Mechanisms of age-associated cardiac heterochromatin dysfunction

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A large body of evidence indicates that 1) epigenetic alterations are a conserved feature of aging, 2) they contribute to age-associated functional decline, and 3) manipulation of epigenetic regulators can markedly increase health- and lifespan. However, knowledge regarding mechanisms of age-associated epigenetic changes is far from complete. This proposal focuses on elucidating the basis for age-associated alterations in cardiac heterochromatin. The major satellite repeats (MSRs) are a key heterochromatin component in the mouse. Twenty-five years ago, it was noted that MSR repression is attenuated with aging in mouse heart. The molecular basis for this effect has never been elucidated. Moreover, the functional impact of this derepression is unclear, although in other contexts loss of MSR silencing is associated with impaired chromosomal segregation and development of aneuploidy, a non-diploid cellular chromosomal content. The long-term goal is to understand both mechanisms and functional consequences of altered chromatin function with age. The objective of this application is to elucidate the basis for age-associated cardiac MSR derepression, and to test its impact on cardiac genomic stability. Supporting studies potentially link this effect to reduced function of the NAD+-dependent deacetylase SIRT1. The hypothesis is that acquired loss of SIRT1 function, due to reductions in NAD+ levels in the aging heart, lead in turn to impaired MSR heterochromatin structure and transcriptional derepression. It is further proposed that dysregulation of MSR heterochromatin contributes to age-associated cardiac aneuploidization. The rationale for these studies is that epigenetic changes are reversible, at least in principle. Therefore, mechanistic insight into age-associated heterochromatin alterations may identify novel therapeutic opportunities in rejuvenative medicine. The work will be carried out in the context of two Specific Aims. First, the potential role of reduced NAD+ levels and SIRT1 activity in age-associated MSR derepression will be assessed. This Aim will be carried out via chromatin immunoprecipitation, immunofluorescence, and micrococcal nuclease studies, analyzing mice of varied ages, and strains with genetic or pharmacological reconstitution of NAD+ levels or SIRT1 function in aged myocardium. Second, the functional impact of impaired MSR heterochromatinization will be assessed in the context of age-associated cardiac aneuploidization. A cell culture-based system will be developed to rigorously test functional relationships between SIRT1 activity, MSR expression, and maintenance of euploidy. This proposal is innovative, since the basis for age-associated loss of cardiac MSR silencing remains unknown. The work is significant, in that it will provide mechanistic insight into a 25-year old mystery in chromatin and aging biology. In light of published data showing that enhanced euploidy maintenance in the heart is associated with improved function at later ages, the studies in this proposal may provide insight into novel therapeutic interventions to ameliorate cardiac health in older individuals.