New Scholar Award in Aging
Sandy Chang, M.D.,Ph.D
M.D. Anderson Cancer Center

Genomic Instability and Aging in Werner-Telomerase Compound Knockout Mice.

Genetic studies of human progeric syndromes have enhanced our understanding the molecular mechanisms of the aging process. Werner Syndrome (WS) is the segmental progeria considered to be most similar to natural senescence. WS fibroblasts display premature replicative senescence in culture and increased rates of telomere shortening. Telomeres function to stabilize and protect chromosomal ends from degradation, prevent end-to-end chromosomal fusions and regulate cellular replicative capacity. It has been postulated that telomere shortening serves as a molecular clock that eventually signals replicative senescence. WS cells senesce while still possessing longer telomeres than control cells, suggesting that this senescence is due to hypersensitivity to telomere shortening. In support of this hypothesis, the replicative senescence phenotype observed in WS cells can be rescued by over statement of telomerase, suggesting that one consequence of the WS defect is the acceleration of normal telomere based replicative senescence.

Mice bearing homozygous null mutations within the WRN gene have been generated, and to date they do not develop an obvious organismal aging phenotype or an increase in cancer incidence. I hypothesize that manifestation of WS phenotype in WRN-/- mice requires the setting of short telomeres, since telomere lengths in the mouse are normally too long for the required attrition to take place during aging. To understand the effects of WRN inactivation on organismal aging, I propose to shorten telomere lengths genetically in the WRN-/- mouse by crossing it with telomerase-/- mice bearing short telomeres. My working hypothesis, based on preliminary data, is that loss of WRN in the setting of short telomeres results in genomic instability and premature aging in vivo. My lab has generated telomerase-WRN compound knockout mice and we have bred them through three generations to shorten their telomeres. Embryonic fibroblasts from these mice display an increased immortalization potential during serial passage, manifested as continuous growth in a NIH3T9 assay and colony formation at low density. Examination of metaphase chromosomes prepared from these lines by spectral karyotyping revealed profound genomic instability, including multiple chromosomal translocations and marked aneuploidy. Cells from control WRN-/- mice bearing long telomeres do not exhibit these phenotypes, supporting the hypothesis that genomic instability in the WRN-/- mouse requires telomere shortening. These results indicate that at least on a cellular level, our mouse model of aging recapitulates some of the WS phenotype observed in human patients. Our immediate goal is to further characterize the level of genomic instability in these mice by examining primary cells and tissues for cytogenetic abnormalities in an age-dependent manner. We will correlate this finding to any signs and symptoms of premature aging that these mice may be experiencing as they grow older by examining various biomarkers associated with advancing age.

I believe that our mouse model offers an unparalleled system to study genomic instability and its role on the aging process in vivo. Our long-term goal is to understand pathways and molecules that may be perturbed in the setting of this global genomic instability, and how these pathways impact on mammalian organismal aging.

Contact Dr. Chang.