Targeted disruption of ATM leads to growth retardation, chromosomal fragmentation during meiosis, immune defects, and thymic lymphoma.

ATM, the gene mutated in the inherited human disease ataxia-telangiectasia, is a member of a family of kinases involved in DNA metabolism and cell-cycle checkpoint control. To help clarify the physiological roles of the ATM protein, we disrupted the ATM gene in mice through homologous recombination. Initial evaluation of the ATM knockout animals indicates that inactivation of the mouse ATM gene recreates much of the phenotype of ataxia-telangiectasia. The homozygous mutant (ATM-/-) mice are viable, growth-retarded, and infertile. The infertility of ATM-/- mice results from meiotic failure. Meiosis is arrested at the zygotene/pachytene stage of prophase I as a result of abnormal chromosomal synapsis and subsequent chromosome fragmentation. Immune defects also are evident in ATM-/- mice, including reduced numbers of B220+CD43- pre-B cells, thymocytes, and peripheral T cells, as well as functional impairment of T-cell-dependent immune responses. The cerebella of ATM-/- mice appear normal by histologic examination at 3 to 4 months and the mice have no gross behavioral abnormalities. The majority of mutant mice rapidly develop thymic lymphomas and die before 4 months of age. These findings indicate that the ATM gene product plays an essential role in a diverse group of cellular processes, including meiosis, the normal growth of somatic tissues, immune development, and tumor suppression.

The Atr and Atm protein kinases associate with different sites along meiotically pairing chromosomes.

A number of cell-cycle checkpoint genes have been shown to play important roles in meiosis. We have characterized the human and mouse counterpart of the Schizosaccharomyces pombe Rad3 protein, named Atr (for ataxia-telangiectasia- and rad3-related), and the protein that is mutated in ataxia-telangiectasia, Atm. We demonstrate that ATR mRNA and protein are expressed in human and mouse testis. More detailed analysis of specific cells in seminiferous tubules shows localization of Atr to the nuclei of cells in the process of meiosis I. Using immunoprecipitation and immunoblot analysis, we show that Atr and Atm proteins are approximately 300 and 350 kD relative molecular mass, respectively, and further demonstrate that both proteins have associated protein kinase activity. Further, we demonstrate that Atr and Atm interact directly with meiotic chromosomes and show complementary localization patterns on synapsing chromosomes. Atr is found at sites along unpaired or asynapsed chromosomal axes, whereas Atm is found along synapsed chromosomal axes. This is the first demonstration of a nuclear association of Atr and Atm proteins with meiotic chromosomes and suggests a direct role for these proteins in recognizing and responding to DNA strand interruptions that occur during meiotic recombination.