Hidrotic ectodermal dysplasia 2, or Clouston syndrome (referred to as HED2 throughout this GeneReview) is characterized by a triad of major clinical features including partial-to-complete alopecia, nail dystrophy, and palmoplantar hyperkeratosis. Sweating is preserved and there are usually no dental anomalies. Sparse scalp hair and dysplastic nails are seen early in life. In infancy, scalp hair is fine, sparse, and brittle. Progressive hair loss may lead to total alopecia by puberty. The nails may be milky white in early childhood; they gradually become dystrophic, thick, and distally separated from the nail bed. Palmoplantar keratoderma may develop during childhood and increases in severity with age. Associated features may include cutaneous hyperpigmentation (particularly over the joints) and finger clubbing. The clinical manifestations are highly variable even within the same family.

Familial porphyria cutanea tarda (F-PCT) is characterized by: skin findings including blistering over the dorsal aspects of the hands and other sun-exposed areas of skin, skin friability after minor trauma, facial hypertrichosis and hyperpigmentation, and severe thickening of affected skin areas (pseudoscleroderma); and an increased risk for hepatocellular carcinoma (HCC).

Many types of nonsyndromic congenital nail disorders (NDNC) have been described. Twenty-nail dystrophy (TND), also known as trachyonychia (from the Greek for 'rough nails'), is an autosomal dominant nail dystrophy characterized by excessive longitudinal striations and numerous superficial pits on the nails, which have a distinctive rough sandpaper-like appearance. Occasionally some nails are spared. The slowly progressive condition is usually apparent at birth and may be self-limiting, with spontaneous resolution in some patients (summary by Sehgal, 2007). TND is referred to here as nonsyndromic congenital nail disorder-1 (NDNC1). Genetic Heterogeneity of Nonsyndromic Congenital Nail Disorders Other nonsyndromic congenital nail disorders include koilonychia (NDNC2; 149300); leukonychia (NDNC3; 151600) caused by mutation in the PLCD1 gene (602142) on chromosome 3p22; anonychia/hyponychia (NDNC4; 206800) caused by mutation in the RSPO4 gene (610673) on chromosome 20p13; partial onycholysis with scleronychia (NDNC5; 164800); anonychia of thumbs with onychodystrophy of other nails (NDNC6; 107000); onychodystrophy mapping to chromosome 17p13 (NDNC7; 605779); toenail dystrophy (NDNC8; 607523) caused by mutation in the COL7A1 gene (120120) on chromosome 3p21; onychodystrophy mapping to chromosome 17q25.1-q25.3 (NDNC9; 614149).

Naxos disease (NXD) is characterized by arrhythmogenic right ventricular cardiomyopathy associated with abnormalities of the skin, hair, and nails. The ectodermal features are evident from birth or early childhood, whereas the cardiac symptoms develop in young adulthood or later. Clinical variability of ectodermal features has been observed, with hair anomalies ranging from woolly hair to alopecia, and skin abnormalities ranging from mild focal palmoplantar keratoderma to generalized skin fragility or even lethal neonatal epidermolysis bullosa (Protonotarios et al., 1986; Cabral et al., 2010; Pigors et al., 2011; Erken et al., 2011; Sen-Chowdhry and McKenna, 2014). Another syndrome involving cardiomyopathy, woolly hair, and keratoderma (DCWHK; 605676) is caused by mutation in the desmoplakin gene (DSP; 125647). Also see 610476 for a similar disorder caused by homozygous mutation in the DSC2 gene (125645).

