Biomedical subjects

Bret R Williams

Publications and source records attributed to Bret R Williams.

3 recordsLinked to original sources

PubMed2003

Checkpoint failure and chromosomal instability without lymphomagenesis in Mre11(ATLD1/ATLD1) mice.

In this study, mice expressing one of the two Mre11 alleles inherited in the human ataxia-telangiectasia like disorder (A-TLD) were derived. The mutation had a profound maternal effect on embryonic viability, revealing an acute requirement for Mre11 complex function in early embryogenesis. Mre11(ATLD1/ATLD1) mice exhibited several indices of impaired ATM function. The mice also exhibited pronounced chromosomal instability. Despite this phenotypic spectrum, the animals were not prone to malignancy. These data indicate that defective cell cycle checkpoints and chromosomal instability are insufficient to significantly enhance the initiation of tumorigenesis. In contrast, the latency of malignancy in p53(+/-) mice was dramatically reduced. We propose that in Mre11(ATLD1/ATLD1) mice, genome instability and cell cycle checkpoint defects reduce viability in early embryos and in proliferating cells, while promoting malignancy in the context of an initiating lesion.

Animals↗

PubMed2002

A murine model of Nijmegen breakage syndrome.

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder characterized by microcephaly, immunodeficiency, and predisposition to hematopoietic malignancy. The clinical and cellular phenotypes of NBS substantially overlap those of ataxia-telangiectasia (A-T). NBS is caused by mutation of the NBS1 gene, which encodes a member of the Mre11 complex, a trimeric protein complex also containing Mre11 and Rad50. Several lines of evidence indicate that the ataxia-telangiectasia mutated (ATM) kinase and the Mre11 complex functionally interact. Both NBS and A-T cells exhibit ionizing radiation (IR) sensitivity and defects in the intra S phase checkpoint, resulting in radioresistant DNA synthesis (RDS)-the failure to suppress DNA replication origin firing after IR exposure. NBS1 is phosphorylated by ATM in response to IR, and this event is required for activation of the intra S phase checkpoint (the RDS checkpoint). We derived a murine model of NBS, the Nbs1(DeltaB/DeltaB) mouse. Nbs1(DeltaB/DeltaB) cells are phenotypically identical to those established from NBS patients. The Nbs1(DeltaB) allele was synthetically lethal with ATM deficiency. We propose that the ATM-Mre11 complex DNA damage response pathway is essential and that ATM or the Mre11 complex serves as a nexus to additional components of the pathway.

Amino Acid Sequence↗

PubMed2002

The DNA damage-dependent intra-S phase checkpoint is regulated by parallel pathways.

To preserve genetic integrity, mammalian cells exposed to ionizing radiation activate the ATM kinase, which initiates a complex response-including the S-phase checkpoint pathways-to delay DNA replication. Defects in ATM or its substrates Nbs1 or Chk2 (ref. 3), the Nbs1-interacting Mre11 protein, or the Chk2-regulated Cdc25A-Cdk2 cascade all cause radio-resistant DNA synthesis (RDS). It is unknown, however, whether these proteins operate in a common signaling cascade. Here we show that experimental blockade of either the Nbs1-Mre11 function or the Chk2-triggered events leads to a partial RDS phenotype in human cells. In contrast, concomitant interference with Nbs1-Mre11 and the Chk2-Cdc25A-Cdk2 pathways entirely abolishes inhibition of DNA synthesis induced by ionizing radiation, resulting in complete RDS analogous to that caused by defective ATM. In addition, Cdk2-dependent loading of Cdc45 onto replication origins, a prerequisite for recruitment of DNA polymerase, was prevented upon irradiation of normal or Nbs1/Mre11-defective cells but not cells with defective ATM. We conclude that in response to ionizing radiation, phosphorylations of Nbs1 and Chk2 by ATM trigger two parallel branches of the DNA damage-dependent S-phase checkpoint that cooperate by inhibiting distinct steps of DNA replication.

Ataxia Telangiectasia Mutated Proteins↗