Genetic Basis of Cutaneous Diseases


Key features

  • Advances in molecular technology have led to elucidation of the genetic bases of many single-gene inherited and mosaic skin disorders, greatly improving our understanding of these conditions

  • McKusick's Online Mendelian Inheritance in Man (OMIM) database provides easily accessible, up-to-date information on human genes and genetic diseases; each gene or phenotype entry is assigned a specific six-digit MIM number

  • Genotype–phenotype correlations in genodermatoses are often complex, with multiple examples of allelic heterogeneity (mutations in a single gene causing more than one disorder) and locus heterogeneity (mutations in different genes causing the same disorder)

  • Molecular classification of genodermatoses into categories such as keratin defects and RASopathies complements traditional morphologic classification, highlighting pathomechanisms and relationships between conditions

  • Certain syndromic associations actually represent contiguous gene syndromes caused by large deletions that affect two or more neighboring genes

  • Types 1 and 2 mosaicism in autosomal dominant disorders have been confirmed on a molecular level, and functional X-chromosome mosaicism can lead to a mosaic distribution of skin lesions in female patients heterozygous for X-linked disorders

  • Determining the molecular basis of monogenic skin disorders can enable the development of targeted therapies and provide insights into the pathogenesis of acquired skin disease

  • Molecular research has begun to pave the way to the ultimate goal of gene therapy for severe inherited skin disorders such as epidermolysis bullosa

Abbreviations

AD, autosomal dominant; AR, autosomal recessive; XD, X-linked dominant; XR, X-linked recessive.

Introduction

In 1987, deletions in the steroid sulfatase gene were found to underlie X-linked recessive ichthyosis . This heralded an era of tremendous progress in the elucidation of the genetic bases of inherited skin disorders, made possible by rapid advances in molecular technology (including the development of next-generation massively parallel sequencing), discovery of candidate genes, utilization of animal models, and sequencing of the human genome, including single nucleotide polymorphisms (SNPs) useful for linkage analysis and homozygosity mapping (see Ch. 54 ) . Better understanding of signaling pathways, molecules involved in cell–cell communication and adhesion, and mechanisms of cutaneous differentiation have also contributed to the momentum.

As a result of this explosion in research, more than 1000 genes were recognized to be responsible for a particular human phenotype by the year 2000, with approximately 300 of these conditions including cutaneous abnormalities ; by 2017, these numbers had increased to nearly 3800 and 1400, respectively . Currently, the molecular genetic bases of the majority of single-gene inherited skin disorders have been established . The emergence of new genomic and proteomic databases has transformed laborious positional cloning approaches and traditional functional studies, circumventing previous obstacles to the study of rare conditions and facilitating identification of candidate genes. For example, microarray (see Ch. 3 )-based SNP genotyping enabled the identification of ABCA12 mutations as the cause of harlequin ichthyosis; although classic linkage analysis was not possible due to the limited size of the pedigrees, homozygosity mapping (utilizing the aforementioned technology) linked this autosomal recessive condition to a region of homozygosity shared by affected individuals from diverse ethnic backgrounds . With whole-exome (the transcribed portion of the genome) and whole-genome sequencing now feasible and relatively affordable, disease-causing genes can potentially be identified without genetic mapping .

In the past, diagnosis of genodermatoses was complicated by the existence of multiple complex classification systems based on various combinations of clinical, histologic, radiographic, and biochemical criteria. Inconsistent nomenclature laden with descriptive terms, eponyms, and synonyms added to the confusion and potential for misdiagnosis . As the genetic bases of genodermatoses have been determined, integration of molecular and clinical data has helped to simplify disease categorization and eliminate redundant terminology. This has been successfully accomplished for disorders such as epidermolysis bullosa (EB) and ichthyoses, but it represents a work in progress, to be continually refined as additional genotype–phenotype correlations are established . In addition, grouping hereditary skin disorders according to their molecular bases ( Table 55.1 ) can supplement traditional morphologic classification, clarifying pathomechanisms and the relationships between conditions.