Graves disease (GRD) is an autoimmune disorder in which antibodies to the thyrotropin receptor (TSHR; 603372) result in constitutive activation of the receptor and increased levels of thyroid hormone. Wilkin (1990) reviewed endocrine disorders of hormone excess and hormone deficiency resulting from receptor autoimmunity. Genetic Heterogeneity of Graves Disease Susceptibility to Graves disease-1 (GRD1) has been mapped to chromosome 14q31. Other susceptibility loci for Graves disease include GRD2 (603388) on chromosome 20q13, GRDX1 (300351) on Xp11, and GRDX2 (see 300351) on Xq21.33-q22. Graves disease has also been mapped to several loci that confer susceptibility to autoimmune thyroid diseases, including Hashimoto thyroiditis (HT; 140300): AITD1 (608173) on 6p11; AITD2 (608174) on 5q31-q33; AITD3 (608175) on 8q24; AITD4 (608176) on 10q, and AITD5 (601941) on 18q21.

A group of rare autosomal recessive forms of ichthyosis with clinical characteristics of superficial, asymptomatic, spontaneous peeling of the skin and histologically by a shedding of the outer layers of the epidermis. Presents with either an acral or a generalised distribution.

Schopf-Schulz-Passarge syndrome (SSPS) is an autosomal recessive disorder characterized by a constellation of multiple eyelid cysts, hypodontia, hypotrichosis, palmoplantar hyperkeratosis, and onychodystrophy (summary by Mallaiah and Dickinson, 2001).

Psoriasis (psoriasis vulgaris; PV) is a chronic inflammatory dermatosis that affects approximately 2% of the population. It is characterized by red, scaly skin patches that are usually found on the scalp, elbows, and knees, and may be associated with severe arthritis. The lesions are caused by abnormal keratinocyte proliferation and infiltration of inflammatory cells into the dermis and epidermis. The usual age of onset of psoriasis is between 15 and 30 years, although it can present at any age (summary by Matthews et al., 1996). Generalized pustular psoriasis (GPP) is a life-threatening disease characterized by sudden, repeated episodes of high-grade fever, generalized rash, and disseminated pustules, with hyperleukocytosis and elevated serum levels of C-reactive protein (123260) (summary by Marrakchi et al., 2011). GPP often presents in patients with existing or prior psoriasis vulgaris; however, GPP can develop without a history of PV (Sugiura et al., 2013). Palmoplantar pustulosis and acrodermatitis continua of Hallopeau represent acral forms of pustular psoriasis that have historically been grouped with GPP (summary by Setta-Kaffetzi et al., 2013). Nestle et al. (2009) provided a detailed review of the pathogenesis and genetics of psoriasis. Genetic Heterogeneity of Psoriasis and Psoriasis Susceptibility PSORS2 (602723) is caused by mutation in the CARD14 gene (607211) on chromosome 17q25, and PSORS14 (614204) is caused by mutation in the IL36RN gene (605507) on chromosome 2q14. Psoriasis susceptibility loci include PSORS1 on 6p21.3; PSORS3 (601454) on 4q; PSORS4 on 1q21; PSORS5 (604316) on 3q21; PSORS6 (605364) on 19p; PSORS7 (605606) on 1p; PSORS8 (610707) on 16q; PSORS9 (607857) on 4q31; PSORS10 (612410) on 18p11; PSORS11 (612599) on 5q31-q33; PSORS12 (612950) on 20q13; PSORS13 (614070), conferred by variation in the TRAF3IP2 gene (607043) on 6q21; and PSORS15 (616106), conferred by variation in the AP1S3 gene (615781) on 2q36. An additional putative psoriasis candidate locus has been reported on 20p (Nair et al., 1997).

Striate palmoplantar keratoderma belongs to a group of skin diseases in which there is thickening of the skin on the palms and soles. The striate form is characterized by longitudinal hyperkeratotic lesions extending the length of each finger to the palm, and hyperkeratotic lesions are restricted to regions of the body where pressure and abrasion are greatest (summary by Hunt et al., 2001). Patients with diffuse or focal forms of keratoderma associated with mutation in the DSG1 gene have also been reported (Keren et al., 2005; Milingou et al., 2006). Genetic Heterogeneity of Keratosis Palmoplantaris Striata Type II PPKS (PPKS2; 612908) is caused by mutation in the DSP gene (125647) on chromosome 6. Type III PPKS (PPKS3; 607654) is caused by mutation in the keratin-1 gene (KRT1; 139350) on chromosome 12q. For a general phenotypic description and a discussion of genetic heterogeneity of palmoplantar keratoderma (PPK), see epidermolytic PPK (144200). Nitoiu et al. (2014) reviewed desmosome biology in cardiocutaneous syndromes and inherited skin disease, including discussion of the involvement of the DSG1 and DSP genes.

