A MUTATION IN REPB, THE DICTYOSTELIUM HOMOLOG OF THE HUMAN XERODERMA-PIGMENTOSUM-B GENE, HAS INCREASED SENSITIVITY TO UV-LIGHT BUT NORMAL MORPHOGENESIS

Nucleotide excision repair (NER) is an important cellular defense mech anism which protects the integrity of the genome by removing DNA damag e caused by UV-light or chemical agents. In humans, defects in the NER pathway result in the disease xeroderma pigmentosum (XP) which is cha racterized by increased UV-sensitivity, with increased propensity for skin cancer, and an array of developmental abnormalities. Some XP pati ents exhibit, in addition, symptoms of Cockayne's syndrome (CS) and tr ichothiodystrophy (TTD), which are characterized by increased UV-sensi tivity, without increased cancer incidence, and an array of developmen tal abnormalities. Some NER genes, including the DNA helicases XPB and XPD, have been shown to function in transcription as well as repair, by virtue of being an integral part of the transcription initiation fa ctor TFIIH. This dual function may account for the above-mentioned wid e pleiotropy of phenotypes associated with defects in NER genes, and m ay explain why some XP patients exhibit developmental abnormalities in addition to XP symptoms. To date, only five XPB patients with three d ifferent mutations in the XPB gene have been reported. One of these mu tations is a C to A transversion at the splice site at the beginning o f the last exon, which resulted in a frameshift throughout the last ex on. This patient shows combined clinical symptoms of XP and CS. The re cent cloning of the repB gene, the Dictyostelium discoideum homolog of XPB, allowed us to generate a similar C-terminal mutation in the Dict yostelium, in order to test whether the defect in this NER gene has an effect on growth or development. To this end, we have constructed a C -terminal deletion repB mutant in Dictyostelium. To avoid the possibil ity that a null mutant would be lethal, we used direct homologous reco mbination to create a 46 amino acid C-terminal deletion mutant. Indeed , we were unable to obtain mutants with a longer 95 amino acid deletio n. The repB Delta C46 mutants showed an increased sensitivity to W-lig ht, but a normal pattern of W-induced expression of repair genes, and no immediately obvious defect in either growth rate or development. Th e results suggest that the associated developmental defects in the hum an XPB patients may be due to mutations in another gene. (C) 1998 Else vier Science B.V. All rights reserved.