A KRT6A Mutation p.Ile462Asn In A Chinese Family With Pachyonychia Congenita, And Identification of Maternal Mosaicism

Background: Pachyonychia congenita (PC, OMIM #167200, #167210, #615726, #615728, and #615735) is a rare autosomal dominant disorder caused by keratin gene mutations in KRT6A , KRT6B , KRT6C , KRT16 or KRT17 . It is characterized with nail dystrophy and palmoplantar keratoderma (PPK). The most prominent manifestation is plantar pain. This is the first reported case of maternal mosaicism in PC. Although very rare, germ cell mosaicism should be considered when providing genetic counselling for unaffected parents of a child with PC. Methods: : Genomic DNA was extracted from peripheral blood samples, hair bulbs, buccal smears and the father’s germ cells. The entire coding and flanking intronic sequences of 5 keratin genes were screened for mutations in every individuals of the family by Sanger sequencing. We used whole exome sequencing (WES) to search for mosaicism in the parents who had no KRT6A mutation identified by Sanger sequencing. Mosaicism was confirmed by SNaPshot sequencing and HiSeq deep sequencing. Results: : A previously reported heterozygous mutation, p.Ile462Asn, was identified in KRT6A in the proband and his affected sister. The variant was detected in one sequencing read from 86 sequencing reads from DNA from the mother’s blood by WES. The mutation was not identified in DNA from the father’s blood. Frequency of reads was 47% and 49% in proband and his sister, respectively. SNaPshot sequencing revealed mosaicism at a level of 2.5% and 4.7% in DNA from blood and hair bulbs from the unaffected mother. HiSeq deep sequencing demonstrated low-grade mosaicism in the patient’s younger sister and parents. Conclusion: These findings indicate the ability of WES and SNaPshot sequencing to detect low-frequency mosaic mutations. Although very rare, germinal mosaicism should be considered when genetic counseling is given to families with presumed spontaneous cases of PC.