Fight Aging!

The degree to which human longevity is inherited is one of a large number of interesting research topics that, while being related to aging, has little to no relevance to the question of how to treat aging as a medical condition. In developing means to repair or resist the cell and tissue damage that causes degenerative aging, the focus must be on the damage, not the differences from individual to individual. How it is that aging progresses somewhat differently from individual to individual will become increasingly irrelevant as therapies to slow and reverse aging emerge.

That said, today's open access paper on the heritability of longevity is quite interesting. The argument put forward by the authors is that previous efforts to quantify the degree to which individual variance in longevity is determined by one's immediate ancestry have produced underestimates because they failed to properly compensate for the effects of premature death resulting from accidents, infectious disease, and the like. If the strategy for assessment used in the paper is employed instead, then human heritability of longevity is higher than past results, and also more in line with the heritability of other physical traits.

At the same time, the big picture on the genetics of aging that has emerged in recent years, with the advent of very large population databases such as the UK Biobank, is that genetics plays only a small role in determining life expectancy. It is far outweighed by lifestyle choice in the vast majority of people. A high heritability but low contribution of genetic variance suggests that heritability largely exists as a result of the cultural transmission of lifestyle choices; parents that take better care of their health tend to have children who take better care of their health, and vice versa.

Heritability of intrinsic human life span is about 50% when confounding factors are addressed

Understanding the heritability of human life span is fundamental to aging research. However, quantifying the genetic contribution to human life span remains challenging. Although specific life span-related alleles have been identified, environmental factors appear to exert a strong effect on life span. Clarifying the heritability of life span could direct research efforts on the genetic determinants of life span and their mechanisms of action.

Previous studies have estimated the heritability of life span in various populations with results ranging from 15 to 33%, with a typical range of 20 to 25%. Recently, studies on large pedigree datasets estimated it at 6 to 16%. These studies contributed to growing skepticism about the role of genetics in aging, casting doubt on the feasibility of identifying genetic determinants of longevity. Current estimates for the heritability of human life span are thus lower than the heritability of life span in crossbred wild mice in laboratory conditions, estimated at 38 to 55%. They are also lower than the heritability of most other human physiological traits, which show a mean heritability of 49%.

Most life-span studies used cohorts born in the 18th and 19th centuries, with appreciable rates of extrinsic mortality. Extrinsic mortality refers to deaths caused by factors originating outside the body, such as accidents, homicides, infectious diseases, and environmental hazards. Another factor that varies between studies is the minimum age at which individuals must be alive to be included, referred to as the cutoff age. To our knowledge, these two factors - extrinsic mortality and cutoff age - have not been systematically investigated for their effect on heritability estimates of life span.

Here, we explored the effects of extrinsic mortality and cutoff age on twin study estimates of heritability. We used model-independent mathematical analysis and simulations of two human mortality models to partition mortality into intrinsic and extrinsic components. We tested our conclusions on data from three different twin studies, including the SATSA (Swedish Adoption/Twin Study of Aging) study, containing data from twins raised apart that have not been previously analyzed for life-span heritability. To test generalizability to non-Scandinavian cohorts, we also analyzed siblings of US centenarians. We found that extrinsic mortality causes systematic underestimates of the heritability of life span and that cutoff age has a mild nonlinear effect on these estimates. When extrinsic mortality is accounted for, estimates of heritability of life span due to intrinsic mortality rise to about 55%, more than doubling previous estimates.