Table 55.1
Molecular classification of genetic skin disorders.
cAMP, cyclic adenosine monophosphate; CANDLE, chronic atypical neutrophilic dermatosis with lipodystrophy and elevated temperature; CHILD, c ongenital h emidysplasia with i chthyosiform nevus and l imb defects; JAK, Janus kinase; MAPK, mitogen-activated protein kinase; NF-κB, nuclear factor-κB; NOD, nucleotide-binding oligomerization domain; NOMID, neonatal-onset multisystem inflammatory disease; PAPA, p yogenic a rthritis, p yoderma gangrenosum, and a cne; SCC, squamous cell carcinoma; STAT, signal transduction and activation of transcription; STING, stimulator of interferon genes; WNT, wingless-type integration site. cAMP, cyclic adenosine monophosphate; MAPK, mitogen-activated protein kinase; SCC, squamous cell carcinoma; WNT, wingless-type integration site.
MOLECULAR CLASSIFICATION OF GENETIC SKIN DISORDERS
Molecular defect Examples of skin disorders that result
Keratin defects See Table 56.4 and Fig. 56.5
Defects in structural proteins of the cornified cell envelope Ichthyosis vulgaris, loricrin keratoderma (variant Vohwinkel syndrome)
Lipid metabolism defects Various ichthyoses, including CHILD syndrome, Conradi–Hünermann–Happle syndrome, neutral lipid storage disease, lamellar ichthyosis/congenital ichthyosiform erythroderma spectrum (e.g. defective lipoxygenase 3 or 12R, cytochrome p450 4F22), self-healing collodion baby (defective lipoxygenase 3 or 12R), late-onset autosomal recessive congenital ichthyosis, Refsum disease, rhizomelic chondrodysplasia punctata, Sjögren–Larsson syndrome, X-linked recessive ichthyosis and ichthyosis, intellectual disability and spastic quadriplegia; various hyperlipidemias; Farber lipogranulomatosis, Gaucher disease type 2, hyperimmunoglobulinemia D syndrome, localized autosomal recessive hypotrichosis II/autosomal recessive woolly hair, Niemann–Pick disease, psoriasiform dermatitis–microcephaly–developmental delay syndrome
Transglutaminase defects Lamellar ichthyosis/congenital ichthyosiform erythroderma spectrum (defective transglutaminase 1), acral peeling skin syndrome
Protease/proteasome defects Dermatosparaxis, familial hidradenitis suppurativa, hereditary angioedema (type III), Howel-Evans syndrome, ichthyosis–hypotrichosis syndrome, IFAP ( i chthyosis f ollicularis– a trichia– p hotophobia) syndrome, KLICK ( k eratosis l inearis– i chthyosis c ongenita–sclerosing k eratoderma) syndrome, Olmsted syndrome, Papillon–Lefèvre and Haim–Munk syndromes, prolidase deficiency, restrictive dermopathy (defective zinc metallopeptidase), hereditary thrombotic thrombocytopenic purpura, MONA ( m ulticentric o steolysis, n odulosis, and a rthropathy), proteasome-associated autoinflammatory/CANDLE/Nakajo-Nishimura syndrome)
Protease inhibitor defects Antithrombin III deficiency, autosomal recessive exfoliative ichthyosis, hereditary angioedema (types I & II), Netherton syndrome
Desmosomal defects (see Table 56.5 and Fig. 56.8 ) Acantholytic epidermolysis bullosa simplex, arrhythmogenic right ventricular dysplasia/cardiomyopathy + palmoplantar keratoderma + woolly hair; Carvajal syndrome, erythrokeratodermia-cardiomyopathy syndrome, generalized inflammatory peeling skin syndrome, hypotrichosis and skin lesions, hypotrichosis simplex of the scalp, localized autosomal recessive hypotrichosis, monilethrix (autosomal recessive), Naxos disease, SAM ( s evere dermatitis, a llergies, and m etabolic wasting), skin fragility-ectodermal dysplasia syndrome, skin fragility/woolly hair syndrome, striate palmoplantar keratoderma
Connexin defects (see Table 58.5 ) Bart–Pumphrey syndrome, Clouston syndrome, erythrokeratodermia variabilis, hereditary lymphedema type IC, ILVEN, keratoderma-hypotrichosis-leukonychia totalis, KID ( k eratitis– i chthyosis– d eafness) syndrome, oculodentodigital dysplasia, palmoplantar keratoderma with deafness, porokeratotic adnexal ostial nevus, Vohwinkel syndrome (classic)
Other defects in cell-to-cell adhesion Ichthyosis–hypotrichosis–sclerosing cholangitis syndrome, cleft lip/palate–ectodermal dysplasia syndrome, ectodermal dysplasia–syndactyly syndrome, ectodermal dysplasia–ectrodactyly–macular dystrophy, hypotrichosis with juvenile macular dystrophy, leukocyte adhesion deficiency type I
Defects in keratinocyte–extracellular matrix (ECM) adhesion Various forms of epidermolysis bullosa (EB) simplex, junctional EB, and dystrophic EB (keratin–ECM linkage); Kindler syndrome (actin–ECM linkage)
ATP-binding cassette (ABC) transporter defects Harlequin ichthyosis, lamellar ichthyosis/congenital ichthyosiform erythroderma spectrum (defective ABCA12 ), pseudoxanthoma elasticum, sitosterolemia, Tangier disease/familial hypoalphaproteinemia
Calcium pump defects Darier disease, Hailey–Hailey disease
