant transformation from porokeratosis increases with exposure to ionizing radiation, duration of the lesion, larger or coalescing lesions, and advanced age.^7,15,16 Histologic studies have provided support for correlation between lesion size and oncogenic potential, with greater numbers of mitotic cells and more abnormal DNA ploidy seen in larger lesions.¹⁷

Histopathology
All subtypes of porokeratosis share certain histopathologic features that aid in the diagnosis of the disorder.¹⁸ Identification of the clinically observed hyperkeratotic ridged border or cornoid lamella is the primary means of definitively diagnosing porokeratosis; however, cornoid lamellae may be observed in other conditions, including verruca vulgaris and actinic keratosis.^4,14

The cornoid lamella appears as a skewed column of densely packed parakeratotic cells with pyknotic basophilic nuclei extending through the stratum corneum from an epidermal invagination.⁴ Directly beneath the cornoid lamella, the granular layer is markedly diminished or absent, and cells of the stratum spinosum may demonstrate vacuolar changes or dyskeratosis.^4,19 The superficial layer of the cornoid lamella may appear to be more centrifugally located and the cornoid lamella may be seen in several locations throughout the lesion.^2,20 The degree of epidermal invagination, which is present under the cornoid lamella, varies by porokeratosis subtype; the central portion of the lesion may contain epidermis that ranges from hyperplastic to atrophic.² Shumack et al²¹ noted that histologic changes under the cornoid lamella may include a lichenoid tissue reaction, papillary dermal lymphocytic infiltrate, vacuolar changes, dyskeratosis, and liquefaction degeneration of the basal layer. Because many of these histologic features also can be identified in lichen planus, a biopsy of the edge of lesions of porokeratosis is essential for making the correct diagnosis.

Heritability
Although linear porokeratosis has no identified pattern of inheritance and appears sporadic in onset, reports have described concomitant occurrence of linear porokeratosis and DSAP as well as linear porokeratosis arising in children of parents who have a diagnosis of DSAP.^5,18,22,23 Based on these findings, it has been hypothesized that linear porokeratosis may represent a mosaic or segmental form of autosomal-dominant inherited subtypes of porokeratosis, such as DSAP.⁵ According to this hypothesis, loss of heterozygosity in patients with a DSAP mutation during early embryogenesis leads to proliferation of cells that are homozygous or hemizygous for the underlying mutation along lines of Blaschko.²⁴ It has been suggested that the allelic loss implicated in the development of linear porokeratosis is the first step in a multistage process of carcinogenesis, which may help to explain the higher rates of malignant transformation that can be seen in linear porokeratosis.²⁴

Management
Several treatment options exist for porokeratosis, including cryotherapy, topical 5-fluorouracil with or without adjunctive retinoid treatment, topical imiquimod, CO₂ laser, shave and linear excision, curettage, dermabrasion, and oral acitretin for widespread lesions.^1,25-29 One case report detailed successful treatment of adult-onset linear porokeratosis with tacrolimus ointment 0.1%.³⁰ Treatments for porokeratosis demonstrate variable degrees of success, with the aim of eradicating the clonal population of mutant keratinocytes.² Additionally, protection from UV radiation should be encouraged, especially in patients who have lesions that occur in areas of high actinic damage.¹

Conclusion

We report of a case of linear porokeratosis with associated multiple SCCs that developed within the lesion. Definitive diagnosis of linear porokeratosis is important due to the higher rate of malignant transformation than the rate seen in other porokeratoses. In larger lesions, appropriate sampling and orientation of the pathology specimen is essential for identifying cornoid lamellae, thus allowing for appropriate follow-up and management. Several treatment options are available, though evidence for the effectiveness of any particular therapy is lacking. Research has shed light on possible genetic and molecular abnormalities in linear porokeratosis, but the exact pathogenesis of the disorder remains unclear.

Lesions of porokeratosis are thought to arise from disordered keratinization, though the exact pathogenesis remains uncertain. At least 5 clinical subtypes of porokeratosis have been identified: porokeratosis of Mibelli, disseminated superficial porokeratosis and disseminated superficial actinic porokeratosis (DSAP), linear porokeratosis, punctuate porokeratosis, and porokeratosis palmaris et plantaris disseminata (PPPD).^1,2 Linear porokeratosis is a rare subtype with a clinical differential diagnosis that includes lichen striatus, linear lichen planus, linear verrucous epidermal nevus, segmental Darier disease, and incontinentia pigmenti.³ Definitive diagnosis of linear porokeratosis is made by histopathologic examination demonstrating a cornoid lamella, defined as a column of parakeratotic cells that lies at 45°to the surface of the epidermis and contains pyknotic basophilic nuclei.⁴ Patients with linear porokeratosis typically develop lesions along the lines of Blaschko in infancy or childhood.^5,6 Among the different subtypes of porokeratosis, linear porokeratosis demonstrates the highest rate of malignant transformation, therefore requiring close clinical observation.⁷

Case Report

An 83-year-old woman presented to the outpatient clinic with a large linear plaque on the right leg that had been present since birth. Ten years prior to presentation, a portion of the lesion started to bleed; biopsy of the area was performed by an outside provider demonstrating squamous cell carcinoma (SCC), which was treated with wide local excision. One year prior to presentation, a separate portion of the plaque was biopsied by an outside provider and another diagnosis of SCC was made.

On examination performed during the initial presentation to our clinic, there was a well-demarcated tan to violaceous linear plaque present at the lower buttock and extending along the posterior leg to the skin overlying the Achilles tendon and dorsal aspect of the right foot. Within the plaque, there were areas of atrophy and areas of inflammation, induration, and hyperkeratosis (Figures 1 and 2). Two punch biopsies were performed: one from the edge of the plaque and one from a hyperkeratotic region within the plaque. Histology from the edge of the plaque demonstrated a cornoid lamella, consistent with a porokeratosis (Figure 3), whereas the histology from the hyperkeratotic region demonstrated a lichenoid infiltrate (Figure 4).

