Complementation of chromosomal aberrations in AT/NBS hybrids: inadequacy of RDS as an endpoint in complementation studies with immortal NBS cells

Nijmegen breakage syndrome (NBS) and ataxia telangiectasia (AT) are rare au tosomal recessive hereditary disorders characterized by radiosensitivity, c hromosomal instability, immunodeficiency and proneness to cancer. Although the clinical features of both syndromes are quite distinct, the cellular ch aracteristics are very similar. Cells from both NBS and AT patients are hyp ersensitive to ionizing radiation (IR), show elevated levels of chromosomal aberrations and display radioresistant DNA synthesis (RDS), The proteins d efective in NBS and AT, NBSI and ATM, respectively, are involved in the sam e pathway, but their exact relationship is not yet fully understood, Stumm et al. (Am. J. Hum. Genet. 60 (1997) 1246) have reported that hybrids of AT and NBS lymphoblasts were not complemented for chromosomal aberrations. In contrast, we found that X-ray-induced cell killing as well as chromosomal aberrations were complemented in proliferating NBS-1LBI/AT5BIVA hybrids, co mparable to that in NBS-1LBI cells after transfer of a single human chromos ome 8 providing the NBS1 gene. RDS observed in AT5BIVA cells was reduced in these hybrids to the level of that seen in immortal NBS-1LBI cells. Howeve r, the level of DNA synthesis, following ionizing radiation, in SV40 transf ormed wild-type cell Lines was the same as in NBS-1LBI cells. Only primary wild-type cells showed stronger inhibition of DNA synthesis. In summary, th ese results clearly indicate that RDS cannot be used as an endpoint in func tional complementation studies with immortal NBS-1LBI cells, whereas the cy togenetic assay is suitable for complementation studies with immortal AT an d NBS cells. (C) 2001 Elsevier Science B.V. All rights reserved.