# Genodermatoses: Ichthyoses and Epidermolysis Bullosa

## Introduction

The ichthyoses and epidermolysis bullosa (EB) represent two major categories of inherited skin disorders with significant morbidity and, in severe forms, mortality. These conditions result from mutations affecting structural proteins essential for epidermal integrity, barrier function, and skin homeostasis. A solid understanding of the molecular basis, clinical subtypes, and management principles is foundational for dermatology residents.

## Ichthyoses

### Overview and Classification

The ichthyoses are a heterogeneous group of disorders of **cornification** characterized by generalized scaling of the skin. They are classified into **non-syndromic** forms (isolated skin involvement) and **syndromic** forms (multisystem involvement). The underlying pathogenic mechanisms include defects in lipid metabolism, cornified envelope formation, desquamation, and keratinization.

### Ichthyosis Vulgaris (IV)

Ichthyosis vulgaris is the **most common ichthyosis**, with a prevalence of 1 in 250 to 1,000 individuals, and follows autosomal semi-dominant inheritance. It is caused by loss-of-function mutations in **FLG** (filaggrin), which lead to impaired epidermal barrier function. Clinically, it presents with fine, white-to-gray scaling predominantly on the extensor extremities while sparing the flexures, and is associated with **keratosis pilaris** and **hyperlinear palms**. There is a strong association with **atopic dermatitis**, as filaggrin deficiency is the strongest known genetic risk factor for AD. Onset occurs in early childhood, and the condition improves in humid environments and with age. Treatment centers on emollients and keratolytics such as urea (10 to 20%), lactic acid, and salicylic acid.

### X-Linked Recessive Ichthyosis (XLI)

X-linked recessive ichthyosis is the second most common ichthyosis, affecting males at a rate of 1 in 2,000 to 6,000. It results from deletion or mutation of the **STS** (steroid sulfatase) gene on Xp22.3. The clinical presentation features large, dark, polygonal ("dirty-appearing") scales with a predilection for the neck, trunk, and extremities while sparing the palms, soles, and flexures. **Corneal opacities** (stromal punctate) occur in affected males and carrier females. An association with **cryptorchidism** exists in approximately 20% of affected males, and carrier mothers may experience **prolonged labor** due to placental sulfatase deficiency and failure to initiate labor. Diagnosis is confirmed by absent steroid sulfatase activity or FISH demonstrating STS deletion.

### Autosomal Recessive Congenital Ichthyoses (ARCI)

The ARCI spectrum includes **lamellar ichthyosis (LI)** and **congenital ichthyosiform erythroderma (CIE)**. Most present at birth as a **collodion baby**, encased in a taut, shiny membrane that sheds over 2 to 4 weeks. Lamellar ichthyosis features large, dark, plate-like scales with ectropion and eclabium but minimal erythema, and is most commonly caused by mutations in **TGM1** (transglutaminase 1). Congenital ichthyosiform erythroderma presents with fine, white scales on an erythematous base with generalized erythroderma, caused by mutations in **ALOX12B**, **ALOXE3**, and others. **Harlequin ichthyosis** is the most severe form of ARCI, presenting at birth with massive, armor-like plates of scale due to mutations in **ABCA12**. Previously uniformly fatal, it is now survivable with intensive neonatal care and retinoids. Complications across the ARCI spectrum include heat intolerance from eccrine duct obstruction, recurrent infections, and failure to thrive.

<image>Clinical comparison photographs showing the phenotypic spectrum of ichthyoses: ichthyosis vulgaris with fine white scaling, X-linked ichthyosis with dark polygonal scales, lamellar ichthyosis with plate-like scales and ectropion, and a collodion baby presentation</image>

| Ichthyosis | Inheritance | Gene | Key Clinical Features | Distinguishing Clue |
|-----------|-------------|------|----------------------|-------------------|
| Ichthyosis vulgaris | AD (semi-dominant) | FLG (filaggrin) | Fine white scaling, spares flexures | Hyperlinear palms; keratosis pilaris; AD association |
| X-linked recessive | XLR | STS (steroid sulfatase) | Dark polygonal scales; spares palms/soles | Corneal opacities; cryptorchidism; prolonged labor |
| Lamellar ichthyosis | AR | TGM1 (most common) | Large plate-like scales; ectropion | Collodion baby; minimal erythema |
| CIE | AR | ALOX12B, ALOXE3 | Fine white scales on erythematous base | Collodion baby; prominent erythema |
| Harlequin ichthyosis | AR | ABCA12 | Armor-like plates at birth | Most severe ARCI; previously fatal |

