Work in our laboratory is focused on understanding the molecular transactions that govern chromosome stability and replication. The association of cancer predisposition and other pathology with mutations that affect chromosomal metabolism forms the basis of our interest in this process. In this regard, we focus on a conserved multiprotein complex that includes Mre11, Rad50, and Nbs1 in mammals or Xrs2 in the budding yeast S. cerevisiae. Our laboratory has isolated and characterized the human Mre11 complex, hMre11, hRad50, and Nbs1. We proved that an analogue of the S. cerevisiae Mre11 complex exists in human cells, and subsequently established definitive evidence that the yeast and human complexes mediate double-strand break repair in S. cerevisiae and mammalian cells, respectively. Our data suggest that in human cells, the complex acts as a sensor of DNA damage that participates in the activation of cell cycle checkpoints following g-irradiation.
This interpretation stems from two observations. First, cytologic analyses indicate that the complex becomes associated with damaged DNA early in the DNA damage response and remains associated until the bulk of DNA repair is complete. Second, we have shown that mutations of the NBS1 and MRE11 genes, encoding members of the human complex, are responsible for the chromosome instability syndromes — Nijmegen breakage syndrome (NBS) and the ataxia telangiectasia-like disorder (A-TLD). DNA damage association of the Mre11 complex is disrupted in NBS cells and A-TLD cells, and these cells are defective in the activation of certain cell-cycle-checkpoint functions, suggesting a disruption in signal transduction following DNA damage.
Two important new directions in the lab grow out of recent exciting findings. First, we have established that the Mre11 complex is linked to DNA synthesis through its association with chromosomal origins and also with DNA replication forks. This is linked to the cell-cycle-checkpoint functions of the complex, as well as its role in maintaining chromosome stability. Second, we have shown that the Mre11 complex is situated at human telomeres via its association with the telomere maintenance protein, TRF2. This observation helps to explain some of the premature aging and cancer predisposition phenotypes associated with NBS and A-TLD, and suggests an important role for the complex in telomere functions and chromosome stability.
In yeast, mutagenesis efforts identified domains of ScMre11 that are important for diverse phenotypic outcomes, such as mitotic recombination, double-strand break repair, and physical association with other members of the complex. We have established that the S. cerevisiae Mre11 complex is critical in particular phases of the cell cycle in controlling the dynamics of chromatid interaction during double-strand break repair. This protein complex is also involved in processes that protect against chromosome loss, in addition to its role in recombinational DNA repair
Finally, we have definitive evidence that, as in mammalian cells, the complex functions with the ATM homologue, Tel1, to control cell cycle progress in following DNA damage. Many aspects of chromosomal maintenance are conserved between mammals and S. cerevisiae, as our previous work on the hMre11 and hRad50 homologues from S. cerevisiae and humans demonstrate. Therefore, we feel that the data we obtain from S. cerevisiae will be relevant to the (human) Mre11 complex, and will provide fundamental insights regarding the eukaryotic cellular DNA damage response.
Mouse models for NBS and A-TLD have been established in our lab, in addition to three different Rad50 mutant mice. These animals provide insight regarding the in vivo roles of the Mre11 complex in meiotic progression, hematopoiesis, and cancer prevention. We expect that the genetic analysis of the DNA damage response permitted by these mutant mice will elucidate the mechanisms of the complex’s diverse influences. Also, cells and tissues obtained from these mice give us a venue for biochemical analysis. These relatively new efforts have already yielded important information (see recent publication list below).
In the long term, we hope to understand what role this complex in particular and the process of recombinational DNA repair in general play in the onset and progression of malignancy. In addition, we hope to acquire a better understanding of the structural organization of chromosome metabolism functions, such as DNA recombination and replication within the cell. This insight will help to elucidate the link between DNA metabolism and the signal transduction pathways that regulate other aspects of the cellular DNA damage response.