Copper transporter defects Menkes disease, occipital horn syndrome, Wilson disease
Defects in other transporters Acrodermatitis enteropathica (zinc transporter), arterial tortuosity syndrome (glucose transporter), autosomal dominant hemochromatosis (iron-regulated transporter), spondylodysplastic Ehlers–Danlos syndrome (zinc transporter), H syndrome (nucleoside transporter), Hartnup disease (neutral amino acid transporter), ichthyosis–prematurity syndrome (fatty acid transporter), oculocutaneous albinism (transporters on lysosome-related organelles)
Collagen defects Various types of dystrophic EB, various types of Ehlers–Danlos syndrome, Ullrich muscular dystrophy, lysyl hydroxylase 3 deficiency
Defects in other ECM proteins Autosomal dominant cutis laxa, autosomal recessive cutis laxa type I, cutis laxa with severe pulmonary/gastrointestinal/urinary abnormalities, classic-like and hypermobile Ehlers–Danlos syndrome, juvenile hyaline fibromatosis/infantile systemic hyalinosis, lipoid proteinosis, Marfan syndrome, stiff skin syndrome
Nuclear membrane defects (see Table 63.10 ) Buschke–Ollendorff syndrome/melorheostosis, Néstor–Guillermo progeria syndrome, Hutchinson–Gilford progeria syndrome, mandibuloacral dysplasia, partial > generalized lipodystrophy (familial and “acquired” forms), restrictive dermopathy
Defects in pyrin/NOD-family members and related proteins (see Fig. 4.2 ) Blau syndrome, NOMID syndrome, familial cold autoinflammatory syndrome, familial Mediterranean fever, Muckle–Wells syndrome, PAPA syndrome
Interferonopathies (see Ch. 45 ) Aicardi-Goutières syndrome, familial chilblain lupus, proteasome-associated autoinflammatory/CANDLE/Nakajo-Nishimura syndrome, SAVI ( S TING- a ssociated v asculopathy with onset in i nfancy), X-linked reticulate pigmentary disorder
RecQ DNA helicase defects Bloom syndrome, Rothmund–Thomson syndrome, Werner syndrome
Classic DNA repair defects Various subtypes of xeroderma pigmentosum, trichothiodystrophy and Cockayne syndrome; Muir–Torre syndrome, constitutional mismatch repair deficiency syndrome
Cell cycle checkpoint defects Ataxia telangiectasia, familial atypical mole and melanoma (FAMM) syndrome, multiple endocrine neoplasia (MEN) 4
Defects in telomere maintenance Dyskeratosis congenita (see Table 67.8 ; 11 genes implicated to date)
RAS–MAPK pathway activation (RASopathies; see Fig. 55.4 ) Capillary malformation–arteriovenous malformation, cerebral capillary malformations (familial), cardio-facio-cutaneous syndrome, congenital melanocytic nevi, Costello syndrome, Legius syndrome, neurofibromatosis type 1, Noonan syndrome (with multiple lentigines), Noonan-like disorder with loose anagen hair, verrucous epidermal nevi, nevus sebaceus, woolly hair nevi
Phosphatidylinositol 3-kinase (PI3K)/AKT pathway activation Epidermal nevi, lymphatic malformations, PIK3CA-related overgrowth spectrum (see Table 104.5 ), Proteus syndrome, PTEN-hamartoma tumor syndrome, tuberous sclerosis complex, venous malformations
G protein activation Diffuse capillary malformation with overgrowth (some patients), extensive dermal melanocytosis, McCune-Albright syndrome (versus loss-of-function in Albright hereditary osteodystrophy), phakomatosis pigmentovascularis, port-wine stains, Sturge-Weber syndrome
cAMP and AMP-activated protein kinase pathway defects (see Fig. 55.3 ) Albright hereditary osteodystrophy, Carney complex, McCune–Albright syndrome, Peutz–Jeghers syndrome, tuberous sclerosis complex
Defects in WNT/β-catenin signaling (see Fig. 55.6 ) Anonychia congenita, ectodermal dysplasia–neoplastic syndrome, Gardner syndrome, Goltz syndrome (focal dermal hypoplasia), hereditary hypotrichosis simplex, palmoplantar keratoderma with cutaneous SCC and sex reversal, WNT10A-related ectodermal dysplasias (including odonto-onycho-dermal dysplasia and Schöpf–Schulz–Passarge syndrome)
Defects in transforming growth factor (TGF)-β signaling Buschke–Ollendorff syndrome/melorheostosis, Ferguson–Smith multiple self-healing squamous epitheliomas, hereditary hemorrhagic telangiectasia, Loeys–Dietz syndrome, Marfan syndrome, stiff skin syndrome
Defective vesicle assembly/protein sorting to vesicles ARC ( a rthrogryposis– r enal d ysfunction– c holestasis) syndrome, Hermansky–Pudlak syndrome, MEDNIK ( m ental retardation, e nteropathy, d eafness, n europathy, i chthyosis, k eratodermia) syndrome
Defective vesicle trafficking or transport Autosomal recessive cutis laxa type IIA, CEDNIK ( ce rebral d ysgenesis, n europathy, i chthyosis and k eratoderma) syndrome, Chédiak–Higashi syndrome, gerodermia osteodysplastica, Griscelli syndrome, ocular albinism type 1, wrinkly skin syndrome, MACS ( m acrocephaly, a lopecia, c utis laxa and s coliosis) syndrome

