The heart is a really fascinating organ. Functionally, it’s pretty simple — a mechanical pump responsible for pushing blood around your body. But this simplistic pump is also responsible for keeping us alive, and when our heart fails, we die. In fact, heart disease is, and has remained, the number one cause of death in the US since the 1930s, and heart attacks kill over 500,000 people a year in the US. However, when considering the total scope of heart disease and how it affects us, you never hear the term “heart cancer”. Ever wonder why your heart doesn’t get cancer? If so, read on!
The link between organ damage and repair
Broadly, if tissues or cells in your body are damaged, they need to be replaced. If you read my previous post, “Stem cells: a primer”, you’ll recall that usually a stem cell or progenitor population is responsible for replacing this dysfunctional or damaged tissue. Different organ systems and tissues have specific stem cells then which are responsible for this repair. For instance, your digestive system cells only live between 2–7 days before being replenished by a progenitor or stem cell population. For your digestive system, these cells are exposed to a harsh environment, like stomach acids and enzymes, that necessitates a constant replenishment of the cells as they become damaged quickly. Similarly, your white blood cells turn over relatively quickly as they patrol your body and fight off infections and are similarly replenished by progenitor cells. It’s reasonable to assume then, that regeneration is linked to the frequency of damage a tissue receives; cells that are constantly exposed to harsh, damaging environments tend to die quickly and need to be replenished. So tissues and organs that are more likely to be damaged generally have pretty active stem cell populations.
A link between regeneration and cancer
I’m not a cancer scientist (I’ll look to expand upon this with a more qualified scientist later) but in general, there is thought to be a link to a tissue’s ability to regenerate and also ability to form cancers or tumors. Each time a cell divides, it splits into two cells. Since there are two cells, a cell needs to copy its DNA. And our genome is pretty massive; we have roughly 3 billion base pairs, which are the information-storing portion of DNA. While our cells have incredibly good error-checking mechanisms to prevent mutations while copying DNA, even an incredibly high accuracy would still lead to an accumulation of mutations just due to the sheer number of cells and cell divisions through the course of our life. Putting that into numbers, our DNA “spellcheck” accuracy is thought to be around ~1/10¹⁰ mutations per base pair per cell division. With 6x10⁹ base pairs (twice the 3 billion, because our DNA is double stranded), that means a normal cell division would not even likely have a mutation, however after two divisions, a mutation would have likely occurred. We have ~37 trillion cells in our body, and some of them divide quite actively so you can imagine mutations do become a problem. There’s a lot of failsafes our bodies have to fight against cancer, but in general, the more a cell divides, the more likely it is to accumulate a mutation, and mutations in specific genes are thought to be the driving force of cancers. So, in general, the more a cell or tissue divides, the greater risk it has of developing cancer.
Does the heart regenerate itself?
So we’ve established that cells and tissues in harsher environments need to replenish themselves. But, conversely, when cells and tissues divide more, that makes them more susceptible to cancer. So what about the heart? Well, the heart, relatively speaking, is not exposed to much more than blood. Blood can carry viruses and bacteria, and exposure to these can actually lead to a heart infection, but this is relatively rare. So for the most part, our hearts are pumping away nonstop, safe deep in our bodies away from enzymes, acids, toxins, and environmental damage like UV rays. Instead of actively dividing, the heart focuses on actively pumping, and regeneration or division of the heart cells is incredible limited. The heart does get exposed to damage from the contraction process, so a little bit of cell replenishment is necessary, but how little is astonishing: a really cool study actually determined that only 50% of our heart cells are ever replaced over the course of our entire lives, meaning that you’re born already with half the heart cells you’ll die with. As a result, heart cancer or tumors from heart cells is incredible rare to the point where there has yet to be a confirmed case of heart cancer arising from heart cells; instead, the only tumors and cancers found the heart (still incredible rare) are from the supporting cells in the heart, or cancers from other parts of the body that have spread to the heart.
Your heart’s lack of regeneration causes it to fail
But as a result, the heart also lacks the ability to repair itself sufficiently after a more serious injury. Again, in general, the heart is not exposed to very much damage or trauma in daily life, so this is usually a non-issue. However, over time, a lot of this damage can accumulate and cause a myriad of problems to your heart. Some issues include high blood pressure and thickening of your heart muscle, “fibrosis” or scarring of your heart tissue just due to normal wear and tear, and “ischemia” or reduction of blood flow due to arteries narrowing/closing up due to cholesterol plaque accumulation. If the heart loses blood supply, since it’s so working so hard and requires tons of oxygen and nutrients it dies quickly. A well-known example of this is a “myocardial infarction”, or heart attack — this happens when an artery supplying blood to the heart becomes blocked and the blood supply to a portion of the heart stops or is dramatically reduced. This, in turn, causes massive cell death in the areas of the heart affected. And since the heart does not have great regenerative capabilities, when we have heart attacks, our hearts are not able to repair themselves adequately — which causes a number of downstream complications even if we survive the initial heart attack, like electrical problems and reduction of contractile function.
There’s a complex system of regeneration in your body. For tissues and cells that are damaged constantly, it makes sense to replenish the tissues and regenerate those cells more quickly. But for other systems, like your heart, it is more important to maintain constant pumping function since it’s so critical to survival. So for these systems, the regeneration is incredibly limited; only 50% of your heart cells are ever replaced. The upside? No heart cancer. The downside? No physiological repair for serious heart damage.