X chromosome inactivation

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X chromosome inactivation

Postby dignan » Thu Nov 01, 2007 9:11 am

Another angle to try and figure out the gender difference in MS incidence.



Eur J Neurol. 2007 Oct 26

X chromosome inactivation in females with multiple sclerosis.

Knudsen GP, Harbo HF, Smestad C, Celius EG, Akesson E, Oturai A, Ryder LP, Spurkland A, Orstavik KH.
Department of Medical Genetics, Faculty Division Rikshospitalet, University of Oslo, Oslo, Norway.

The aetiology of multiple sclerosis (MS) is unknown. Autoimmune mechanisms are most probably involved. Loss of immunological tolerance to self-antigens is a common feature of autoimmune disorders. Response to X-linked self-antigens could be influenced by X-chromosome inactivation, and contribute to the gender bias observed in autoimmune disorders. Previous studies have indicated an association between skewed X inactivation and autoimmune thyroid disease and scleroderma. To investigate a potential role of X inactivation in MS, we compared the X-inactivation pattern in 568 female MS patients with controls. We found no difference in degree of skewing between patients (median 64%) and controls (median 65%) (P = 0.474). The X-inactivation pattern did thus not explain the female predominance of MS patients in general. As the aetiology of different subgroups of MS may differ, patients were grouped according to disease course: relapsing-remitting (RR-MS), secondary progressive (SP-MS) and primary progressive (PP-MS). A comparison of the X-inactivation pattern between subgroups indicated a possible difference in degree of skewing between patients with a progressive versus a relapsing course (P = 0.05).

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Postby dignan » Sat May 02, 2009 5:41 pm

Here is an abstract about X chromosome inactivation from this year's AAN meeting:


Challenging the Role of Chromosome X Inactivation in Multiple Sclerosis Pathogenesis

Nicolas Couturier, Pierre-Antoine Gouraud, Pierre Antoinne Gourraud, Céline Rabadeux, Laurie Parmentier, Florence Bucciarelli, Toulouse, France, Isabelle Rebeix, Claire Gout, Jussieu, France, Renato Colamarino, Vichy, France, Florent Borgel, Genoble, France, Serge Mrejen, Paris, France, Philippe Cabre, Martinique, France, Christine Lebrun Frenay, Nice, France, Jacqueline Yaouanq, Rennes, France, Patrick Hautecoeur, Lilles, France, Marc Debouverie, Nancy, France, Gilles Edan, Rennes, France, Michel Clanet, Toulouse, France, Bertrand Fontaine, Paris, France, David Brassat, Toulouse, France

OBJECTIVE: To determine if X chromosome inactivation (XCI) is different between female monozygotic twins discordant for MS.

BACKGROUND: XCI is a phenomenon that occurs in female mammals, whereby one of the two X-chromosomes is randomly inactivated in early embryonic life. Thus females are mosaics for two cell lines that differ in the active X-chromosome. Typically, maternally- and paternally-derived X chromosomes are inactivated at the same frequency and females usually exhibit a random 50:50 ratio of the two cell lines. A skewed X-chromosome inactivation (XCI) is a deviation from this ratio and is arbitrarily defined as a pattern where 80% or more of the cells inactivate the same X-chromosome. Skewed XCI has been reported to occur more frequently in women with scleroderma and autoimmune thyroid disease compared to women without autoimmune diseases.

DESIGN/METHODS: XCI was tested by enzymatic predigestion of genomic DNA with a methylation-sensitive enzyme (Hha1) followed by PCR of the polymorphic CAG repeat of the androgen receptor (AR) gene. This site is methylated on the inactive X-chromosome, so this allele would be amplified by PCR. The active X-chromosome would not be amplified because the sequence is cleaved by Hha1. The relative amounts of the methylated/unmethylated AR alleles were quantitatively determined by migration of the PCR products on a capillary sequencer. We investigated and compared the XCI pattern in DNA from blood cells and epithelial cells from MS patients and from control subjects. This approach allowed us to assess whether MS patients and non-MS persons shared the same degree of XCI.

RESULTS: We found a different XCI intrapair correlation in a population of 20 female monozigotic twin pairs (FMTP) discordant for MS than in 10 FMPT concordant for MS and 30 FMTP with no disease.

