Lyonization vs Genetic Imprinting

Question

Lyonization is the process in which there is inactivation of an X chromosome in females. This process is implicated in mosaic forms of turner's syndrome (in this case the altered X chromosome is Lyonised, thus allowing the normal X Chromosome to express itself).

1. Is Lyonization considered as a form of genetic imprinting? What is the difference between the two (if no)?

2. Lyonization is a random process (according to the extent of my knowledge which X chromosome is inactivated is not predicatable). Is genetic imprinting also random?

3. Is there any known aberration of Lyonization in males? (meaning the single X Chromosome or Y Chromosome gets silenced)

Answer 1

1. I use the term "X-inactivation" instead of "lyonization". Anyway, X-inactivation is a very specific process by which an entire X chromosome (or equivalent sex chromosome) in a female mammal is completely silenced.

"Imprinting" is typically applied to single genes, wherein genes inherited from a specific parent are always epigenetically silenced. The genes are usually autosomal. Thus, we're talking about scales : entire sex chromosome composed of thousands of genes (x-inactivation) vs single genes (imprinting)

2. The choice for which X-chromosome is inactivated is sporadic and different in different cells within the body. It is controlled by opposing expression of different non-coding RNAs (XIST on the inactive X and TSIX on the active X).

Imprinting is very specific to the allele that is inherited from a specific parent (for example, some genes are paternally imprinted, while others can be maternally imprinted).

3. Males with aberrant X-inactivation will lack any expression of genes carried on their single X-chromosome, a condition that will not result in a viable fetus. However, whether there are aberrant X-inactivations in single cells or tissues of ageing adult males....perhaps likely yes, given all the other aberrant somatic mutation events that occur over the course of life.

Answer 2

I guess, we are talking about mammals here (because in other organisms leveling up of expression on sex chromosomes happens differently: e.g. in flies females express both X chromosomes, while fly males express their single X at twice as high level).

In embryo proper the decision of whether to inactivate maternal or paternal X seems to be at random. This inactivation adjusts level of expression of genes on X to be in the same ball park as on autosomes and in the same ballpark between males and females. Neither farther nor mother cares about which copy of X gets inactivated in individual cells of an embryo.

When we say that a gene is imprinted, that means only one copy of the gene is expressed but, to the contrary of what happens with X, maternal or paternal origin of the expressed copy is predetermined (so there is no randomness in which copy is inactive). Imprinting is usually a property of genes expressed during embryonic development and it is related to tug of war between farther and mother's interests: the farther "wants" embryo to take as much resources from the mother as possible (so paternal copies of some genes tend to be highly expressed), while mother wants not to give too much to one embryo, so that she can raise many more progeny (so maternal copies of some other genes tend to be highly expressed). This whole thing is interestingly described Matt Ridley book The Red Queen: Sex and the Evolution of Human Nature.

Note, that to confuse matters a little bit, in placenta of female embryos it is always the paternally inherted X that gets silenced, so the whole X chromosome is imprinted in extra-embryonic tissues of female embryos.

And in males X chromosomes do not get inactivated, since many genes are present solely on X chromosome and not on Y, so inactivating the only copy of X chromosome is lethal.