Several treatment options directed at the entire lesion were offered to the patient, but she declined these therapies and opted to address only those areas with clinical features of SCC, such as hyperkeratosis, bleeding, and rapid growth. Although biopsies performed by an outside provider were consistent with SCC, it had not been detected on biopsy performed during her initial visit to our clinic.

The patient was educated on the risk associated with her condition and instructed to follow up every 6 months to monitor for the development of SCC.

Comment

Porokeratosis is a disorder of keratinization with at least 5 clinical subtypes that share histologic similarities: porokeratosis of Mibelli, disseminated superficial porokeratosis and DSAP, linear porokeratosis, punctate porokeratosis, and PPPD.^1,2 Other less common variants of porokeratosis include porokeratosis ptychotropica (a verrucous variant confined to the perianal area) and congenital unilateral linear porokeratosis.^8,9

Linear porokeratosis appears in infancy or childhood with plaques that follow the lines of Blaschko.^5,6 Most commonly, it presents unilaterally with annular plaques and linear hyperkeratotic papules that preferentially affect the extremities, though it also may present in a more generalized form or appear in a zosteriform pattern.^10,11 Linear porokeratosis affects fewer than 20,000 individuals in the United States and accounts for fewer than 13% of all porokeratosis cases.^12,13

Despite its relatively low prevalence, early identification of linear porokeratosis is important due to its high oncogenic potential, with malignant transformation to basal cell carcinoma or, more commonly, SCC reported in 19% of reported cases.^1,5,7,14 The malignant transformation rate of linear porokeratosis is reported to be higher than rates seen in other porokeratosis subtypes (9.5%, 7.6%, and 3.4% for PPPD, porokeratosis of Mibelli, and DSAP, respectively).⁷ The risk of malignant transformation from porokeratosis increases with exposure to ionizing radiation, duration of the lesion, larger or coalescing lesions, and advanced age.^7,15,16 Histologic studies have provided support for correlation between lesion size and oncogenic potential, with greater numbers of mitotic cells and more abnormal DNA ploidy seen in larger lesions.¹⁷

Histopathology
All subtypes of porokeratosis share certain histopathologic features that aid in the diagnosis of the disorder.¹⁸ Identification of the clinically observed hyperkeratotic ridged border or cornoid lamella is the primary means of definitively diagnosing porokeratosis; however, cornoid lamellae may be observed in other conditions, including verruca vulgaris and actinic keratosis.^4,14

The cornoid lamella appears as a skewed column of densely packed parakeratotic cells with pyknotic basophilic nuclei extending through the stratum corneum from an epidermal invagination.⁴ Directly beneath the cornoid lamella, the granular layer is markedly diminished or absent, and cells of the stratum spinosum may demonstrate vacuolar changes or dyskeratosis.^4,19 The superficial layer of the cornoid lamella may appear to be more centrifugally located and the cornoid lamella may be seen in several locations throughout the lesion.^2,20 The degree of epidermal invagination, which is present under the cornoid lamella, varies by porokeratosis subtype; the central portion of the lesion may contain epidermis that ranges from hyperplastic to atrophic.² Shumack et al²¹ noted that histologic changes under the cornoid lamella may include a lichenoid tissue reaction, papillary dermal lymphocytic infiltrate, vacuolar changes, dyskeratosis, and liquefaction degeneration of the basal layer. Because many of these histologic features also can be identified in lichen planus, a biopsy of the edge of lesions of porokeratosis is essential for making the correct diagnosis.

Heritability
Although linear porokeratosis has no identified pattern of inheritance and appears sporadic in onset, reports have described concomitant occurrence of linear porokeratosis and DSAP as well as linear porokeratosis arising in children of parents who have a diagnosis of DSAP.^5,18,22,23 Based on these findings, it has been hypothesized that linear porokeratosis may represent a mosaic or segmental form of autosomal-dominant inherited subtypes of porokeratosis, such as DSAP.⁵ According to this hypothesis, loss of heterozygosity in patients with a DSAP mutation during early embryogenesis leads to proliferation of cells that are homozygous or hemizygous for the underlying mutation along lines of Blaschko.²⁴ It has been suggested that the allelic loss implicated in the development of linear porokeratosis is the first step in a multistage process of carcinogenesis, which may help to explain the higher rates of malignant transformation that can be seen in linear porokeratosis.²⁴

Management
Several treatment options exist for porokeratosis, including cryotherapy, topical 5-fluorouracil with or without adjunctive retinoid treatment, topical imiquimod, CO₂ laser, shave and linear excision, curettage, dermabrasion, and oral acitretin for widespread lesions.^1,25-29 One case report detailed successful treatment of adult-onset linear porokeratosis with tacrolimus ointment 0.1%.³⁰ Treatments for porokeratosis demonstrate variable degrees of success, with the aim of eradicating the clonal population of mutant keratinocytes.² Additionally, protection from UV radiation should be encouraged, especially in patients who have lesions that occur in areas of high actinic damage.¹

Conclusion

We report of a case of linear porokeratosis with associated multiple SCCs that developed within the lesion. Definitive diagnosis of linear porokeratosis is important due to the higher rate of malignant transformation than the rate seen in other porokeratoses. In larger lesions, appropriate sampling and orientation of the pathology specimen is essential for identifying cornoid lamellae, thus allowing for appropriate follow-up and management. Several treatment options are available, though evidence for the effectiveness of any particular therapy is lacking. Research has shed light on possible genetic and molecular abnormalities in linear porokeratosis, but the exact pathogenesis of the disorder remains unclear.