Germline intergenic duplications at Xq26.1 underlie Bazex-Dupre-Christol syndrome, an inherited basal cell carcinoma susceptibility condition

Background: Bazex-Dupre-Christol syndrome (BDCS; MIM301845) is a rare X-linked dominant genodermatosis characterized by follicular atrophoderma, congenital hypotrichosis and multiple basal cell carcinomas (BCCs). Previous studies have linked BDCS to an 11.4 Mb interval on chromosome Xq25-27.1. However, the genetic mechanism of BDCS remains an open question. Methods: To investigate the genetic etiology of BDCS, we ascertained eight families with individuals affected with BDCS (F1-F8). Whole exome (F1 and F2) and genome sequencing (F3) were performed to identify putative disease-causing variants within the linkage region. Array-comparative genomic hybridization and quantitative PCR were used to explore copy number variations (CNV) in BDCS families, followed by long-range gap-PCR and Sanger sequencing to amplify duplication junction and define the precise head-tail junctions, respectively. Immunofluorescence was performed in hair follicles, BCCs and trichoepitheliomas from BDCS patients and sporadic BCCs to detect the expression of corresponding genes. The ACTRT1 variant (p.Met183Asnfs*17), previously proposed to cause BDCS, was evaluated with allele frequency calculator. Results: In eight BDCS families, we identified overlapping 18-135kb duplications (six inherited and two de novo) at Xq26.1, flanked by ARHGAP36 and IGSF1. We detected ARHGAP36 expression near the control hair follicular stem cells compartment, and found increased ARHGAP36 levels in hair follicles in telogen, BCCs and trichoepitheliomas from patients with BDCS. ARHGAP36 was also detected in sporadic BCCs from individuals without BDCS. Our modelling showed the predicted ACTRT1 variants maximum tolerated minor allele frequency in control populations to be orders of magnitude higher than expected for a high-penetrant ultra-rare disorder, suggesting loss-of-function of ACTRT1 is unlikely to cause BDCS. Conclusions: Our data support the pathogenicity of intergenic duplications at Xq26.1, most likely leading to dysregulation of ARHGAP36, establish BDCS as a genomic disorder, and provide a potential therapeutic target for both inherited and sporadic BCCs.

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