There is new evidence linking left- or right-handedness to microtubule formation in human cells. Microtubules are protein threads that form the ‘framework’ of a cell. These filaments are also involved in cell division and the transport of substances through the cell.
A Nijmegen team of brain researchers led by Clyde Francks (Max Planck Institute Nijmegen) has now discovered that certain rare variants in a gene for microtubules (TUBB4B) occur much more often in left-handed people than in right-handed people. Some variants even only occur in left-handed people. The research was conducted with the help of the UK Biobank, with the DNA data of almost 40,000 left-handed people and more than 310,000 right-handed people. It was published on Tuesday in Nature Communications.
The gene variants now found do not play a major role in the heredity of left-handedness. In an explanation by e-mail, Francks writes that the variants found only occur in one in a thousand left-handed people. These conclusions do confirm that there is a connection with the microtubules, and that is what matters. These connections with microtubules also appeared in previous genetic research into left-handedness. The largest genetic study to date (including 200,000 left-handed and 1.1 million right-handed people) found that at least eight of the 41 genes relevant to left-handedness were related to microtubules. This outcome is also important for research into other differences between the hemispheres of the brain. “The different development of the cerebral hemispheres starts early in embryonic development, chance plays a major role in this, but the mechanism behind it is still unknown,” says Francks.
Crossed nerves
Hand preference is the best-known example of brain asymmetry. In left-handed people, hand control is dominated by the right hemisphere of the brain, while in right-handed people it is the left hemisphere. (That reversal occurs because the nerves are crossed between the brain and the rest of the body, except for smell). Around the world, about 10 percent of people are left-handed, and there are not many differences per culture (at least if things like forced right-hand writing, which was also common in the Netherlands for a long time, are taken into account). There is a connection with autism, Parkinson’s and schizophrenia, for example, which is slightly more common in left-handed people. Most left-handed people are perfectly healthy, but if there were no disadvantages to being left-handed, you would expect half the population to be left-handed, a researcher recently wrote in a major review. Francks says: “In certain sports – such as tennis – it can be an advantage, because the opponent is used to right-handed opponents. There are stories that left-handed people are less or more creative or skilled than right-handed people, but I have never seen any real evidence for that.”
The difference in hand preference is not very hereditary. Twin research shows that approximately 25 percent of the difference in left- and right-handedness must have a genetic cause. Differences in right-left development in humans arise in the early embryo. Already at ten weeks, prenatal ultrasound scans show whether a baby prefers to move its left or right arm. Francks: “Right-handedness is the standard development, which is genetically controlled. We think that most left-handed people owe their hand preference to random variations during embryonic development, due to random fluctuations of certain substances in certain phases of brain development.”
Threaded organelles
It is still unclear how microtubules, those ‘framework proteins’ of the cell, influence right-left development, but their influence on the asymmetry within a cell may play a role. This left-right division in a cell can in turn influence the formation of the tissue of which the cell is a part. The fact that the microtubules in the cell are connected to cilia may also play a role. These are filamentous organelles on the cell surface that can set the fluid outside the cell in motion, possibly affecting the asymmetry during the growth of the embryo.
It is not the case that left-handedness is irrevocably related to all kinds of other symmetry differences in the body, although there is much more variation in left-right distribution in the brain among left-handed people than among right-handed people. For example, more often than in right-handed people, the language center of left-handed people is not in the left half, but in the right half, but this is certainly not standard.
Correction (April 3, 2024) Initially, this article reported that the nerves were not crossed when hearing only. That is incorrect: only with smell is information from the right nostril processed in the right hemisphere of the brain. This has been improved.
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