### Syndromic Ichthyoses

**Netherton syndrome** features ichthyosis linearis circumflexa, trichorrhexis invaginata ("bamboo hair"), and an atopic diathesis, caused by mutations in **SPINK5** (LEKTI protease inhibitor). **Sjogren-Larsson syndrome** combines ichthyosis with spastic diplegia and intellectual disability, resulting from mutations in **ALDH3A2** (fatty aldehyde dehydrogenase). **Refsum disease** presents with ichthyosis, retinitis pigmentosa, peripheral neuropathy, and cerebellar ataxia from defective phytanic acid metabolism. **KID syndrome** (keratitis-ichthyosis-deafness) is caused by mutations in **GJB2** (connexin 26).

### Management of Ichthyoses

**Emollients** are the cornerstone of management, including petrolatum-based creams and ceramide-containing moisturizers. **Keratolytics** such as urea (10 to 40%), lactic acid (12%), ammonium lactate, and salicylic acid (avoiding use in infants due to toxicity risk) help reduce scaling. **Oral retinoids** such as acitretin improve scaling in severe ARCI but do not cure the condition and require monitoring for skeletal toxicity in children. **Collodion baby management** involves a humidified incubator, emollients, and monitoring for sepsis, electrolyte imbalances, and temperature instability. Genetic counseling and prenatal diagnosis are important components of comprehensive care.

## Epidermolysis Bullosa

### Overview

Epidermolysis bullosa comprises a group of inherited mechanobullous disorders characterized by skin and mucosal fragility with blister formation in response to minimal trauma. EB is classified by the level of skin cleavage into four major types: **EB simplex (EBS)**, **junctional EB (JEB)**, **dystrophic EB (DEB)**, and **Kindler EB**. Over 20 subtypes have been described, with mutations in more than 16 genes encoding structural proteins of the dermoepidermal junction.

### EB Simplex

EB simplex involves **intraepidermal** cleavage within basal keratinocytes. It is most commonly autosomal dominant and caused by mutations in **KRT5** and **KRT14** (keratins 5 and 14). **EBS-localized (Weber-Cockayne)** is the mildest form, with blisters limited to the hands and feet that are often worsened by heat and friction. **EBS-generalized severe (Dowling-Meara)** presents with grouped, herpetiform blisters, may cause neonatal blistering, and develops keratoderma over time. Electron microscopy shows cytolysis of basal keratinocytes with clumping of tonofilaments.

### Junctional EB

Junctional EB involves cleavage within the **lamina lucida** of the basement membrane zone. It is autosomal recessive, with mutations in **LAMA3**, **LAMB3**, and **LAMC2** (encoding laminin-332) and **COL17A1** (encoding type XVII collagen/BP180). **JEB-severe (Herlitz type)** is devastating, with generalized blistering, granulation tissue around the mouth, nares, and tracheostomy sites, and enamel hypoplasia. Most patients with Herlitz JEB die in infancy from sepsis or failure to thrive. **JEB-generalized intermediate** is milder, featuring chronic blistering with scarring alopecia and nail dystrophy. Absent or severely diminished **laminin-332** staining on immunofluorescence mapping is diagnostic of Herlitz JEB.

### Dystrophic EB

Dystrophic EB involves cleavage below the lamina densa within the **sublamina densa/upper papillary dermis** and is caused by mutations in **COL7A1** (type VII collagen, the component of anchoring fibrils). **Dominant DEB (DDEB)** is milder, with blistering over acral sites, nail dystrophy, and milia formation. **Recessive DEB-severe (RDEB-sev)** causes generalized blistering from birth, with progressive scarring leading to **mitten deformities** (pseudosyndactyly), esophageal strictures, corneal erosions, chronic anemia, and growth retardation. RDEB carries a **dramatically elevated risk of aggressive squamous cell carcinoma**, with a cumulative risk exceeding 90% by age 55, making SCC the leading cause of death in adult RDEB patients.

