Chapter 11: Gene Expression Biomarkers and Longevity

Chronological age, a count of how many orbits of the sun an individual has made as a passenger of planet earth, is a useful but limited proxy of aging processes. Some individuals die of age related diseases in their sixties, while others live to double that age. As a result, a great deal of effort has been put into identifying biomarkers that reflect the underlying biological changes involved in aging. These markers would provide insights into what processes were involved, provide measures of how much biological aging had occurred and provide an outcome measure for monitoring the effects of interventions to slow ageing processes. Our DNA sequence is the fixed reference template from which all our proteins are produced. With the sequencing of the human genome we now have an accurate reference library of gene sequences. The recent development of a new generation of high throughput array technology makes it relatively inexpensive to simultaneously measure a large number of base sequences in DNA (or RNA, the molecule of gene expression). In the last decade, array technologies have supported great progress in identifying common DNA sequence differences (SNPs) that confer risks for age related diseases, and similar approaches are being used to identify variants associated with exceptional longevity [1]. A striking feature of the findings is that the majority of common disease-associated variants are located not in the protein coding sequences of genes, but in regions of the genome that do not produce proteins. This indicates that they may be involved in the regulation of nearby genes, or in the processing of their messages. While DNA holds the static reference sequences for life, an elaborate regulatory system influences whether and in what abundance gene transcripts and proteins are produced. The relative abundance of each transcript is a good guide to the demand for each protein product in cells (see section 2 below). Thus, by examining gene expression patterns or signatures associated with aging or age related traits we can peer into the underlying production processes at a fundamental level. This approach has already proved successful in clinical applications, for example using gene signatures to classify cancer subtypes [2]. In aging research, recent work conducted in the InCHIANTI cohort has identified gene-expression signatures in peripheral leucocytes linked to several aging phenotypes, including low muscle strength, cognitive impairment, and chronological age itself. In the sections that follow we provide a brief introduction to the underlying processes involved in gene expression, and summarize key work in laboratory models of aging. We then provide an overview of recent work in humans, thus far mostly from studies of circulating white cells.