As the “morbid anatomy of the dermatologic genome” continues to be established, new challenges will arise and additional questions will be answered . Because of the rarity of many genodermatoses, their full clinical spectra have yet to be elucidated. However, with the increasing availability of genetic and biochemical testing ( www.genetests.org ), dermatologists can establish the diagnosis in patients with mild or atypical presentations, thereby expanding the range of phenotypes. Continuing to decipher heritable skin disorders will require close interactions between basic scientists and clinicians . Hopefully, such translational research will continue to lead to better understanding of cutaneous structure and function; insights into the pathogenesis of common multifactorial disorders; and effective therapies as well as diagnostic and prognostic information, improved genetic counseling and DNA-based prenatal/preimplantation testing for patients with genodermatoses.

In this chapter, Tables 55.3 to 55.8 divide selected monogenic skin disorders with a known genetic basis into categories according to their primary clinical features. Groups of monogenic conditions similarly listed in tables elsewhere in the book are noted in Table 55.2 .

Table 55.2
Groups of monogenic skin disorders listed in tables elsewhere in the book.
In addition, other chapters (especially in the genodermatosis section) have tables that present more detailed information on monogenic skin disorders.
GROUPS OF MONOGENIC SKIN DISORDERS LISTED IN TABLES ELSEWHERE IN THE BOOK
Category Table(s)
Congenital insensitivity to pain and related neuropathies 6.10
Epidermolysis bullosa and other blistering disorders 32.1 & 32.3
Sclerodermoid disorders 43.7
Autoinflammatory diseases 45.2 & 45.7
Porphyrias 49.3
Keratinopathies and desmosomal disorders 56.4 ; Figs 56.5 & 56.8
Ichthyoses 57.1 & 57.4
Connexin disorders and keratodermas 58.5 & 58.6
Primary immunodeficiencies 60.4 , 60.6 , 60.8 , 60.10 , 60.13 , 60.15 , & 60.16
Disorders associated with multiple café-au-lait macules 61.4
Epidermal nevi and other mosaic disorders 62.4 & 62.7
Progeroid syndromes/inherited poikilodermas and nuclear membrane disorders 63.9 & 63.10
Ectodermal dysplasias 63.11 , 63.12 , 63.13
Aplasia cutis congenita, cleft lip/palate, and digital anomalies 64.3 , 63.4 , 63.5 , 63.6
Disorders of hypopigmentation (diffuse or circumscribed) 65.1 , 66.4
Disorders with reticulated hyperpigmentation, dyskeratosis congenita, and dyschromatoses 67.7 , 67.8 , & 67.10
Hypotrichosis, hair shaft disorders, and hypertrichosis 69.8 ; 70.1 & 70.2
Xeroderma pigmentosa and other photosensitivity disorders 86.2 , 87.4
Hyperlipidemias 92.2
Ehlers–Danlos syndrome, cutis laxa, and other extracellular matrix disorders 95.5 ; 97.1 , 97.2 , & 97.6
Lipodystrophy syndromes 101.1
Vascular anomalies and overgrowth syndromes 104.2 & 104.5
Disorders with multiple lentigines 112.2

Table 55.3
Selected hereditary disorders of the nails.
Nail abnormalities can also be observed in patients with disorders of keratinization and ectodermal dysplasias. Pachydermoperiostosis is included in Table 55.8 .
SELECTED HEREDITARY DISORDERS OF THE NAILS
Disorder Mode of inheritance Gene or gene product Gene symbol
Anonychia congenita AR R-spondin family, member 4 RSPO4
Isolated AR nail dysplasia AR Frizzled 6 FZD6
Isolated congenital nail clubbing AR Hydroxyprostaglandin dehydrogenase 15-(NAD) HPGD
Isolated toenail dystrophy AD Collagen VII, α1 chain COL7A1
Leukonychia totalis AD, AR Phospholipase C delta 1 PLCD1
Nail–patella syndrome AD LIM homeobox transcription factor 1β LMX1B
Pachyonychia congenita-6a AD Keratin 6a KRT6A
Pachyonychia congenita-16 AD Keratin 16 KRT 16
Pachyonychia congenita-6b AD Keratin 6b KRT6B
Pachonychia congenita-17 AD Keratin 17 KRT17
Tricho-rhino-phalangeal syndrome 1 AD TRPS1 (zinc finger protein) TRPS1

Table 55.4
Selected hereditary metabolic skin disorders.
SELECTED HEREDITARY METABOLIC SKIN DISORDERS
Disorder Mode of inheritance Gene or gene product Gene symbol
Disorders due to enzyme deficiencies (see Ch. 63 )
Alkaptonuria AR Homogentisate 1,2-dioxygenase (homogentisate oxidase) HGD
Biotinidase deficiency AR Biotinidase BTD
Holocarboxylase synthetase deficiency AR Holocarboxylase synthetase HLCS
Fabry disease * XR α-Galactosidase A GLA
Fucosidosis * AR α- l -Fucosidase FUCA1
Farber lipogranulomatosis AR Acid ceramidase ASAH
Gaucher disease types I–III AR Acid β-glucosidase (glucocerebrosidase) GBA
Hereditary angioedema Types I & II AD Serpin peptidase inhibitor, clade G, member 1 (C1 esterase inhibitor) SERPING1
Type III AD (only females affected) Factor XII (Hageman factor) F12
Homocystinuria AR Cystathionine β-synthase CBS
Niemann–Pick disease type A AR Acid sphingomyelinase SMPD1
Phenylketonuria AR Phenylalanine hydroxylase PAH
AR Quinoid dihydropteridine reductase QDPR
AR 6-Pyruvoyltetrahydropterin synthase PTS
Prolidase deficiency AR Prolidase (peptidase D) PEPD
Disorders due to defective transporters/related proteins
Acrodermatitis enteropathica AR Solute carrier family 39, member 4 (zinc transporter) SLC39A4
H syndrome AR Solute carrier family 29, member 3 (nucleoside transporter) SLC29A3
Hartnup disease AR Solute carrier family 6, member 19 (neutral amino acid transporter) SLC6A19
Hemochromatosis AR Hemochromatosis HFE
AR Transferrin receptor 2 TFR2
AD Solute carrier family 40, member 1 (ferroportin) SLC40A1
Hemochromatosis, juvenile AR Hepcidin antimicrobial peptide HAMP
AR Hemojuvelin HFE2
Wilson disease AR Cu 2+ -transporting P-type ATPase 7B ATP7B

* Additional metabolic disorders that can present with angiokeratoma corporis diffusum are presented in Table 63.7 .