CONCLUSIONS/RELEVANCE: We suggest that X-chromosome inactivation may participate in MS pathogenesis in females. Supported by: ARSEP

http://www.abstracts2view.com/aan2009se ... 9L_P08.030
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Postby BioDocFL » Sun May 03, 2009 7:28 am

Dignan,

I was asked by a journal to write a review of X inactivation and autoimmunity. It should be coming out soon. What one finds in the literature with regards to skewing is that there are a few autoimmune diseases in which skewing appears to be significant but for most diseases, it is still inconclusive. The group that wrote the first article you posted did find some significance of skewing relative to the severity of MS, but not between MSers and normal controls. We discussed that on ThisIsMS last year when that paper came out. It may not be the initial choice in cells whether to inactivate the paternally- or maternally-derived X chromosome that gives a skewed pattern in the progressive MSer. The choice may have been random in the embryo and gave a roughly 50/50 split between inactivating the paternal or maternal X. Rather, it may be that over time cells with one parentally-derived X die off at a faster rate for some reason. This then leads to a skewed pattern in the progressive MSer. The skewing develops later in life rather than in the embryo. The question I have on these skewing studies is: which cells should they be looking at? Often researchers simply look at blood cells. But skewing can vary from one tissue to another in the same individual.

My feeling is that skewing is not really the issue or is only part. I think the X chromosome is involved but it has very complicated epigenetic controls in the X inactivation process. These can get messed up and allow for reactivation of some genes throwing off expression levels.

Anyway, in writing the article it got me to thinking about my old theory that we discussed 4 years ago about X-linked genes and polyamines. I put a few sentences in the article about it so we'll see if there is any further interest when the article gets published. In the meantime, I am going to start writing another article on another area that I think has not been properly understood. From my thinking and reading, it is an area that has a lot of unexplored possibilities for understanding autoimmune diseases. I have to get a couple of cancer-related grants written before I can get to writing my next article but I am really anxious to begin.

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Postby dignan » Sun May 03, 2009 10:09 am

Wesley, thanks for that information. I was hoping you would chime in on this. So it's far too early to say there is definitive evidence that X inactivation is playing a key role in MS?
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Postby BioDocFL » Sun May 03, 2009 10:55 am

My feeling is that the X chromosome is involved. The X inactivation process is part of what goes on with the X chromosome but male's don't do X inactivation, yet they can get MS. They do have one X. So there has to be more to the X involvement in MS than just problems or biases in X inactivation.

Most genes on the X are not sex specific so you would expect males and females to have the same overall expression in cells for those non-sex specific genes. Since females have two X chromosomes, they have to inactivate one to achieve the expression equal to males. And so this is where the discussion begins about skewing, which X (father's or mother's) do they choose to inactivate in each cell?

The X, and especially the inactivated X, has a lot of epigenetic markers established to control the gene expression. The X also has a lot of ways this can go wrong, like fragile sites that can break. And so that needs to be explored more with regards to autoimmune diseases. And it is not just immune system cells (T cells or B cells) that need to be studied. Non-immune cells should be considered as to the autoimmune consequences that could arise when they lose X-related epigenetic control.

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Postby BioDocFL » Sun May 03, 2009 3:26 pm

I thought I should add that, with regards to skewing, it's not necessarily that the cells with one parentally-derived X inactivated die off quicker than the other. They may well survive. It may be that one parentally-derived X can maintain the X inactivation state better than the other X. The one that is not as good at maintaining it, may wind up with two active X chromosomes and begin overexpressing some genes. In the article I wrote (in press now), I explain one way in which the X inactivation state can be easily lost.

So when you think of X inactivation skewing, you might want to picture several categories of cells in the analysis:
1) Cells with the maternally-derived X inactivated, expected to be 50%;
2) Cells with the paternally-derived X inactivatied, expected to be 50%;
3) Cells that have lost X inactivation and have more than one active X,
expected to be near 0%;
4) Cells that have lost X inactivation and only have one complete X left,
which is active, the other X is fragmented or was degraded by
nucleases due to damage;
5) Cells that have died and therefore can not be counted.

The first two types are what we are expecting to see and count. The other types would be difficult or impossible to pick out of the data since the methods used (PCR on some genes that are methylated or not) would have difficulty getting the sensitivity for differentiating those types. The dead cells would be gone and not contribute to the data and we don't know how significant cell loss is when loss of XCI occurs. The cells that have more than one X active may be exaggerating the signal but I think there are ways to figure that out, possibly using cell counts and quantitative PCR. It's been a long time since I did any PCR work so I would have to go back and brush up on the different protocols.
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