Understanding Skewed X-Inactivation: Mechanisms and Clinical Impact

Normal X-inactivation occurs in a random manner within female embryos.
The resulting distribution is typically a $50/50$ split between cell populations.
- Approximately half of the cells in a female's body will inhabit an active paternally derived X chromosome.
- Approximately half of the cells will inhabit an active maternally derived X chromosome.
This random process ensures that females are mosaics, expressing genes from both parental X chromosomes across different tissues.

Skewed X-inactivation refers to a situation where the random distribution is disrupted.
Instead of a $50/50$ split, the inactivation pattern is heavily biased toward one chromosome.
In extreme cases, a female may have $100\%$ skewed X-inactivation, where the same X chromosome is inactivated in every single cell.
This state is heritable through future mitotic events; once the X-inactivation pattern is established in a progenitor cell, all daughter cells resulting from mitosis will maintain the same active and inactive X chromosomes.
The transcript notes that while the examples often show the maternal chromosome being inactivated, the skewing could just as easily affect the paternal chromosome.

In a normal scenario involving a recessive mutation on one X chromosome, a female is protected by her mosaicism. Half of her cells express the wild-type (normal) version of the gene from the other X chromosome, preventing the disease phenotype.
If skewed X-inactivation occurs and the active X chromosome in every cell is the one carrying a mutation, the protective effect of the wild-type X is lost.
Under these conditions, a female will express the mutation in all of her cells.
This leads to the female developing a phenotype that is exactly the same as an affected male with that specific condition.
This lack of protective effect from random X inactivation explains why some carrier females manifest symptoms of X-linked recessive disorders.

Skewed X-inactivation can occur purely by chance without underlying genetic flaws or selective pressures.
X-inactivation is established very early in embryonic development, specifically at the $16$ to $64$ cell stage.
If the inactivation event happens at a very early point (e.g., the $16$ cell stage), it is statistically possible—though rare—for every cell to coincidentally inactivate the same X chromosome.
This random occurrence results in the individual having a $100\%$ skewed X-inactivation pattern from the outset.

Skewing can be caused by mutations in the genes responsible for the X-inactivation process itself.
The $Xist$ gene is the primary driver of X-inactivation.
If there is a mutation in the $Xist$ gene that prevents it from being switched on (activated) on a particular chromosome, the process cannot proceed normally.
This results in the X-inactivation being set up in a skewed manner in the very first instance, leading to the same X chromosome being expressed in every cell.

In this scenario, X-inactivation is initially established in a normal, random fashion.
Skewing occurs subsequently because one X chromosome carries a mutation that negatively impacts cell growth or survival.
The cells are initially a mix of those expressing the mutation and those expressing the wild-type version.
Selection Process:
- Cells inactivating the chromosome with the mutation (thereby expressing the wild-type version) have a growth advantage.
- Cells expressing the mutation (indicated in the transcript as "pink cells") are at a disadvantage and are outcompeted.
- After several rounds of mitotic division, the cells expressing the mutation are selected against.
- The cells expressing the wild-type version are selected for and eventually dominate the population.
Consequently, the individual ends up with a skewed profile, where the majority or all of the remaining cells express only the wild-type or non-mutated X chromosome.