A New Nail Genodermatosis: Familial Onychocytic Hamartoma Presenting as Polydactylous Longitudinal Leukonychia. Implications in the Early Detection of BAP1 Tumor Predisposition Syndrome

Onychocytic matricoma (OCM) is an acquired benign onychogenic tumor producing a localized thickening of the nail plate that usually presents as longitudinal pachymelanonychia. However, longitudinal pachyleukonychia as the presenting sign of OCM is increasingly recognized in clinical practice. To our knowledge, polydactylous OCM has not previously been reported in the literature. This study aims to describe a new nail genodermatosis presenting as polydactylous longitudinal leukonychia with histology typical of OCM. Four cases were identified. The previously unreported familial nail abnormality occurred in at least 2 first-degree relatives. Longitudinal white bands located exclusively on the fingernails were the only characteristic of this disease. The longitudinal bands varied in number from 2 to 4 per nail. Nail changes may appear during childhood, but most cases were observed between the ages of 20 and 50 years. The multiple longitudinal bands corresponded histologically to multiple OCM. Three cases in this series had no relevant personal of family history of neoplasm, but only 2 generations were analyzed. In 1 family, polydactylous OCM was associated with a family history of melanoma and renal cell carcinoma. The genomic profile of this single family allowed a diagnosis of OCM associated with BRCA1-associated protein ( BAP1 ) tumor predisposition syndrome (TPDS). Our results are limited by incomplete follow-up and the small size of this retrospective case series. As such tumors are rare, additional cases need to be collected to clarify the role of onychocytic hamartoma as an early indicator of BAP1 TPDS.