Identification of a ERCC5 c.2333T>C (L778P) Variant in Two Tunisian Siblings With Mild Xeroderma Pigmentosum Phenotype

Abstract

Xeroderma pigmentosum (XP) is a rare autosomal recessive disorder due to a defect in the nucleotide excision repair (NER) DNA repair pathway, characterized by severe sunburn development of freckles, premature skin aging, and susceptibility to develop cancers at an average age of eight. XP is an example of accelerated photo-aging. It is a genetically and clinically heterogeneous disease. Eight complementation groups have been described worldwide. In Tunisia, five groups have been already identified. In this work, we investigated the genetic etiology in a family with an atypically mild XP phenotype. Two Tunisian siblings born from first-degree consanguineous parents underwent clinical examination in the dermatology department of the Charles Nicolle Hospital on the basis of acute sunburn reaction and mild neurological disorders. Blood samples were collected from two affected siblings after written informed consent. As all mutations reported in Tunisia have been excluded using Sanger sequencing, we carried out mutational analysis through a targeted panel of gene sequencing using the Agilent HaloPlex target enrichment system. Our clinical study shows, in both patients, the presence of achromic macula in sun exposed area with dermatological feature suggestive of Xeroderma pigmentosum disease. No developmental and neurological disorders were observed except mild intellectual disability. Genetic investigation shows that both patients were carriers of an homozygous T to C transition at the nucleotide position c.2333, causing the leucine to proline amino acid change at the position 778 (p.Leu778Pro) of the ERCC5 gene, and resulting in an XP-G phenotype. The same variation was previously reported at the heterozygous state in a patient cell line in Europe, for which no clinical data were available and was suggested to confer an XP/CS phenotype based on functional tests. This study contributes to further characterization of the mutation spectrum of XP in consanguineous Tunisian families and is potentially helpful for early diagnosis. It also indicates that the genotype-phenotype correlation is not always coherent for patients with mild clinical features. These data therefore suggest that targeted NGS is a highly informative diagnostic strategy, which can be used for XP molecular etiology determination.

Human XPG nuclease structure, assembly, and activities with insights for neurodegeneration and cancer from pathogenic mutations

“…XP40GO, a cell line with no associated patient information but biochemical analysis of which was consistent with XP-G/CS (24), also lacked detectable XPG. We did not have cells for analysis from sibling patients XP174-1 and -2, which carry the same L788P mutation as XP40GO but are homozygous for it and are XP in phenotype (SI Appendix, Table S4) (50). Nonetheless, our results suggest that both XP-G and XP-G/CS mutations disrupt XPG protein stability.…”

“…Structural Biochemistry of Pathogenic Mutation Sites. To examine XPG pathogenic mutations in their molecular context, we mapped all known missense mutations onto our structure (nine XP-G, five XP-G/CS, and one mixed/uncertain) (24,25,(50)(51)(52)(53)(54)(55)(56)(57)(58)(59). Although deletions and truncations that cause XP-G/CS map throughout the ERCC5 gene, 14 of 15 missense mutations in both XP-G and XP-G/CS map to the N and I catalytic domains (Fig.…”

“…The XP-G/CS mutations were P72H, G805R, W814S, and L858P. The mutation L778P was defined as mixed, since it exhibits as XP in homozygous patients, but cells from a phenotypically uncharacterized patient in which L778P is heterozygous with a frameshift mutation in ERCC5 have biochemical properties that are CS-like (SI Appendix, Table S4) (24,50). All 10 pathogenic mutations reduced recombinant expression of XPGcat to different extents compared to the WT (Fig.…”

“…It is also possible that heteroallelic effects may contribute to disease outcome. Indeed, the mutation L778P exhibits as XP in homozygous patients, but cells from a patient in which that missense mutation is heterozygous with a truncating frameshift mutation in Xpg have biochemical properties that are CS-like (SI Appendix, Table S4) (24,50). The structure of a protein is intimately linked to its stability, function, and interactions, and the effect of many human missense variants can be accurately interpreted from an analysis of experimental structures (71).…”

The crystal structure of human XPG, the xeroderma pigmentosum group G endonuclease, provides insight into nucleotide excision DNA repair

“…The structures of XPGn reported here provide an atomic framework to rationalize XPG mutations found in XP and CS patients (summarized in ( 39 )). From the thirteen different point mutations identified to date, only I290N lies in the spacer region, absent in our constructs ( Supplementary Table S1 ).…”