Assessment of Chronic γ Radiosensitivity as an in Vitro Assay for Heterozygote Identification of Ataxia-Telangiectasia

Abstract

Ataxia-telangiectasia (A-T) is a rare human autosomal recessive disorder characterized by, among other symptoms, catastrophic reaction to conventional radiotherapy. A-T heterozygotes are clinically asymptomatic and their fibroblasts are intermediate in radiosensitivity between homozygotes and normals. We have attempted to identify heterozygotes by assaying for cellular hypersensitivity to chronic gamma irradiation. Cultured dermal fibroblast strains from 13 control subjects and 55 members from a large Amish pedigree segregating for A-T were assayed for loss of colony-forming ability (CFA) in response to 137Cs gamma radiation delivered at a dose rate of 0.8 cGy/min. For each strain, multiple dose-response curves were summarized in a composite D10 value (dose, in cGy, reducing colony survival to 10%). The D10's of the clinically normal controls and of those pedigree members with known A-T genotype formed a trimodal distribution, with the seven obligate heterozygotes displaying an average value (516 cGy) intermediate between that of the 10 healthy controls (797 cGy) and that of the two affected patients (154 cGy). The D10's were modeled statistically using Gaussian penetrance functions. The most parsimonious model yielded a significant difference in D10 means for heterozygotes and normal homozygotes, a significant donor age effect, but no sex effect. We compared probabilistic identification of heterozygotes based on D10 values with identification based on linkage data for two markers, THY1 and D11S144, closely linked to the A-T gene. This comparison revealed that the D10 data were appreciably less informative than the linked markers. Indeed, the extensive overlap between D10 values for heterozygotes and normal homozygotes precludes the use of postirradiation CFA for either accurate identification of heterozygotes or chromosomal mapping of the A-T gene.

Cellular responses to ionising radiation of AT heterozygotes: differences between missense and truncating mutation carriers

“…Many authors have reported a level of cell survival intermediate between that seen in normal and ATM homozygote cells, but, in other studies, some overlap or no statistically significant differences between normal and AT het cell types have been detected (Chen et al, 1978;Kinsella et al, 1982;Nagasawa et al, 1985;Weeks et al, 1991;West et al, 1995;Shigeta et al, 1999;Sun et al, 2002). Many of these studies have used a fibroblast-cloning assay, which requires the trypsinising and replating of treated cells, which may lead to additional cell death, particularly in radiationsensitive cell types.…”

Comparison between radiation-induced cell cycle delay in lymphocytes and radiotherapy response in head and neck cancer

“…Defined disorders with radiosensitive phenotypes, such as the ataxia-telangiectasia (AT) syndrome, show that genetic factors can strongly determine the radiosensitivity of humans (Thacker, 1994). In addition, studies have revealed that cells from AT heterozygotes show increased radiosensitivity (Cole et al, 1988;Weeks et al, 1991;West et al, 1995). There are ongoing efforts to evaluate a possible role of AT heterozygosity for over-reaction to RT, particularly in breast cancer patients (Weeks et al, 1991;West et al, 1995).…”

“…In addition, studies have revealed that cells from AT heterozygotes show increased radiosensitivity (Cole et al, 1988;Weeks et al, 1991;West et al, 1995). There are ongoing efforts to evaluate a possible role of AT heterozygosity for over-reaction to RT, particularly in breast cancer patients (Weeks et al, 1991;West et al, 1995). From these studies, it can be observed that in vitro sensitivity of normal cells from some cancer patients may fall into a similar range, as AT heterozygotes do.…”

Cell cycle arrest defect in Li – Fraumeni Syndrome: a mechanism of cancer predisposition?

“…After high dose rate (HDR) irradiation, as used in the present investigation, only one LFS strain (FH1) showed a signi®cantly radioresistant phenotype compared with normals. However, after low dose rate (LDR) exposure, which is known to amplify small di erences in survival response seen with HDR (Weeks et al, 1991;Burnet et al, 1994), all the LFS strains were signi®cantly more resistant than the normal strains (Sproston et al, 1996). The quantitative relationship between G 1 arrest and these radiosensitivity data (Sproston et al, 1996) was determined by linear regression analysis (Table 2 and Figure 3).…”