Roles of chromosome based signal transduction in ensuring genome stability

Lead Research Organisation: MRC National Inst for Medical Research

Abstract

Ataxia Telengiectasia (A-T) is a heritable disorder characterized by cancer predisposition, infertility, neuronal degeneration, and immunodeficiency. A-T is caused by lack or inactivation of the ATM protein and is found in 1/40,000 -1/100,000 live births in various populations. One of the defects observed in cells taken from A-T patients is that they are unable to repair DNA damage and accumulate chromosome breaks. We are investigating how the loss of ATM protein leads to these defects using bakers yeast. Yeast is an excellent model system to study the ways in which our DNA is processed and repaired because the basic mechanism and the genes involved in these processes, including the ATM, are conserved. Findings of our studies will provide invaluable insights into the mechanisms underlying chromosome abnormality, genome instability, infertility, and cancer predisposition in A-T patients.

Technical Summary

The evolutionarily conserved ATM/ATR family proteins are chromosome bound signal transduction molecules that play essential roles in DNA replication, repair, recombination, and checkpoint regulation. Mutations in the ATM gene in humans lead to the genetic disorder Ataxia-Telengiectasia (AT), which is characterized by cancer, neuronal degeneration, immunodeficiency, and sterility. At the cellular level, inactivation of ATR or ATM frequently leads to chromosome breakage and cell death, phenotypes that likely contribute to clinical symptoms of AT patients. To better understand the molecular mechanisms underlying these defects, we have isolated and characterized mutants of the budding yeast ATM/ATR homologue, MEC1. We found that Mec1 is required for promoting replication fork progression and that defects in this process lead to genome-wide fork stalling followed by chromosome fragmentation and cell death. Further analyses revealed that the occurrence of chromosomal breakage is confined to specific regions in the genome where the rate of replication progression is notably slow. These break-susceptible regions were named Replication Slow Zones (RSZs) and proposed to be analogous to the mammalian fragile sites. Recent studies linking the inactivation of ATR to chromosome fragmentation at a fragile site indicate that the mechanisms underlying chromosome breakage in the yeast and mammals are conserved, and that mec1 mutants are an excellent model system in which to study the genome instability and cancer predisposition in ATM and ATR individuals. Our current research is focused on three major areas: Hotspots for chromosome breakage in the eukaryotic genome. The budding yeast RSZs and the mammalian fragile sites are regions in the genome with hypersensitivity to chromosome breakage. Such break-susceptible regions are implicated in a number of important biological phenomena including genome instability, evolution, and cancer. We wish to better understand why certain regions of the genome are particularly fragile. Specifically, we will be examining the primary structure of DNA sequence, chromatin structure, and the nature of biochemical processes targeted to these regions. Mec1 mediated replication fork progression. The formation of lethal chromosomal breaks in mec1 mutants stems from a defect in promoting replication fork progression. To better understand the precise mechanism of Mec1s action, we are utilizing genetic and biochemical approaches to identify factors that are involved in this process, and the ways in which each component is regulated by Mec1. Roles of Mec1 in coupling chromosome morphogenesis and DNA replication. Previously, we have shown that the status of replication-dependent chromosome morphogenesis (e.g. the establishment of sister chromatid cohesion) can either lengthen or shorten the overall duration of S-phase. Based on this and other observations, we proposed the existence of an intra-S-phase regulatory network that monitors the status of chromosome morphogenesis and regulates ongoing DNA replication accordingly. We will be exploring the possibility that Mec1 is the central regulatory component of this feedback system.

Publications

10 25 50
publication icon
Carballo JA (2007) Meiotic roles of Mec1, a budding yeast homolog of mammalian ATR/ATM. in Chromosome research : an international journal on the molecular, supramolecular and evolutionary aspects of chromosome biology

publication icon
Machín F (2006) Transcription of ribosomal genes can cause nondisjunction. in The Journal of cell biology

 
Description Roles of ATM/Tel1 on meiotic DSB-catalysis 
Organisation Memorial Sloan Kettering Cancer Center
Department Molecular Biology Program
Country United States 
Sector Academic/University 
PI Contribution Providing unpublished observations in yeast so that the collaborators can confirm the mechanical conservation in the mouse.
Collaborator Contribution Translating findings in yeast to mammalian system.
Impact A substantive manuscript should be ready for submission in 3-6 months.
Start Year 2010