Defects in this gene represent the most common cause of homocystinuria.

Including cases reported as pigmented hypertrichotic insulin-dependent diabetes mellitus syndrome and familial histiocytosis syndrome.

Table 55.5
Selected hereditary skin disorders characterized by benign skin tumors.
These conditions are discussed in Chapter 61 , Chapter 62 , Chapter 104 , Chapter 117 , Chapter 118 ). GTP, guanine triphosphate; mt, mitochondrial inheritance; TGF, transforming growth factor.
SELECTED HEREDITARY DISORDERS CHARACTERIZED BY BENIGN SKIN TUMORS
Disorder Mode of inheritance Gene or gene product Gene symbol
Familial cylindromatosis; Brooke–Spiegler syndrome; multiple familial trichoepitheliomas AD CYLD (deubiquitinating enzyme) CYLD
Familial mastocytosis ± gastrointestinal stromal tumors * AD KIT proto-oncogene (stem cell factor receptor) KIT
Leiomyomatosis, cutaneous and uterine (Reed syndrome) AD Fumarate hydratase FH
Lipomatosis, familial multiple AD
Lipomatosis, benign symmetric (Madelung disease) mt Mitochondrial tRNA-lysine MT-TK
AR Mitofusin 2 MFN2
AR Lipase E, hormone sensitive LIPE
Ferguson–Smith multiple self-healing squamous epitheliomas AD Transforming growth factor-β receptor 1 (loss-of-function mutations) TGFBR1
Neurofibromatosis type 1 AD Neurofibromin 1 (GTPase activating protein) NF1
Neurofibromatosis type 2 AD Neurofibromin 2 (merlin) NF2
Tuberous sclerosis complex AD Hamartin TSC1
AD Tuberin TSC2

* Occasionally malignant.

Also associated with renal cell carcinoma.

Associated with myoclonic epilepsy with ragged red fibers ( MT-TK mutations), neuropathy ( MFN2 mutations), or partial lipodystrophy and myopathy ( LIPE mutations).

Table 55.7
Selected hereditary skin disorders associated with extracutaneous cancer.
These conditions are discussed in Chapter 60 , Chapter 63 , Chapter 67 , Chapter 87 . CNS, central system.
SELECTED HEREDITARY SKIN DISORDERS ASSOCIATED WITH EXTRACUTANEOUS CANCER
Disorder Mode of inheritance Gene or gene product Gene symbol
Ataxia–telangiectasia AR Ataxia–telangiectasia mutated (phosphatidylinositol 3-kinase-like serine/threonine protein kinase) ATM
Birt–Hogg–Dubé syndrome AD Folliculin FLCN
Bloom syndrome AR RecQ protein-like 3 (DNA helicase) BLM (RECQL3)
Costello syndrome (see text) AD v-Ha-ras Harvey rat sarcoma viral oncogene homolog HRAS
AD v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog KRAS
Cowden-like syndrome AD Succinate dehydrogenase complex, subunit B, iron sulfur SDHB
AD Succinate dehydrogenase complex, subunit D, integral membrane protein SDHD
Dyskeratosis congenita (see Table 67.8 )
Ectodermal dysplasia–neoplastic syndrome AD Axis inhibitor 2 AXIN2
Gardner syndrome AD Adenomatous polyposis coli APC
Howel–Evans syndrome (tylosis–esophageal carcinoma) AD Rhomboid 5 homologue 2 RHBDF2
Muir–Torre syndrome AD MutS homolog 2 (mismatch repair enzyme) MSH2
AD MutS homolog 6 (mismatch repair enzyme) MSH6
AD MutL homolog 1 (mismatch repair enzyme) MLH1
MUTYH-associated polyposis AR MutY homolog MUTYH
Constitutional mismatch repair deficiency syndrome (childhood tumor syndrome with neurofibromatosis type 1 phenotype) AR MutS homolog 2 (mismatch repair enzyme) MSH2
AR MutS homolog 6 (mismatch repair enzyme) MSH6
AR MutL homolog 1 (mismatch repair enzyme) MLH1
AR Postmeiotic segregation increased 2 (mismatch repair enzyme) PMS2
Multiple endocrine neoplasia (MEN) MEN 1 AD Menin MEN1
MEN 2A AD Ret proto-oncogene (cysteine-rich extracellular domain) RET
MEN 2B AD Ret proto-oncogene (particularly Met918Thr in substrate recognition pocket) RET
MEN 4 AD Cyclin-dependent kinase inhibitor 1B (p27, Kip1) CDKN1B
PTEN hamartoma tumor syndrome (including Cowden and Bannayan–Riley–Ruvalcaba syndromes) AD Phosphatase and tensin homolog PTEN
Rothmund–Thomson syndrome AR RecQ protein-like 4 (DNA helicase) RECQ4

