Chromosomal instability

Almost all AT patients have translocations involving the T cell receptor gene complexes at 14qll, 7q35 or 7pl4, making up about 10% of metaphases counted. AT heterozygotes have similar translocations in about 1% of their T cells. Normals also have such translocations in about 0.1% of their T cells. Chromosomal aberrations in fibroblasts and amnio-cytes of AT patients are random and do not show the characteristic translocations. The T-PLL leukemia of AT patients is frequently associated with translocations or inversions involving the same sites described above. These translocations juxtapose part of a T cell receptor gene to a site just proximal to either the TCL-1 (T cell leukemia 1) gene at 14q32 or the c6.1B/MTCP-l (mature T cell proliferation 1) gene at Xq28. Other factors must also be necessary for leukemia to develop since some AT patients have these clonal expansions in 100% of their T cells for many years and yet do not have leukemia. The frequencies of both mitotic and meiotic recombination are increased in AT cells, as is intrachromosomal recombination. Gametogenesis may be abnormal. ATM knockout mice have gonadal streaks, female animals do not ovulate, and male animals are sterile. Despite this, most AT patients reach normal sexual maturity, with normal menstrual cycles in females, and the ability to produce sperm in at least some males.

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