This is a topic that I have been somewhat reluctant to write about for couple of reasons. First, since this is something that directly affects women, perhaps a woman’s perspective would be more appropriate. Second, while there is some interesting information that has emerged on this topic, most of it is fairly tentative at this point, and there are few well-established facts. Nevertheless, there have been a couple of recent developments that make me feel that this might be a good time to summarize some of the information that is available. First, because some women in affected families have been requesting this information. Second, because a recent research article has helped shed a little more light on the topic. So, I will try to summarize some of the available information here.
Who are carriers?
In most cases, the mothers of BOYS with MECP2 Duplication Syndrome also share exactly the same duplication on one of their two X chromosomes. This is not usually the case for most girls with MECP2 Duplication Syndrome. Since the mothers who carry this duplication do not have the major symptoms, we do not say that have the syndrome, but because they can pass the duplication on to their offspring, we refer to them as carriers.
Of course, some women who are carriers do not have sons with the syndrome. If a carrier has a son, there is only about a 50% chance that he will have this syndrome. Some would argue that the chances are even a bit less than 50% for reasons explained later. In the vast majority of cases, daughters of carriers will not have the syndrome, although there can be rare exceptions, however, there is 50% chance that daughters will also get the duplication and be carriers.
Why don’t carriers have major symptoms of the syndrome?
Because females have two X chromosomes and always inactivate one in each cell, they can protect themselves by selectively inactivating the X with an extra copy of MECP2. In most cases, some process allows the body to detect the X chromosomes with the extra MECP2 and selectively inactivate it. This is sometimes referred to skewing, and in most cases females carriers are able to selectively inactivate nearly 100% of the Xs with the duplication.
Can men be carriers?
Men cannot be carriers in this sense. Since men only have one X chromosome, they can not inactivate it. If they have the duplication, they will have the syndrome. To the bast of my knowledge, no male with the syndrome has fathered children, so they do not pass it on. If a man who had the syndrome fathered children, boys would presumably be unaffected, because men do not pass an X chromosme to their sons (the exception might be if the duplication was translocated to another chromosome. If they fathered a girl, she presumably would be a carrier, since she would get the affected X from her father.
This is Part 1 of a series. . Part 2: “Do Carriers have symptoms” will follow