Increased radiosensitivity and the basic defect in ataxia telangiectasia

Various cellular defects have been found in ataxia telangiectasia (A-T) cells including increased radiosensitivity, increased sensitivity to various chemical agents, a probable DNA repair defect and a defect in DNA synthesis. How these different features are related to each other is at present unknown. It has been suggested that there is a defect in A-T that acts in tissue differentiation as well as during growth and in the mature adult. This hypothesis is supported by the observations, for example, of an immature thymus present in patients, the production of alpha-fetoprotein, which results in a high serum level, and ovarian dysgenesis. A gene for A-T has recently been localized to chromosome region 11q22-23, a site involved in chromosomes translocations in some non-lymphoid leukaemias. At the chromosomal level the spontaneous abnormalities in A-T include, first, an increased frequency of cells showing chromosome translocations involving immune system genes that normally undergo rearrangement to form a functional product; secondly, the formation of telometric dicentrics in both lymphocytes and fibroblasts; and thirdly formation of long-lived chromosome damage following exposure to ionizing radiation and radiomimetic drugs. The gene defect underlying this disorder is unknown and distinguishing between primary and secondary effects of the mutant gene is difficult. We consider alternative models for retention of translocation T cells. First, it is possible that there is a defect in recognition of site-specific damage leading to retention of translocation cells that might otherwise be removed. Secondly, a feature common to the production of illegitimate T-cell receptor gene rearrangements and to formation of telomeric dicentric chromosomes in A-T cells is an increased period of time available for chromosome interchange, possibly due to a site-specific defect in strand break repair. It is possible that this defect may also prevent chromosome restitution following exposure of cells to ionizing radiation.