Thymus of the essence?

We've considered cancer and its relationship to aging on a number of previous occasions. Studies published in the British Journal of Cancer in 2011 and 2018 concluded that around 40% of cases are attributable to known modifiable lifestyle and environmental factors, which is a substantial minority. Whilst risk for specific cancer subtypes will be more or less amenable to lifestyle and environment interventions than this, it is beyond doubt that the longer we live, the higher our risk of cancer becomes. But why?

A 2014 analysis of analysis of age and cancer risk proposed the view that "For most adults, age is coincidentally associated with preventable chronic conditions, avoidable exposures, and modifiable risk behaviors that are causally associated with cancer". This seems a sensible preventative outlook for individuals, since conditions such as diabetes, metabolic syndrome and chronic infections (such as hepatitis C) do increase cancer risk. However that proposal does not address direct causality in a meaningful way. A prevailing view has been that the growing burden of cellular mutation with time is the primary driver of cancer. This has come to be known as the multiple-hit hypothesis whereby accumulated mutations leads to cancer. This idea has been explored using both power-law and Erlang distributions to establish the best way to describe the number of such aberrations in tumours of various types. This hypothesis can be summarised simply: as we age we accumulate DNA damage, and ultimately this causes cancer.

However, recent research posits that the accumulation of mutations is not the whole story and that the bulk of increasing cancer risk with age arises from immunosenescence, i.e. the decreasing function of the immune system with increasing age. When a cell becomes damaged and potentially cancerous, this should be handled by apoptosis (cell death) or cellular senescence (that latter process bringing its own issues). Cells that evade those mechanisms should be prevented from causing harm by the immune system. This latest proposal is that cancers arise from a failure in the immune system to contain cancerous cells, and the researchers used models to demonstrate that their hypothesis offers a better explanation of the available data than the multiple-hit approach. Their model is based around the rate of shrinkage (involution) in the thymus as we age. The thymus is responsible for maturation of immune system T cells, and it begins to shrink as soon as we reach sexual maturity. As the thymus shrinks so does its ability to deliver naive T cells, resulting in a reduced ability to respond to new threats.

We see the impact of immunosenescence in other areas, such as the decreased effectiveness of vaccines and increased susceptability to novel pathogens. Therapies to prevent thymic involution or even regenerate the thymus and related organ systems, could prove important in multiple ways. While certain drastic approaches would likely prove effective they are unlikely to achieve wide-spread acceptance for humans. More realistically, evidence is building that we could add a reduction in immunosenescence to the numerous benefits of exercise. Indeed, a recent study showed highly physically active cyclists aged 55-79 could maintain thymic outputs comparable with healthy 20-36 year-olds. However, the ability to regularly cycle 100km in your sixties and seventies sets a high bar, defining a very select group the wider population are unlikely to become members of. For this reason there is serious work underway on medical rejuvination of the thymus, with a research team in Arizona being allocated a $10 million grant to study what they describe as a "holy grail of aging".