A Rare Replication of a Human Longevity-Associated Gene Variant in Different Study Populations

It isn't often at all that researchers find an association between longevity and genetic variants in humans that holds up in different study populations. This is quite the contrast with shorter-lived species such as mice, where significant effects on longevity via genetic variants are near commonplace. Nonetheless, here I point out a recent paper in which TXNRD1 variants show longevity associations in two European groups - it is a result of interest purely because of its rarity.

There are as of yet no human single gene manipulations that produce effects on longevity anywhere near as impressive as those achieved in laboratory species such as flies, worms, and mice. In one sense this is a part of a larger theme: approaches to altering the operation of metabolism in short-lived species produce sometimes dramatic extension of healthy life, and the shorter the normal life span the larger the gain. None of these have more than modestly beneficial short-term effects in humans, and nor are they expected to do better than adding just a few years to human life expectancy. Calorie restriction is a great example: it can extend life in mice by 40% or so, but certainly doesn't do that in humans. The rationale for this is that shorter-lived species tend to evolve a much greater plasticity of life span because events that might require a postponement of reproduction until later tend to take place over a much greater proportion of their life span. A seasonal famine is a large fraction of a mouse life span, but not so for humans - and hence only the mouse evolves a large extension of healthy life in response to reduced calorie intake.

It isn't just that there is an absence of large effects from human longevity-related genes, however. It is that there is a near absence of any human longevity-related genes backed by defensible data in multiple study populations. Many studies have found small effects and statistically significant associations between a wide variety of genetic variants and human longevity in one study population, but when following up in a different group of people, even in the same part of the world, researchers find that these correlations cannot be replicated. This strongly suggests that the genetic determinants of natural variations in humans longevity and health in later life are very complex, consisting of the interaction of hundreds or thousands of genes, each producing individually tiny effects, varying widely with environmental circumstances, and the whole network of interactions very different for different groups of people. At this point we probably shouldn't expect the study of genetics in aging to be a good path towards enhanced human longevity, and this simply because we're not finding the same sort of results in people, a plethora of defensible associations between specific genes and longevity, that easily fall out of the data in mice.

So all this said, here is one of the rare small effects and genetic associations with longevity that is replicated in different study populations. There are all too few of these beyond the well known APOE and FOXO3A associations. None have large effects. If you have the beneficial variant, you may have a slightly better chance of reaching extreme old age in the environment of today's medical technology - but in absolute terms your odds are still terrible, and something like three quarters of the people with these beneficial variants are still dead by 90. Improved understanding in biology is always a good thing, but this is not the road to rejuvenation and greatly extended healthy life spans:

Antioxidants and Quality of Aging: Further Evidences for a Major Role of TXNRD1 Gene Variability on Physical Performance at Old Age

The role of oxidative stress response in the susceptibility to longevity is a hot topic in aging research. Comparisons among species with different rates of aging suggested that long lived species tend to show reduced oxidative damage, reduced mitochondrial free radicals production, increased antioxidant defenses, and increased resistance to oxidative stress. Indeed, centenarians generally show a lower degree of oxidative stress. However, a direct cause-and-effect relationship between the accumulation of oxidative mediated damage and aging has not been strongly established. The overall cellular oxidative stress during aging is determined not only by ROS generation but also by a reduced defense capacity of antioxidant systems.

The thioredoxin system is a most important antioxidant frontier of the cell, able to regulate its reduction/oxidation (redox) status. Thioredoxin (Trx) plays an essential role in the antioxidant defense, both directly, acting as redox regulator of intra- and extracellular signalling pathways and transcription factors, and indirectly, by protein-protein interactions with key signaling molecules such as thioredoxin-interacting protein (TXNIP). Furthermore, Trx protects the cell against lipid and protein peroxidation by controlling the protein folding through the catalysis of sulfur-exchange reactions among protein complexes. Its endogenous regulator, TrxR1, is a key selenoprotein antioxidant enzyme as well, able to reduce Trx (its main substrate) and other compounds, thus detoxifying cells from oxidative injuries. Highly conserved along the evolution, the system has also a pivotal role in growth promotion, neuroprotection, inflammatory modulation, antiapoptosis, immune function, and atherosclerosis.

The variability of encoding gene (TXNRD1) was previously found associated with physical status at old age and extreme survival in a Danish cohort. To further investigate the influence of the gene variability on age-related physiological decline, we analyzed 9 tagging single nucleotide polymorphisms (SNPs) in relation to markers of physical and cognitive status, in a Southern-Italian cohort of 64-107 aged individuals. We replicated the association of TXNRD1 variability with physical performance, with three variants (rs4445711, rs1128446, and rs11111979) associated with physical functioning after 85 years of age. In addition, we found two SNPs borderline influencing longevity (rs4964728 and rs7310505) in our cohort, the last associated with health status and survival in Northern Europeans too. Overall, the evidences of association in a different population here reported extend the proposed role of TXNRD1 gene in modulating physical decline at extreme ages, further supporting the investigation of thioredoxin pathway in relation to the quality of human aging.

Comments

"Improved understanding in biology is always a good thing, but this is not the road to rejuvenation and greatly extended healthy life spans"

Disagree, one thing we can conclude from the negligible senescence species if we compare with others (still a lot of research is necessary in this aspect), is that small differences in some pathways could lead to huge differences in life/healthspan. From my point of view, knowing better the known pathways involved in aging and the possible drug treatments that could modulate them, can make a difference. Of course, the affected pathways, even if not directly, would involve an effect in autophagy, proteostasis, damage repair... that would lead to the beneficial effect.

I would say that this is a great discovery, and in a not so distant future maybe diets or drugs could increase-induce the thioredoxin system, that would help against oxidative damage. Of course, there are many other issues leading to aging, modulation of other pathways could also solve them. And yes, I agree, the interactome is a mess, but further understanding of the pathways and targeting downstream effectors could reduce secondary effects, we just need to keep digging.

Moreover, in my opinion, last paper by Partridge┬┤s Lab was also of great relevance and it was quite undervalued here. The fact that the drug treatment was leading to a moderate life extension is the less important result of the paper, they are linking the insuling-like signalling pathway with Ras and that could have a big impact in the future aging research.

Every time I read you I feel like what you believe is unquestionable and the rest is crap. I am not criticizing that you express your opinion, but could be more tactful :)

Posted by: Paul Ferdinand at July 2nd, 2015 9:41 AM

"Disagree, one thing we can conclude from the negligible senescence species if we compare with others (still a lot of research is necessary in this aspect), is that small differences in some pathways could lead to huge differences in life/healthspan."

For example?

Posted by: Antonio at July 2nd, 2015 11:17 AM

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