Having errors in your genes isn't all that unusual. Our genetic counselor told us that everyone has errors in their genome, typically around 20. Genetic errors aren't usually problematic because you have two copies of each gene and most of the time, at least one of those copies is fine. The body uses the "good" copy to do what it needs to do. Unfortunately, BOTH of LB's genes that tell his body how to make MCAD either aren't there or are broken, so his body honestly doesn't know how to make the MCAD enzyme. It's kind of like his body's recipe book has that page ripped out or somebody spilled something all over it and it's illegible. As much as he may want to make that enzyme, he just doesn't know how.
How did he end up with two bad copies of the same gene? It's his parents' fault!! Husband and I just happen to have the same errors in our genetic code and we both passed the "bad" copy of the MCAD gene onto LB. It's likely that Husband and I are both just carriers of the condition. We have one good copy of the MCAD gene and one bad copy, but our bodies can use the good copy to make the enzyme and we are fine. Unfortunately, between the two of us, we didn't give LB a single good copy to work with. With both of us being carriers, the chances of us having a baby with MCADD are 1 in 4. The process looks something like this...
We know through newborn screening that LB has MCADD. That is, he has two bad copies of the gene. We now know that both Husband and I are at least carriers of the condition (Rr) since we produced offspring with one bad copy from each of us (rr). If we are both carriers, then there is a chance that our older son, LM, may also have inherited a copy of our bad gene. He could also be a carrier (Rr) and there is a 50% chance of that. The best case scenario would be that he inherited only our good gene (RR) and there is a 25% chance of that. Since the state was screening for MCADD two years ago when he was born, it is unlikely that the LM has MCADD, but they suggested we re-test him just to be sure -- his sample card may have been mishandled or the wrong test done on the wrong blood spot. Again, it is unlikely that he has it, but we're going to go for "better safe than sorry."
Our genetic counselor told us that in some situations, infant screening has identified the condition in a parent (since testing for it has been only relatively recent). With MCADD, the parent happens to be lucky enough to not have ever been very ill to where their life was in danger due to fasting, but knowing they have the condition going forward can be very helpful for future illnesses, surgeries, etc. Knowing that there is also a possibility that one or both of us could have MCADD (although unlikely), we are both going to be tested to be sure. If one of us ends up testing positive for MCADD, our genetic situation changes slightly...
If one of us were affected with MCADD, that person could only pass a bad copy of the gene onto our child. The unaffected parent could pass on either a good copy or a bad copy. Thus, this would give us a 50-50 chance of having a child with MCADD and all of our children would at least be carriers of the condition.
I doubt either of us has MCADD, but it's also so statistically rare that two people with the same genetic errors have children together, so I guess anything is possible. I'm still really thankful for newborn screening, though. Without it, we would never have known LB had this condition and we certainly wouldn't have know that we were carriers of it either.
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