Atypical Clinical Presentation of Xeroderma Pigmentosum in a Patient Harboring a Novel Missense Mutation in the XPC Gene: The Importance of Clinical Suspicion

Familial Melanoma Phenotype With Xeroderma Pigmentosum Group C (XP-C) Genotype - The Putative Role of MC1R Polymorphism as Modifier

INTRODUCTION

Xeroderma pigmentosum (XP), a rare inherited condition, hallmarked by extreme sensitivity to sun exposure resulting in multiple skin cancers and non-malignant skin alterations is attributed to homozygous inactivating pathogenic variants (PVs) in DNA repair genes, predominantly the XPC gene.

OBJECTIVES

Report a unique phenotypic expression of mutant XPC allele that may be compatible with a putative modifier role for MC1R polymorphism.

METHODS

A family of 13 siblings, seven of whom were diagnosed with at least one cutaneous melanoma (N = 53) and non-melanoma skin cancers (N = 9) was studied. Of seven melanoma-affected cases, five consented for genetic analysis. CDKN2A revealed no PV in any case and subsequent whole-exome sequencing (WES) identified a rare homozygous missense PV (c.919C>T; p.Arg307Trp) in exon 8 of the XPC gene in all affected individuals. Notably, XPC PV carriers who co-harbored the p.I155T MC1R variant (N = 3) exhibited larger number of tumors, deeper Breslow indexes, higher rates of invasive melanomas and earlier age at diagnosis compared with non MC1R variant carriers (N = 2).

CONCLUSIONS

Familial malignant melanoma phenotype may, in fact, be an unusual clinical presentation of XPC, and MC1R may be a genetic modifier of penetrance and phenotype of mutant XPC alleles.

Structural modeling and analyses of genetic variations in the human XPC nucleotide excision repair protein

Xeroderma pigmentosum C (XPC) is a key initiator in the global genome nucleotide excision repair pathway in mammalian cells. Inherited mutations in the XPC gene can cause xeroderma pigmentosum (XP) cancer predisposition syndrome that dramatically increases the susceptibility to sunlight-induced cancers. Various genetic variants and mutations of the protein have been reported in cancer databases and literature. The current lack of a high-resolution 3-D structure of human XPC makes it difficult to assess the structural impact of the mutations/genetic variations. Using the available high-resolution crystal structure of its yeast ortholog, Rad4, we built a homology model of human XPC protein and compared it with a model generated by AlphaFold. The two models are largely consistent with each other in the structured domains. We have also assessed the degree of conservation for each residue using 966 sequences of XPC orthologs. Our structure- and sequence conservation-based assessments largely agree with the variant's impact on the protein's structural stability, computed by FoldX and SDM. Known XP missense mutations such as Y585C, W690S, and C771Y are consistently predicted to destabilize the protein's structure. Our analyses also reveal several highly conserved hydrophobic regions that are surface-exposed, which may indicate novel intermolecular interfaces that are yet to be characterized.Communicated by Ramaswamy H. Sarma.

A novel missense variant and multiexon deletion causing a delayed presentation of xeroderma pigmentosum, group C

Pathogenic variants in the XPC complex subunit, DNA damage recognition, and repair factor (XPC) are the cause of xeroderma pigmentosum, group C (MIM: 278720). Xeroderma pigmentosum is an inherited condition characterized by hypersensitivity to ultraviolet (UV) irradiation and increased risk of skin cancer due to a defect in nucleotide excision repair (NER). Here we describe an individual with a novel missense variant and deletion of exons 14-15 in XPC presenting with a history of recurrent melanomas. The proband is a 39-yr-old female evaluated through the Mayo Clinic Department of Clinical Genomics. Prior to age 36, she had more than 60 skin biopsies that showed dysplastic nevi, many of which had atypia. At age 36 she presented with her first melanoma in situ, and since then has had more than 10 melanomas. The proband underwent research whole-exome sequencing (WES) through the Mayo Clinic's Center for Individualized Medicine and a novel heterozygous variant of uncertain significance (VUS) in XPC (c.1709T > G, p.Val570Gly) was identified. Clinical confirmation pursued via XPC gene sequencing and deletion/duplication analysis of XPC revealed a pathogenic heterozygous deletion of ∼1 kb within XPC, including exons 14 and 15. Research studies determined the alterations to be in trans Although variants in XPC generally result in early-onset skin cancer in childhood, the proband is atypical in that she did not present with her first melanoma until age 36. Review of the patient's clinical, pathological, and genetic findings points to a diagnosis of delayed presentation of xeroderma pigmentosum.

Xeroderma Pigmentosum with Severe Neurological Manifestations/De Sanctis-Cacchione Syndrome and a Novel XPC Mutation

Several genetic disorders caused by defective nucleotide excision repair that affect the skin and the nervous system have been described, including Xeroderma Pigmentosum (XP), De Sanctis–Cacchione syndrome (DSC), Cockayne syndrome, and Trichothiodystrophy. Cutaneous photosensitivity with an increased risk of skin malignancy is a common feature of these disorders, but clinical manifestations commonly overlap these syndromes. Several genes have been found to be altered in these pathologies, but we lack more genotype-phenotype correlations in order to make an accurate diagnosis. Very few cases of DSC syndrome have been reported in the literature. We present a case of a 12-year-old Colombian male, with multiple skin lesions in sun-exposed areas from the age of 3 months and a history of 15 skin cancers. He also displayed severe neurologic abnormalities (intellectual disability, ataxia, altered speech, and hyperreflexia), short stature, and microcephaly, which are features associated with DSC. Genetic testing revealed a novel germline mutation in the XP-C gene (c.547A>T). This is the first case of an XP-C mutation causing De Sanctis–Cacchione syndrome. Multigene panel testing is becoming more widely available and accessible in the clinical setting and will help rapidly unveil the molecular etiology of these rare genetic disorders.