Autosomal Recessive Transmission of a Rare KRT74 Variant Causes Hair and Nail Ectodermal Dysplasia: Allelism with Dominant Woolly Hair/Hypotrichosis

Abstract

Pure hair and nail ectodermal dysplasia (PHNED) comprises a heterogeneous group of rare heritable disorders characterized by brittle hair, hypotrichosis, onychodystrophy and micronychia. Autosomal recessive (AR) PHNED has previously been associated with mutations in either KRT85 or HOXC13 on chromosome 12p11.1-q14.3. We investigated a consanguineous Pakistani family with AR PHNED linked to the keratin gene cluster on 12p11.1 but without detectable mutations in KRT85 and HOXC13. Whole exome sequencing of affected individuals revealed homozygosity for a rare c.821T>C variant (p.Phe274Ser) in the KRT74 gene that segregates AR PHNED in the family. The transition alters the highly conserved Phe274 residue in the coil 1B domain required for long-range dimerization of keratins, suggesting that the mutation compromises the stability of intermediate filaments. Immunohistochemical (IHC) analyses confirmed a strong keratin-74 expression in the nail matrix, the nail bed and the hyponychium of mouse distal digits, as well as in normal human hair follicles. Furthermore, hair follicles and epidermis of an affected family member stained negative for Keratin-74 suggesting a loss of function mechanism mediated by the Phe274Ser substitution. Our observations show for the first time that homozygosity for a KRT74 missense variant may be associated with AR PHNED. Heterozygous KRT74 mutations have previously been associated with autosomal dominant woolly hair/hypotrichosis simplex (ADWH). Thus, our findings expand the phenotypic spectrum associated with KRT74 mutations and imply that a subtype of AR PHNED is allelic with ADWH.

Autosomal Recessive Hypotrichosis with Woolly Hair Caused by a Mutation in the Keratin 25 Gene Expressed in Hair Follicles

“…The mutations in these genes were previously observed in individuals with hypotrichosis with the WH phenotype. Further functional analysis of cell lines validated these mutations as pathogenic, as they disrupted the formation of heterodimers between the two types of keratins and led to cytoskeletal damage (Fujimoto et al, 2012;Rasool et al, 2010;Raykova et al, 2014;Shimomura et al, 2010b;Wasif et al, 2011).…”

Molecular Genetics of Alopecias

“…Similar clinical findings were also observed in a Pakistani family with an autosomal recessive inheritance pattern, but with a relatively mild clinical expression. In this family, a missense variant in KRT74 was found to be responsible for the disorder [23]. Therefore, it has been suggested that autosomal dominant woolly hair/hypotrichosis and this subtype of autosomal recessive PHNED are allelic disorders [23].…”

Genetics of syndromic and non‐syndromic hereditary nail disorders

“…The condition is inherited in an autosomal recessive pattern with mutations of both KRT85 and HOXC13 on chromosome 12p11.1‐q14.3 . A recent study has also identified a homozygous mutation in KRT74 leading to PHNED . K74 is found in the distal digits during mouse claw development and in the inner root sheath of hair follicles .…”