Bazex-Dupré-Christol syndrome: an ectodermal dysplasia with skin appendage neoplasms

Germline intergenic duplications at Xq26.1 underlie Bazex-Dupré-Christol basal cell carcinoma susceptibility syndrome

BACKGROUND

Bazex-Dupré-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-q27.1. However, the genetic mechanism of BDCS remains an open question.

OBJECTIVES

To investigate the genetic aetiology and molecular mechanisms underlying BDCS.

METHODS

We ascertained multiple individuals from eight unrelated families 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 polymerase chain reaction (PCR) were used to explore copy number variations, followed by long-range gap PCR and Sanger sequencing to amplify the duplication junctions and to define the head-tail junctions. Hi-C was performed on dermal fibroblasts from two affected individuals with BDCS and one control. Public datasets and tools were used to identify regulatory elements and transcription factor binding sites within the minimal duplicated region. Immunofluorescence was performed in hair follicles, BCCs and trichoepitheliomas from patients with BDCS and sporadic BCCs. The ACTRT1 variant c.547dup (p.Met183Asnfs*17), previously proposed to cause BDCS, was evaluated with t allele frequency calculator.

RESULTS

In eight families with BDCS, we identified overlapping 18-135-kb duplications (six inherited and two de novo) at Xq26.1, flanked by ARHGAP36 and IGSF1. Hi-C showed that the duplications did not affect the topologically associated domain, but may alter the interactions between flanking genes and putative enhancers located in the minimal duplicated region. We detected ARHGAP36 expression near the control hair follicular stem cell compartment, and found increased ARHGAP36 levels in hair follicles in telogen, in BCCs and in trichoepitheliomas from patients with BDCS. ARHGAP36 was also detected in sporadic BCCs from individuals without BDCS. Our modelling showed the predicted maximum tolerated minor allele frequency of ACTRT1 variants in control populations to be orders of magnitude higher than expected for a high-penetrant ultra-rare disorder, suggesting loss of function of ACTRT1 variants to be an unlikely cause for BDCS.

CONCLUSIONS

Noncoding Xq26.1 duplications cause BDCS. The BDCS duplications most likely lead to dysregulation of ARHGAP36. ARHGAP36 is a potential therapeutic target for both inherited and sporadic BCCs. What is already known about this topic? Bazex-Dupré-Christol syndrome (BDCS) is a rare X-linked basal cell carcinoma susceptibility syndrome linked to an 11·4-Mb interval on chromosome Xq25-q27.1. Loss-of-function variants in ACTRT1 and its regulatory elements were suggested to cause BDCS. What does this study add? BDCS is caused by small tandem noncoding intergenic duplications at chromosome Xq26.1. The Xq26.1 BDCS duplications likely dysregulate ARHGAP36, the flanking centromeric gene. ACTRT1 loss-of-function variants are unlikely to cause BDCS. What is the translational message? This study provides the basis for accurate genetic testing for BDCS, which will aid precise diagnosis and appropriate surveillance and clinical management. ARHGAP36 may be a novel therapeutic target for all forms of sporadic basal cell carcinomas.

Overview of familial syndromes with increased skin malignancies

The vast majority of skin cancers can be classified into two main types: melanoma and keratinocyte carcinomas. The most common keratinocyte carcinomas include basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). Multiple familial syndromes have been identified that can increase the risk of developing SCC, BCC, and/or melanoma. The major syndromes include oculocutaneous albinism for SCC, basal cell nevus syndrome for BCC, familial atypical multiple mole-melanoma syndrome, and hereditary breast and ovarian cancer syndrome for melanoma. In addition, familial syndromes that can predispose individuals to all three major skin cancers include xeroderma pigmentosum and Li-Fraumeni syndrome. This review highlights the epidemiology, risk factors, pathogenesis, and etiology of the major and minor syndromes to better identify and manage these conditions. Current investigational trials in genomic medicine are making their way in revolutionizing the clinical diagnosis of these familial syndromes for earlier preventative measures and improvement of long-term prognosis in these patients.

ARP-T1-associated Bazex-Dupré-Christol syndrome is an inherited basal cell cancer with ciliary defects characteristic of ciliopathies

Actin-Related Protein-Testis1 (ARP-T1)/ACTRT1 gene mutations cause the Bazex-Dupré-Christol Syndrome (BDCS) characterized by follicular atrophoderma, hypotrichosis, and basal cell cancer. Here, we report an ARP-T1 interactome (PXD016557) that includes proteins involved in ciliogenesis, endosomal recycling, and septin ring formation. In agreement, ARP-T1 localizes to the midbody during cytokinesis and the basal body of primary cilia in interphase. Tissue samples from ARP-T1-associated BDCS patients have reduced ciliary length. The severity of the shortened cilia significantly correlates with the ARP-T1 levels, which was further validated by ACTRT1 knockdown in culture cells. Thus, we propose that ARP-T1 participates in the regulation of cilia length and that ARP-T1-associated BDCS is a case of skin cancer with ciliopathy characteristics.

Park et al. characterise the interactome, localisation and function of Actin-Related Protein-Testis1 protein (ARP-T1), encoded by the ACTRT1 gene, associated with inherited basal cell cancer. They find that ARP-T1 is localised to the primary cilia basal body in epidermal cells, interacts with the cilia machinery, and is needed for proper ciliogenesis.

Congenital hypotrichosis, eruptive milia, and palmoplantar pits: a case report with review of literature

We report a ten-month-old male infant, presenting with congenital hypotrichosis of scalp, absence of eyebrows and eyelashes. He also had multiple milia over face and extremities along with palmoplantar pits. The infant was born to third degree consanguineous marriage. None of the family members had similar disease. Histopathology was consistent with milia. Based on these findings, a diagnosis of unclassified ectodermal dysplasia was made.

Hereditary tumour syndromes featuring basal cell carcinomas

Basal cell carcinoma (BCC) is the most frequent cutaneous malignancy worldwide. This skin tumour is characterized by a broad phenotypic variability and distinct histopathological subtypes. It shows slow, locally invasive growth and only rarely metastasizes. BCCs can occur either sporadically or in the context of genetic syndromes, including Gorlin syndrome, Bazex-Dupré-Christol syndrome, Rombo syndrome, Oley syndrome and xeroderma pigmentosum. Different genes and signalling routes have been shown to play an important role in the development and growth of these tumours, including the hedgehog and Wnt/β-catenin pathway. In some of the aforementioned hereditary disorders the underlying genetic defect is still unknown whereas in others several genes have been demonstrated to be involved. Currently, most therapeutic approaches are based on surgical measures. In the case of superficial BCCs, photodynamic therapy, 5-fluorouracil cream, imiquimod or radiotherapy also may be an option. Elucidation of the molecular mechanisms governing the manifestation of BCCs in monogenetically inherited tumour syndromes will not only contribute to a better understanding of the complex pathogenesis of these tumours but might pave the way to the development of noninvasive, specific and molecule-based therapeutic strategies in the near future.