Table 55.8
Other selected inherited skin disorders.
ERCC, excision repair cross-complementing; SADDAN, s evere a chondroplasia with d evelopmental d elay and a canthosis n igricans; LAMB, l entigines, a trial myxomas, m ucocutaneous myxomas, b lue nevi; LEOPARD, l entigines, E CG changes, o cular hypertelorism, p ulmonary stenosis, a bnormal genitalia, r etardation of growth, d eafness; NAME, n evi, a trial myxomas, m yxoid neurofibromas, e phelides.
OTHER SELECTED INHERITED SKIN DISORDERS
Disorder Mode of inheritance Gene or gene product Gene symbol
Disorders characterized by excessive extracellular matrix protein deposition (also see Table 43.7 )
Inherited systemic hyalinosis (juvenile hyaline fibromatosis, infantile systemic hyalinosis) AR Anthrax toxin receptor 2 (capillary morphogenesis protein 2) ANTXR2
Lipoid proteinosis AR Extracellular matrix protein 1 ECM1
Psoriasiform conditions (also see Table 45.7 for pustular psoriasis variants)
Psoriasiform dermatitis–microcephaly–developmental delay AR Sterol-C4-methyl oxidase-like SC4MOL
Seborrhea-like dermatitis with psoriasiform elements AD Zinc finger protein 750 ZNF750
Familial pityriasis rubra pilaris or (pustular) psoriasis AD Caspase recruitment domain family member 14 CARD14
Craniosynostosis/skeletal dysplasia syndromes
Apert syndrome AD Fibroblast growth factor receptor 2 FGFR2
Beare–Stevenson cutis gyrata syndrome AD Fibroblast growth factor receptor 2 FGFR2
Crouzon syndrome with acanthosis nigricans; SADDAN; thanatophoric dysplasia AD Fibroblast growth factor receptor 3 * FGFR3
Oral–facial–digital syndrome XD Oral–facial–digital syndrome 1 OFD1
Disorders with prominent neurologic abnormalities (also see Table 6.10 )
Cockayne syndrome CSA AR ERCC8 (transcription-coupled repair protein) ERCC8
CSB AR ERCC6 (transcription-coupled repair protein) ERCC6
Cold-induced sweating syndrome AR Cytokine receptor-like factor 1 CRLF1
AR Cardiotrophin-like cytokine factor 1 CLCF1
Primary erythromelalgia AD Sodium channel, voltage-gated, type IX, α-subunit SCN9A
Familial primary localized cutaneous amyloidosis AD Oncostatin M receptor OSMR
Interleukin-31 receptor A IL31RA
Defects in coagulation or platelet plugging (see Ch. 23 , Table 105.9 )
Activated protein C resistance (factor V Leiden) AD Factor V F5
Protein C deficiency AD Protein C PROC
Protein S deficiency AD Protein S PROS1
Antithrombin III deficiency AD Serpin peptidase inhibitor C1 (antithrombin III) SERPINO
Hyperprothrombinemia AD Prothrombin, G20210A polymorphism F2
ADAMTS13 deficiency-mediated thrombotic microangiopathy (thrombotic thrombocytopenic purpura) AR ADAM metallopeptidase with thrombospondin type 1 motif, 13 (von Willebrand factor-cleaving protease) ADAMTS13
Disorders with cutaneous calcification or ossification (see Ch. 50 )
Albright hereditary osteodystrophy AD Stimulatory G protein, α-subunit (G s α; inactivating mutations) GNAS
Progressive osseous heteroplasia AD Stimulatory G protein, α-subunit (G s α; inactivating mutations) GNAS
Fibrodysplasia ossificans progressiva AD Activin A receptor, type 1 ACVR1
Hyperphosphatemic familial tumoral calcinosis AR UDP-N-acetyl-αD-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3 GALNT3
AR Fibroblast growth factor 23 FGF23
AR Klotho KL
Normophosphatemic familial tumoral calcinosis AR Sterile alpha motif domain-containing 9 SAMD9
Pachydermoperiostosis AR 15-hydroxyprostaglandin dehydrogenase HPGD
AR Solute carrier organic anion transporter, member 2A1 SLCO2A1

* Somatic FGFR3 mutations have been found to underlie a subset of epidermal nevi.

Patients with mutations in ERCC3 (XPB), ERCC2 (XPD) and ERCC5 (XPG) may have phenotypes with features of both xeroderma pigmentosum and Cockayne syndrome (XP/CS).

Mckusick's Mendelian Inheritance in MAN

McKusick's Mendelian Inheritance in Man (MIM) database was first published in 1966 as “Catalogs of Autosomal Dominant, Autosomal Recessive and X-linked Phenotypes”. In the 1994 edition, the subtitle was changed to “A Catalog of Human Genes and Genetic Disorders”, reflecting the progress that had been made in the field. Online MIM (OMIM; https://omim.org/ ) has been widely available on the Internet for over 30 years, providing immediate access to current information on human genes and genetic diseases . This database is updated continuously and can be searched by entering a constellation of clinical features as well as the name of a gene or syndrome.

