A founder large deletion mutation in Xeroderma pigmentosum-Variant form in Tunisia: implication for molecular diagnosis and therapy

Abstract

Xeroderma pigmentosum Variant (XP-V) form is characterized by a late onset of skin symptoms. Our aim is the clinical and genetic investigations of XP-V Tunisian patients in order to develop a simple tool for early diagnosis. We investigated 16 suspected XP patients belonging to ten consanguineous families. Analysis of the POLH gene was performed by linkage analysis, long range PCR, and sequencing. Genetic analysis showed linkage to the POLH gene with a founder haplotype in all affected patients. Long range PCR of exon 9 to exon 11 showed a 3926 bp deletion compared to control individuals. Sequence analysis demonstrates that this deletion has occurred between two Alu-Sq2 repetitive sequences in the same orientation, respectively, in introns 9 and 10. We suggest that this mutation POLH NG_009252.1: g.36847_40771del3925 is caused by an equal crossover event that occurred between two homologous chromosomes at meiosis. These results allowed us to develop a simple test based on a simple PCR in order to screen suspected XP-V patients. In Tunisia, the prevalence of XP-V group seems to be underestimated and clinical diagnosis is usually later. Cascade screening of this founder mutation by PCR in regions with high frequency of XP provides a rapid and cost-effective tool for early diagnosis of XP-V in Tunisia and North Africa.

Figure 1

Pedigree and haplotype analysis for the XPV families (the disease haplotype is indicated by shading) and clinical photograph of each affected patient.

Figure 2

Agar gel electrophoretic analysis of the PCR POLH gDNA of exon 10 and its intronic boundaries showed difference in the size between affected individuals (XPV17B-1 and XPV91) compared to healthy parents (XPV(P)) and a healthy control. (Marker: 1 kb DNA ladder molecular size marker (GeneRuler).)

Figure 3

Characterization of the deletion breakpoints. (a) Electropherogram demonstrating the junction fragment resulting from the large deletion in the XP-V patients. Partial representation of introns 9 and 10 with the 35 bp breakpoint overlap framed in red. (b) Nucleotide sequence alignment of the genomic sequence of introns 9 and 10 of the POLH gene. Short vertical lines indicate matched bases between both introns. (c) Schematic representation of the deletion breakpoints and their flanking Alu Sq2 elements. (1) represents a normal gDNA fragment and (2) schematizes the mutated gDNA with a deletion of 3925 bp.