Alternative titles; symbols
ORPHA: 2309;
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
Gene/Locus |
Gene/Locus MIM number |
|---|---|---|---|---|---|---|
| 17q21.2 | Pachyonychia congenita 2 | 167210 | Autosomal dominant | 3 | KRT17 | 148069 |
A number sign (#) is used with this entry because of evidence that pachyonychia congenita-2 (PC2) is caused by heterozygous mutation in the KRT17 gene (148069) on chromosome 17q21.
Pachyonychia congenita (PC) is an autosomal dominant genodermatosis with the main clinical features of hypertrophic nail dystrophy, painful and highly debilitating plantar keratoderma, oral leukokeratosis, and a variety of epidermal cysts. Although the condition had previously been subdivided clinically into Jadassohn-Lewandowsky PC type 1 and Jackson-Lawler PC type 2, patients with PC were later found to have a mixed constellation of both types, leading to a classification of PC based on genotype (summary by Sybert, 2010; Eliason et al., 2012; McLean et al., 2011).
For a discussion of genetic heterogeneity of pachyonychia congenita, see 167200.
Historical Classification of Pachyonychia Congenita
Gorlin et al. (1976) suggested that 2 distinct syndromes are subsumed under the designation pachyonychia congenita. PC type 1, the Jadassohn-Lewandowsky type, shows oral leukokeratosis. PC type 2, the Jackson-Lawler type, has natal teeth and epidermoid cysts (cylindromas), but no oral leukoplakia. Corneal dystrophy may be a feature exclusively of the Jackson-Lawler type.
Smith et al. (1998) stated that PC type 2, in contrast to PC type 1, has minimal oral involvement and milder keratoderma, and multiple steatocystomas (184500) is a major clinical feature. Steatocystoma, also known as eruptive vellus cyst, is a cystic hamartoma lined by sebaceous ductal epithelium.
On the basis of a study of 13 patients with PC type 1 or type 2, Terrinoni et al. (2001) concluded that the presence of pilosebaceous cysts following puberty is the best indicator of PC type 2; prepubescent patients are more difficult to classify due to the lack of cysts. Natal teeth are indicative of PC type 2, although their absence does not preclude the PC type 2 diagnosis.
The form of PC caused by mutation in the KRT17 gene, here designated PC2, has also been designated PC-17 (Eliason et al., 2012) and PC-K17 (Shah et al., 2014).
Jackson and Lawler (1951-52) reported 6 affected members of 3 generations of a family with pachyonychia congenita. In 4 generations of a family, Vineyard and Scott (1961) observed steatocystoma associated with pachyonychia congenita. Hodes and Norins (1977) reported the same association. Soderquist and Reed (1968) described the same association but thought that the cysts were epidermal cysts. They presented an interesting, illustrated newspaper clipping that described neonatal teeth in persons of 3 generations. The adult teeth were sound.
Clementi et al. (1986) described a family in which 3 members in 2 generations had pachyonychia congenita, hyperkeratosis, and hyperhidrosis of the palms and soles, follicular keratosis, neonatal teeth, and epidermoid cysts. They pointed out that this is the rarer form of pachyonychia congenita. Armpit folliculitis was described in the 21-year-old proposita and in her 61-year-old mother. Cysts in the metacarpal and elbow regions were observed at puberty in the daughter. They were noted to adhere to the overlying skin on the elbows, dorsal surface of the proximal phalanges, and on the knees and pretibial regions.
Covello et al. (1998) reported a 3-generation British Caucasian family with pachyonychia congenita and mutation in the KRT17 gene. The 18-year-old proband presented with rough skin due to follicular keratoses, and thickened nails and plantar skin. Although she did not have clinically obvious cysts, and had mild hyperkeratosis of the buccal mucosa, other family members had abnormal nails, blistering of the feet, and multiple cysts. Her mother had a history of painful keratoses and blistering on the feet, with thickened nails, yellowish cysts on the trunk and limbs, and recurrent flexural abscesses. She also had multiple milia. A lesion excised from the labium majus proved to be an epidermoid cyst. This family had previously been reported as having pachyonychia congenita with hidradenitis suppurativa (Todd et al., 1990). Another family consisted of an affected mother and son. The mother had a history of subcutaneous yellow nodules on the flexor surfaces of the arms, abdomen, and legs since puberty, and also had thickened nails of the feet. Covello et al. (1998) stated that within a single large pedigree (Munro et al., 1994), and indeed in a single individual with multiple cysts, they had found that some were true steatocysts and others keratinous cysts, some of which contained vellus hair. Scrotal and vulvar cystomatosis also form part of the syndrome.
Celebi et al. (1999) reported a mother and son of Caribbean origin with PC and mutation in the KRT17 gene. The proband was a 27-year-old woman who developed thick dystrophic nails of the fingers and toes in childhood. In the second decade of life, she developed numerous cysts on the trunk and hidradenitis suppurativa in both axillae. Biopsies of the cysts showed a variety of histologic findings, including epidermoid cyst, eruptive vellus hair cyst, and steatocystoma. The proband's 5-year-old son had had natal teeth and similar cutaneous findings including thick dystrophic nails, follicular keratosis, cysts, and hidradenitis. Neither the proband nor her son displayed keratoderma, leukokeratosis of the tongue, or hair abnormalities.