A particular six-digit number (MIM number) is assigned to each OMIM entry. The first digit of the MIM number indicates the mode of inheritance of the corresponding genetic defect: 1 for autosomal dominant (entries before May 1994); 2 for autosomal recessive (entries before May 1994); 3 for X-linked; 4 for Y-linked; 5 for mitochondrial; and 6 for autosomal dominant or recessive (entries after May 1994). Phenotype entries describe the clinical and biochemical features, inheritance, mapping, and molecular genetics of a given disease or trait; a “#” is used to designate those for which the molecular basis is known. Gene entries are marked by a “*”, while gene plus phenotype entries are indicated with a “+”; both are appended with important disease-causing allelic variants (each of which is given a four-digit extension, beginning with .0001).

Genotype–Phenotype Correlations

Genotype–phenotype correlations in genodermatoses are often complex. Mutations in a single gene can cause more than one clinical disorder, a phenomenon referred to as allelic or clinical heterogeneity (see Table 54.2 ). This can result either from the same mutation occurring in patients with different genetic backgrounds or ages at presentation (e.g. identical PTEN mutations leading to Cowden syndrome and Bannayan–Riley–Ruvalcaba syndrome) or from different mutations (e.g. distinct mutations in the gene encoding lamin A/C leading to Hutchinson–Gilford progeria and familial partial lipodystrophy). Mutations that affect different domains of a protein can produce divergent phenotypes; for example, mutations in the sterile alpha motif (SAM) domain of the p63 protein typically cause AEC ( a nkyloblepharon, e ctodermal dysplasia and c left lip/palate) syndrome, while those in the DNA-binding domain give rise to EEC ( e ctrodactyly, e ctodermal dysplasia and c left lip/palate) syndrome. Various mutations in a particular gene can even lead to disorders with different modes of inheritance, such as X-linked dominant incontinentia pigmenti (a male-lethal condition due to a genomic rearrangement resulting in a partial deletion of the n uclear factor-κB e ssential mo dulator [NEMO] gene) and X-linked recessive hypohidrotic ectodermal dysplasia with immunodeficiency (due to milder, “hypomorphic” mutations in the NEMO gene). Conditions found to represent unexpected examples of allelic heterogeneity in the transforming growth factor-β (TGF-β) signaling pathway include mutations in the TGF-β receptor 1 gene ( TGFBR1 ) in Ferguson–Smith multiple self-healing squamous epithelioma (loss-of-function) as well as Loeys–Dietz syndrome (gain-of-function), and fibrillin 1 gene ( FBN1 ) mutations in generalized stiff skin syndrome as well as Marfan syndrome.

The type of mutation can also affect the severity of a genodermatosis. The potentially lethal generalized severe form of junctional EB (JEB, an autosomal recessive disorder) is typically caused by mutations in the LAMB3 gene that lead to a premature termination codon (resulting in a complete absence of the LAMB3 protein), whereas milder forms of JEB are produced by missense or splice-site mutations in the same gene (resulting in a LAMB3 protein with decreased function). In contrast, in autosomal dominant conditions where there is dimerization of the mutated protein product (e.g. the KIT tyrosine kinase receptor that is defective in piebaldism), dominant negative missense mutations often result in more severe disease (via abnormal proteins binding to and disrupting normal proteins) than do mutations that lead to premature termination codons and thereby result in haploinsufficiency (complete loss of half of the proteins).

In other instances, mutations in different genes produce the same clinical disorder; this is referred to as locus or genetic heterogeneity . Locus heterogeneity can occur when mutated proteins serve a similar function (e.g. components of various b iogenesis of l ysosome-related o rganelle c omplexes [BLOCs] in Hermansky–Pudlak syndrome) or interact with one another in a complex (e.g. hamartin and tuberin in tuberous sclerosis), as a ligand and receptor (e.g. endothelin-3 and the endothelin-B receptor in type 4 Waardenburg syndrome), or in a signaling pathway (e.g. various RAS–mitogen-activated protein kinase [MAPK] pathway proteins in cardio-facio-cutaneous syndrome; see below). However, phenotypic differences can arise when these proteins also have distinct functions and/or tissue distributions; for example, the neurologic abnormalities in Griscelli syndrome 1 result from myosin Va expression in neurons, and the immunodeficiency and hemophagocytic syndrome that characterize Griscelli syndrome 2 reflect RAB27A expression in hematopoietic cells.

Molecular Classification of Hereditary Skin Disorders

Molecular classification (see Table 55.1 ) represents a useful approach to the categorization of genodermatoses. Based on the pathogenetic defect rather than the clinical presentation, it can highlight similarities between conditions with seemingly disparate phenotypes (e.g. classic ichthyoses and metabolic disorders such as Gaucher disease, both caused by defects in lipid metabolism) and facilitate better understanding of important cellular pathways (e.g. RAS signaling) and responses to external stimuli (e.g. pyrin/NOD family members that function in innate immunity). Several groups of genodermatoses for which molecular classification has been instructive are discussed below.