### Kindler EB

Kindler EB features a mixed cleavage plane that may be intraepidermal, junctional, or sublamina densa, caused by mutations in **FERMT1** (kindlin-1). Clinical features include acral blistering in childhood, progressive poikiloderma, photosensitivity, mucosal involvement, and risk of SCC.

| EB Type | Cleavage Level | Gene(s) | Inheritance | Key Features | Prognosis |
|---------|---------------|---------|-------------|-------------|-----------|
| EBS-localized | Intraepidermal (basal) | KRT5, KRT14 | AD | Acral blisters; heat-worsened | Good |
| EBS-generalized severe | Intraepidermal (basal) | KRT5, KRT14 | AD | Herpetiform blisters; keratoderma | Moderate |
| JEB-severe (Herlitz) | Lamina lucida | LAMA3, LAMB3, LAMC2 | AR | Perioral granulation tissue; enamel defects | Fatal in infancy |
| JEB-intermediate | Lamina lucida | COL17A1, LAMB3 | AR | Chronic blistering; scarring alopecia | Variable |
| DDEB | Sub-lamina densa | COL7A1 | AD | Acral blisters; milia; nail dystrophy | Good |
| RDEB-severe | Sub-lamina densa | COL7A1 | AR | Mitten deformities; esophageal strictures | Poor (SCC >90% by age 55) |
| Kindler EB | Mixed | FERMT1 | AR | Poikiloderma; photosensitivity | Variable (SCC risk) |

<image>Schematic diagram of the dermoepidermal junction showing the structural proteins affected in each type of epidermolysis bullosa: keratins 5/14 in EBS (intraepidermal), laminin-332 and BP180 in JEB (lamina lucida), type VII collagen in DEB (sub-lamina densa), and kindlin-1 in Kindler EB</image>

### Diagnosis of EB

Skin biopsy of a fresh, induced blister is submitted for immunofluorescence antigen mapping (IFM) and/or transmission electron microscopy (TEM). IFM identifies the level of cleavage and which proteins are absent or reduced. **Genetic testing** is now the gold standard for definitive subtype classification and is essential for determining gene therapy eligibility. Prenatal diagnosis is available via chorionic villus sampling or amniocentesis for families with known mutations.

### Management of EB

Wound care relies on non-adherent dressings (Mepitel, Mepilex), gentle bathing, and avoidance of adhesives. Pain management follows a multimodal approach including topical and systemic analgesics. Nutritional support with a high-protein, high-calorie diet and iron and zinc supplementation addresses the demands of chronic wounds and anemia. Surveillance for SCC in RDEB requires regular full-body skin examinations starting in adolescence. Emerging therapies include gene therapy (beremagene geperpavec, a topical COL7A1 gene therapy now FDA-approved for DEB), bone marrow transplant, protein replacement, and cell-based therapies. Comprehensive management requires multidisciplinary care involving dermatology, gastroenterology, nutrition, orthopedics, ophthalmology, and palliative care.

<image>Clinical photographs showing the spectrum of epidermolysis bullosa: EBS with acral blisters, JEB-severe with perioral granulation tissue, and RDEB with mitten deformities of the hands due to progressive scarring</image>

## Key Clinical Pearls

Filaggrin mutations link ichthyosis vulgaris and atopic dermatitis, and screening for AD in IV patients (and vice versa) is warranted. Collodion babies may evolve into various ARCI subtypes, and "self-healing collodion baby" is a distinct entity with an excellent prognosis. In EB, the level of cleavage determines the type, and immunofluorescence mapping or genetic testing should always be obtained before counseling families. Squamous cell carcinoma surveillance in RDEB patients is life-saving, and any non-healing wound or verrucous lesion warrants biopsy. Beremagene geperpavec represents a landmark in dermatologic gene therapy for dystrophic EB.

## References

1. Oji V, Tadini G, Akiyama M, et al. Revised nomenclature and classification of inherited ichthyoses: results of the First Ichthyosis Consensus Conference. *J Am Acad Dermatol*. 2010;63(4):607-641.
2. Has C, Bauer JW, Bodemer C, et al. Consensus reclassification of inherited epidermolysis bullosa and other disorders with skin fragility. *Br J Dermatol*. 2020;183(4):614-627.
3. Guide SV, Gonzalez ME, Bagci IS, et al. Trial of beremagene geperpavec (B-VEC) for dystrophic epidermolysis bullosa. *N Engl J Med*. 2022;387(24):2211-2219.
4. Richard G, Ringpfeil F. Ichthyoses, erythrokeratodermas, and related disorders. In: Bolognia JL, Schaffer JV, Cerroni L, eds. *Dermatology*. 4th ed. Elsevier; 2018:888-923.