We report the spectrum of 59 ATM mutations observed in ataxia-telangiectasia (A-T) patients in the British Isles. Of 51 ATM mutations identified in families native to the British Isles, 11 were founder mutations, and 2 of these 11 conferred a milder clinical phenotype with respect to both cerebellar degeneration and cellular features. We report, in two A-T families, an ATM mutation (7271T-->G) that may be associated with an increased risk of breast cancer in both homozygotes and heterozygotes (relative risk 12.7; P=. 0025), although there is a less severe A-T phenotype in terms of the degree of cerebellar degeneration. This mutation (7271T-->G) also allows expression of full-length ATM protein at a level comparable with that in unaffected individuals. In addition, we have studied 18 A-T patients, in 15 families, who developed leukemia, lymphoma, preleukemic T-cell proliferation, or Hodgkin lymphoma, mostly in childhood. A wide variety of ATM mutation types, including missense mutations and in-frame deletions, were seen in these patients. We also show that 25% of all A-T patients carried in-frame deletions or missense mutations, many of which were also associated with expression of mutant ATM protein.

Ataxia telangiectasia (AT) is characterized by neurological deterioration, immunodeficiency, spontaneous chromosomal instability, hypersensitivity to ionizing radiation, predisposition to cancer, particularly T cell leukaemia and lymphoma, and premature ageing. The most commonly observed defect affecting telomeres in humans is telomeric fusions, particularly in T lymphocytes in AT patients. Rarely, some tumour cells, like senescent cells, have dicentric chromosomes that may arise as a result of telomeric sequence loss. We show that the AT mutation in the homozygous state confers a predisposition to accelerated telomere shortening with increasing age in peripheral blood lymphocytes (PBLs), which may be linked to premature senescence. We also show that telomeric fusions are associated with large (> 90%) preleukaemic translocation clones in T cells. We propose that these fusions may result from a compound effect of accelerated telomere shortening, together with a growth advantage of cells in large clones which leads to further telomere loss. Fusions are not observed in leukaemic cells in these patients. There is no evidence that either accelerated telomere loss per se or telomeric fusions are important in tumourigenesis. Telomerase is present in both normal and AT lymphocytes and so neither telomere shortening nor telomeric fusions can be explained by the absence of telomerase.

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Background:Immunodeficiency in ataxia telangiectasia (A-T) is less severe in patients expressing some mutant or normal ATM kinase activity. We, therefore, determined whether expression of residual ATM kinase activity also protected against tumour development in A-T.Methods:From a total of 296 consecutive genetically confirmed A-T patients from the British Isles and the Netherlands, we identified 66 patients who developed a malignant tumour; 47 lymphoid tumours and 19 non-lymphoid tumours were diagnosed. We determined their ATM mutations, and whether cells from these patients expressed any ATM with residual ATM kinase activity.Results:In childhood, total absence of ATM kinase activity was associated, almost exclusively, with development of lymphoid tumours. There was an overwhelming preponderance of tumours in patients <16 years without kinase activity compared with those with some residual activity, consistent with a substantial protective effect of residual ATM kinase activity against tumour development in childhood. In addition, the presence of eight breast cancers in A-T patients, a 30-fold increased risk, establishes breast cancer as part of the A-T phenotype.Conclusion:Overall, a spectrum of tumour types is associated with A-T, consistent with involvement of ATM in different mechanisms of tumour formation. Tumour type was influenced by ATM allelic heterogeneity, residual ATM kinase activity and age.

Ataxia telangiectasia (A-T) is a rare recessively inherited disorder resulting in a progressive neurological decline. It is caused by biallelic mutation of the ATM gene that encodes a 370 kDa serine/threonine protein kinase responsible for phosphorylating many target proteins. ATM is activated by auto(trans)phosphorylation in response to DNA double strand breaks and leads to the activation of cell cycle checkpoints and either DNA repair or apoptosis as part of the cellular response to DNA damage. The allelic heterogeneity in A-T is striking. While the majority of mutations are truncating, leading to instability and loss of the ATM protein from the allele, a significant proportion of patients carry one of a small number of mutations that are either missense or leaky splice site mutations resulting in retention of some ATM with activity. The allelic heterogeneity in ATM, therefore, results in an equally striking clinical heterogeneity. There is also locus heterogeneity because mutation of the MRE11 gene can cause an obvious A-T like disorder both clinically and also at the cellular level and mutation of the RNF168 gene results in a much milder clinical phenotype, neurologically, with the major clinical feature being an immunological defect.

Ataxia telangiectasia is a recessive disorder in which patients show a progressive cerebellar degeneration leading to ataxia, abnormal eye movements and deterioration of speech. Other features include ocular telangiectasia, high serum AFP levels, immunodeficiency, growth retardation and an increased predisposition to some tumours, particularly T cell leukaemia and lymphoma. We report the 1348 amino acid sequence of the N-terminal half of the A-T gene product which, together with the previously published C-terminal half, completes the sequence of the A-T protein. No homologies with other genes have been found within the N-terminal half of the A-T protein. We have also identified six mutations affecting the N-terminal half of the protein. One of these mutations was found to be associated with a haplotype that is common to four apparently unrelated families of Irish descent. All the patients so far examined for both A-T alleles were shown to be compound heterozygotes. None of these mutations affected a putative promoter region which may direct divergent transcription of both the A-T gene and a novel gene E14. The ability to recognise mutations across the entire coding sequence of the A-T gene provides a practical advantage to A-T families since a DNA based prenatal diagnosis will be possible in families where the mutations are identified irrespective of the level of radiosensitivity in these families.

Patients with the autosomal recessive disorder ataxia telangiectasia (A-T) show the biallelic inactivation of the ataxia telangiectasia mutated (ATM) gene. A-T patients exhibit a predisposition to the development of a wide range of lymphoid tumours, suggesting that the ATM protein normally plays an important role in the prevention of both T and B cell malignancies. The ATM protein is a 370 kDa protein kinase implicated in the integration of different cellular responses to particular forms of DNA damage. Several recent studies have reported the possibility that the ATM gene can act as a tumour suppressor gene in non A-T individuals. Frequent ATM inactivation was confirmed in three sporadic lymphoid tumours of mature phenotype: T cell prolymphocytic leukaemia (T-PLL), B-cell chronic lymphocytic leukaemia (B-CLL) and mantle cell lymphoma (MCL). Here, we provide a summary of the published ATM mutations in sporadic lymphoid tumours, including our own study on the role of ATM mutations in the pathogenesis of sporadic B-CLL. The published results suggest possible differences in the origin, the nature and distribution of ATM mutations between sporadic B-CLL, MCL and T-PLL. While ATM mutations in mature B cell tumours (B-CLL and MCL) represent a mixture of missense and truncating errors distributed across the whole of the ATM coding sequence, mutations in sporadic T-PLL appear to be predominantly missense, clustering in the region encoding the PI-3 kinase catalytic domain of the protein. The reason for this difference is unclear, but the difference itself supports the notion that the pathogenesis of B and T cell tumours on an ATM deficient background might be different. In addition, in both B-CLL and MCL ATM mutation carriers have been reported, raising the possibility that ATM mutation carriers may have an increased risk of developing these tumours. The existence as well as magnitude of the risk, however, remains to be established. Furthermore, our own studies indicate that the presence of ATM mutations in sporadic B-CLL causes a distinctive defect in response to DNA damaging agents, offering a possible explanation for the poor response of ATM mutant tumours to standard treatment. Therefore, one of the future challenges will be to devise strategies to bypass the existing defect in response to DNA damage and activate apoptosis in ATM mutant sporadic lymphoid tumours.