Keratin Defects

In 1991, EB simplex (EBS) became the first human disease shown to be caused by intermediate filament mutations. Since that time, molecular defects in keratins have been identified in a diverse group of hereditary disorders affecting the skin, leading to mechanical fragility and/or abnormal keratinization, depending on the layer of the epidermis in which the defective protein is expressed; the hair, nails, and oral mucosa may also be involved (see Table 55.2 ) . Dowling–Degos disease and Naegeli–Franceschetti–Jadassohn (NFJ) syndrome, which are characterized by reticulated pigmentation and (for NFJ) ectodermal dysplasia, represent entities more recently recognized as belonging to the keratin disorder category.

Keratin intermediate filaments, which are expressed in a tissue- and differentiation-specific fashion, are composed of heterodimeric subunits formed by the specific pairing of a type I (acidic; KRT9–20, 25–28 [inner root sheath], 31–40 [hair]) and type II (basic–neutral; KRT1–8, 71–75 [inner root sheath], 81–86 [hair]) keratin (see Ch. 56 ). Keratin genes are clustered at two loci in the human genome, 17q21 (type I keratins) and 12q13 (type II keratins). Many keratin disorders can be caused by mutations (often dominant negative) in either the type I or type II component of a particular keratin pair, thus exhibiting locus heterogeneity (e.g. KRT10 or KRT1 for epidermolytic ichthyosis; KRT14 or KRT5 for EBS). Allelic heterogeneity is also observed, with mutations in the highly conserved helix initiation and termination motifs leading to more severe phenotypes than those in other regions (e.g. the generalized severe form of EBS versus localized EBS) (see Fig. 56.5 ).

Defects in Intercellular Junctions

Mutations in genes encoding components of desmosomes and gap junctions underlie a variety of genodermatoses, several of which have prominent extracutaneous manifestations.

Desmosomal defects

Desmosomes are intercellular junctions that provide mechanical integrity to tissues by anchoring intermediate filaments to the cell surface and mediating strong cell–cell adhesion (see Ch. 56 ). They are particularly important in stratified squamous epithelia and the myocardium, tissues that are subjected to substantial mechanical stress . It is therefore not surprising that impaired keratinization (especially palmoplantar keratoderma), skin fragility, and cardiomyopathy represent features of disorders caused by defects in desmosomal proteins such as desmoplakin, plakoglobin and plakophilin 1 (see Fig. 56.8 and Table 56.5 ). Abnormal hair (oftentimes woolly in nature) can also result from abnormalities in these proteins, and defects in other desmosomal components such as desmoglein 4 and corneodesmosin (which are highly expressed within the hair follicle) can lead to hypotrichosis and (for desmoglein 4) an autosomal recessive form of monilethrix ( Fig. 55.1 ). Additional consequences of desmosomal defects include inflammatory peeling skin (corneodesmosin) and s evere dermatitis, a llergies, and m etabolic wasting (SAM; desmoglein 1, desmoplakin).

Fig. 55.1, Autosomal recessive monilethrix due to desmoglein 4 mutations.

Connexin defects

Gap junctions are intercellular channels that connect the cytoplasm of neighboring cells, facilitating communication that coordinates cellular growth, differentiation and responses to stimuli as well as tissue morphogenesis and homeostasis (see Ch. 58 ). Transmembrane connexin proteins undergo oligomerization to form the connexons that compose gap junctions. Connexin proteins such as Cx26, Cx30, Cx31, and Cx43 are preferentially expressed in ectoderm-derived epithelia of the inner ear and cornea as well as in the epidermis and its appendages . This accounts for the sensorineural deafness, keratitis, and cutaneous abnormalities ranging from keratoderma to erythrokeratoderma to ectodermal dysplasias affecting the hair and nails that are observed in various connexin disorders (see Table 55.1 ). Several connexins are expressed in lymphatic cells, and mutations in the gene encoding Cx47 cause a hereditary type of lymphedema .

Defects in Keratinocyte–Extracellular Matrix Adhesion

The hemidesmosome links keratin intermediate filaments (KRT5, KRT14) within basal keratinocytes to proteins within the lamina densa (basement membrane proper) and sublamina densa regions of the epidermal basement membrane zone (see Ch. 28 ). Defects in protein components of these structures lead to various forms of EB (simplex, junctional and dystrophic; see Ch. 32 ) . The basement membranes of epithelia in other sites such as the eye, oral cavity, gastrointestinal tract, and genitourinary tract may also be affected in patients with EB. Additional clinical manifestations can result when the abnormal protein has important functions in extraepithelial tissues, such as the skeletal muscle for plectin (leading to EBS with muscular dystrophy).

Focal adhesions are structurally defined sites of linkage between the intracellular actin cytoskeleton and the extracellular matrix (ECM; see Fig. 28.3B ). Mutations in the gene encoding the focal adhesion protein kindlin-1 cause Kindler syndrome, an autosomal recessive disorder characterized by acral blistering and photosensitivity early in life, progressive poikiloderma and erosive periodontal disease (see Ch. 63 ). Kindlin-1 is found within basal keratinocytes along the cell surface that faces the basement membrane, and it has roles in keratinocyte adhesion, polarity, proliferation and motility. To date, Kindler syndrome represents the only skin fragility disorder known to be caused by disruption of linkage between actin microfilaments (rather than keratin intermediate filaments) and the ECM .

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