Patients with familial cutaneous telangiectasia and cancer syndrome (FCTCS) develop cutaneous telangiectases in infancy with patchy alopecia over areas of affected skin, thinning of the lateral eyebrows, and mild dental and nail anomalies. Affected individuals are at increased risk of developing oropharyngeal cancer, and other malignancies have been reported as well (Tanaka et al., 2012).

Some ectodermal dysplasias are here classified as congenital disorders characterized by abnormal development in 2 or more ectodermal structures (hair, nails, teeth, and sweat glands) without other systemic findings. Ectodermal dysplasia of the hair/nail type is a rare congenital condition characterized by hypotrichosis and nail dystrophy without nonectodermal or other ectodermal manifestations.

Some ectodermal dysplasias are here classified as congenital disorders characterized by abnormal development in 2 or more ectodermal structures (hair, nails, teeth, and sweat glands) without other systemic findings. Ectodermal dysplasia of the hair/nail type is a rare congenital condition characterized by hypotrichosis and nail dystrophy without nonectodermal or other ectodermal manifestations.

A rare genetic skin disease characterized by generalized skin peeling, leukonychia, acral punctate keratoses coalescing into focal keratoderma on the weight-bearing areas, angular cheilitis and knuckle pads with multiple hyperkeratotic micropapules. The skin appears dry and scaly with superficial exfoliation and underlying erythema. Histopathologic examination of affected skin areas shows hyperkeratosis, acanthosis and intraepidermal clefting with irregular acantholysis. Additional systemic abnormalities are absent.

Singleton-Merten syndrome (SGMRT) is an uncommon autosomal dominant disorder characterized by abnormalities of blood vessels, teeth, and bone. Calcifications of the aorta and aortic and mitral valves occur in childhood or puberty and can lead to early death. Dental findings include delayed primary tooth exfoliation and permanent tooth eruption, truncated tooth root formation, early-onset periodontal disease, and severe root and alveolar bone resorption associated with dysregulated mineralization, leading to tooth loss. Osseous features consist of osteoporosis, either generalized or limited to distal extremities, distal limb osteolysis, widened medullary cavities, and easy tearing of tendons from bone. Less common features are mild facial dysmorphism (high anterior hair line, broad forehead, smooth philtrum, thin upper vermilion border), generalized muscle weakness, psoriasis, early-onset glaucoma, and recurrent infections. The disorder manifests with variable inter- and intrafamilial phenotypes (summary by Rutsch et al., 2015). Genetic Heterogeneity of Singleton-Merten Syndrome An atypical form of Singleton-Merten syndrome (SGMRT2; 616298) is caused by mutation in the DDX58 gene (609631) on chromosome 9p21.

Junctional epidermolysis bullosa-7 with interstitial lung disease and nephrotic syndrome (JEB7), also known as ILNEB, is an autosomal recessive multiorgan disorder that includes congenital interstitial lung disease, nephrotic syndrome, and epidermolysis bullosa. The respiratory and renal features predominate, and lung involvement accounts for the lethal course of the disease (summary by Has et al., 2012).

Arrhythmogenic cardiomyopathy with variable ectodermal abnormalities (ARCME) is characterized by severe dilated cardiomyopathy resulting in death or cardiac transplantation in childhood. Ventricular tachycardia, sustained or nonsustained, has been reported. In addition, some patients exhibit ectodermal manifestations including woolly or wiry hair, dental anomalies, dry skin, and/or dystrophic nails. Cleft lip and palate and corneal abnormalities have also been observed (Robinson et al., 2020; Henry et al., 2022).