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2. How is FSHD Inherited?
We now know that there are two forms of FSHD, FSHD type 1 (FSHD1) and FSHD type 2 (FSHD2). Both types are clinically identical as far as how they affect muscle function but they are genetically different (see section 8 below).
About 95% of individuals with symptoms suggestive of FSHD have FSHD1. This form of FSHD is dominantly inherited. This means that inheriting one defective copy of a segment of DNA from a parent with FSHD is sufficient to cause disease. The genetic defect in FSHD occurs on one end of chromosome number 4. Each parent carries two copies of their DNA in each cell but pass on a single copy to their offspring. A parent with FSHD carries the FSHD genetic defect on one of two copies of chromosome 4 as illustrated in red in the diagram below. