Functional significance of XPD polymorphic variants: Attenuated apoptosis in human lymphoblastoid cells with the XPD 312 Asp/Asp genotype

Recent molecular epidemiological studies have identified polymorphisms in t he XPD gene that are associated with increased risk of brain gliomas and he ad, neck, lung, and skin cancers. However, the functional significance of t hese polymorphic variants in altering cell processes such as cell cycle che ckpoints, DNA repair, and apoptosis is uncertain. We have cloned the XPD va riants Lys751Gln, Asp312Asn, and Lys751Gln-Asp312Asn into a pcDNA-3.1-expre ssion vector. Using these constructs, we did not rind any detectable differ ence in either in vitro binding with wild-type p53 or in DNA repair profici ency as measured by host cell reactivation assay. We then genotyped 34 diff erent lymphoblastoid cell lines from six Centre d'Etude du Polymorphisme Hu maine (CEPH)/Utah pedigree families and a CEPH/French pedigree family for p olymorphisms at codons 751 and 312 and assessed their apoptotic response af ter either UV or ionized radiation exposure. The lymphoblastoid cell lines with homozygous or heterozygous Asp at codon 312 have similar apoptotic rat es, whereas cell lines with homozygous Asn at codon 312 showed a 2.5-fold i ncreased response to UV (P = 0.005; Student's t test). This is the first re port known to us of a functional polymorphism in a gene involved in DNA dam age-induced apoptosis. However, the presence of Lys or Gln at codon 751 did not influence the apoptotic response to UV. The diminished apoptotic respo nse of cells containing the 312 Asp allele could both allow the survival an d selective clonal expansion of carcinogen-damaged cells and be a mechanist ic explanation for the increased risk of cancer at diverse tissue sites.