Hashiguchi et al. (2002) reported a 19-year-old Japanese man with PC and mutation in the KRT17 gene. Within 2 months after birth, all of his nails had became thickened. Multiple papules and cysts on his scalp and axillae had been noted since the age of 5 years, and hyperkeratosis with blister formation on his soles began at age 10. At 16, the cysts on his scalp and axillae became painful and were repeatedly infected. Physical examination showed many cysts and nodules all over his body, pachyonychia involving all fingernails and toenails, stiff eyebrows, pili torti (twisted hair), and plantar hyperkeratosis with bullae. His mouth, tongue, and teeth were unaffected. Histologic examination of a subcutaneous cyst showed that the cyst wall was composed of several layers of epithelial cells accompanied by sebaceous gland lobules, consistent with steatocystoma.
Kanda et al. (2009) reported a large 5-generation Asian family with PC and mutation in the KRT17 gene. The proband was a 36-year-old woman who had had natal teeth and developed hypertrophy of fingernails and toenails during childhood. Follicular hyperkeratosis at the knees and elbows was noted at puberty, although it resolved with age. She had focal hyperkeratosis on her soles, and subcutaneous axillary cysts. Histopathology from one of the cysts showed that the wall consisted of several thin epithelial cell layers without granular layers, with sebaceous glands near the cyst wall. Large basophilic granules were scattered in the cytoplasm of the uppermost-layer cells in the cyst walls; immunohistochemistry showed that those upper layer cells expressed keratin-17. Ultrastructural analysis showed large, irregularly shaped keratin clumps in the cytoplasm of epithelial cells in the cyst wall. The proband's 3-year-old daughter had follicular keratosis on the knees, nail deformities, pilosebaceous cysts on the face, and focal hyperkeratosis on the soles. Her 62-year-old father had nail dystrophy, numerous steatocystomas on the trunk, and hyperkeratosis of the soles since adolescence. Family history revealed that the proband's paternal grandmother and 5 of her sibs, as well as their mother, were reported to have nail hypertrophy and steatocystoma multiplex.
The transmission pattern of PC2 in the family reported by Munro et al. (1994) and McLean et al. (1995) was consistent with autosomal dominant inheritance.
In a large 5-generation Scottish kindred extensively affected with pachyonychia congenita of the Jackson-Lawler type, Munro et al. (1994) found tight linkage of the disease locus to markers mapping within, or very close to, the keratin type I cluster at 17q12-q21. Maximum lod scores for linkage of the disease to a KRT10 (148080) polymorphism and to D17S800, a marker known to be very tightly linked to KRT10, were, respectively, 4.51 and 7.73, both at theta = 0.00. The findings suggested a role for keratins in the phenomenon of natal dentition.
In a large 5-generation Scottish kindred with pachyonychia congenita of the Jackson-Lawler type, mapped to chromosome 17q12-q21 by Munro et al. (1994), McLean et al. (1995) showed that a heterozygous asn92-to-asp mutation (148069.0001) in the helix initiation motif of keratin-17 cosegregated with the disease.
Smith et al. (1997) reported heterozygous KRT17 missense mutations within the same conserved protein motif in a further 5 families described as having the Jackson-Lawler type of PC (see, e.g., 148069.0002 and 148069.0003). They also found heterozygous missense mutations in KRT17 in 2 families diagnosed with steatocystoma multiplex (148069.0004 and 148069.0005). On review, mild nail changes were observed in some, but not all, of these patients. They concluded that phenotypic variation is observed with KRT17 mutations as is the case with other keratin disorders.
KRT17 is expressed in the nail bed, hair follicle, sebaceous glands, and other epidermal appendages. Covello et al. (1998) described 3 unrelated kindreds carrying KRT17 mutations, including 2 families with disparate phenotypes but the same missense mutation in the 1A domain of KRT17 (R94C; 148069.0006): affected members of 1 of the families showed classic features of Jackson-Lawler PC, whereas affected persons in the other family exhibited the steatocystoma multiplex phenotype. The third family was a 3-generation British Caucasian family with pachyonychia congenita, in which affected individuals were heterozygous for a previously reported missense mutation (N92S; 148069.0002).
In a mother and son of Caribbean origin with Jackson-Lawler pachyonychia congenita, Celebi et al. (1999) identified heterozygosity for a missense mutation in KRT17 (M88T; 148069.0007). The mutation was not found in the proband's unaffected mother or 50 unrelated controls; DNA from her father was unavailable.
In 3 unrelated probands with PC2, Smith et al. (2001) identified heterozygosity for mutations in exon 1 of the KRT17 gene, including a 15-bp deletion in the proband from an Italian Caucasian family (148069.0008), which the authors noted was the first report of a deletion in KRT17. The other 2 probands had missense mutations, an R94P substitution in an Australian Caucasian female (148069.0009), and an L95Q substitution in a French Caucasian female (148069.0010). The authors observed that the deletion produced a similar clinical phenotype to that seen with missense mutations in the same region in KRT17.
Terrinoni et al. (2001) studied 13 patients with clinically heterogeneous pachyonychia congenita and found mutations in K6A (148041), K16 (148067), or K17 (see 148069.0005 and 148069.0011-148069.0013) in all cases. They concluded that K6A or K16 mutations produce the Jadassohn-Lewandowsky phenotype, whereas K17 or K6B (148042) mutations produce the Jackson-Lawler phenotype.
In a 19-year-old Japanese man with PC2, Hashiguchi et al. (2002) directly sequenced the KRT17 gene and identified identified heterozygosity for a de novo missense mutation (V102M; 148069.0014). The variant was not found in his unaffected parents or in 50 Japanese controls.
In an affected father, daughter, and granddaughter from a large 5-generation Asian family with PC2, Kanda et al. (2009) sequenced the KRT6B (148042) and KRT17 genes and identified heterozygosity for the previously reported L99P mutation in the KRT17 gene (148069.0013). The mutation was not found in 50 control individuals.
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