It’s funny how often we make assumptions that are not only wrong, but that we are completely unaware of making. Having spent more than twenty years dealing with the clinical implications of cell aging, telomeres come to mind as an immediate example of this mistake.
Hardly a week goes by without another claim that some particular intervention alters telomere lengths in human patients. Without exception, they are measuring telomeres in peripheral white blood cells. It’s easy to get blood samples and measure telomeres in circulating white cells. Unfortunately, not only are these telomeres the ones that matter least, but (if you’re trying to prove the value of your intervention) they’re almost worthless.
Measuring telomeres in your blood to see how old you are is a bit like looking at your hat size to figure out how tall you are. Whether it’s your peripheral blood telomeres or your hat size, it’s still the wrong measurement for the job.
There are two problems with measuring telomeres in blood cells (even totally ignoring arguments about technical methods, unreliable laboratories, and the mean length versus the shortest lengths of those telomeres).
The first problem is that the blood cells aren’t the key cells when it comes to aging and age-related diseases. If you really want to know where you stand clinically, you should be measuring the telomeres in the endothelial cells lining your coronary arteries, the glial cells in your brain, the chondrocytes in your joints, or several other places more closely related to the most common (and fatal) aging diseases. Few of us are willing to have biopsies taken from our coronary arteries, our brain, or our joints, but just because we are a lot more relaxed about giving a blood sample doesn’t mean that the blood sample is worth getting. It barely reflects what’s going on in your white cells, let alone what is going to end up causing disease and death.
The second problem is a more subtle, but more important. It boils down to this: most of your white cells aren’t circulating in your blood and the ones that do circulate are changing and dividing all the time, making them a poor reflection of what’s happening to the stem cells in your marrow. I wrote an academic review article about this in 2012 and discussed it in The Telomerase Revolution, but let’s look at it here. Imagine you can instantly and accurately measure every telomere in the body, including those in the bone marrow and peripheral venous circulation. Oddly enough, you’d discover that the blood tests aren’t reliable indicators of what’s happening in the marrow.
Let’s say that you measure all of the telomeres at time A and again at time B. In between A and B, you use an intervention such as gene therapy, TA65, mediation, dietary change, or whatever you think might be effective. At time A, you find that the telomeres in the hematopoietic cells of the marrow are 12 kbp long. At the same time (due to stress, infection, poor diet, inflammation, and generally poor health habits) there is rapid peripheral turnover, cell division, and telomere loss in the peripheral blood. As a result, the mean telomere length in the blood sample is only 8 kbp.
We then intervene.
At time B, you find that the telomeres in the hematopoietic stem cells in the marrow are now only 11 kbp long (showing that the patient has gotten older). Also at time B, since we might now have lowered stress, removed infections, decreased inflammation, and generally made the patient “healthier” with whatever intervention we may have chosen, their peripheral cells are now turning over more slowly, dividing less frequently, and losing less telomere lengths once they leave the marrow and enter peripheral circulation, so that the mean telomere length in the peripheral blood sample is now 9 kbp.
We could claim (as many articles do) that our clinical intervention “lengthened the peripheral telomeres!” The truth is that our intervention didn’t lengthen anything and we’re deluding ourselves (and whoever believes our claims). The peripheral telomeres that we sample at time B might be longer than the ones we sampled at time A, but the telomeres of the cells back in the marrow now have shorter telomeres. Our intervention may well have made the patient healthier and we might actually have slowed down the rate of telomere loss, but we definitely didn’t lengthen any telomeres, no matter how proudly we pat ourselves on the back.
Peripheral leukocytes are routinely used to assess telomere lengths (which is fine as far as it goes) and then used to assess clinical interventions, which is overreaching. If we do serial measures of peripheral telomeres every few months for a few years, then the validity will increase somewhat, but peripheral telomere measurements (no matter how often you measure them) are intrinsically an unreliable and invalid biomarker for what we really want to assess, which is “whole body telomere changes” or at least “marrow telomere changes” (in the case of blood cells).
Most of the available literature which suggests that we can slow or reverse telomere losses is – if it’s based on peripheral blood samples – misleading at best and unethical at worst.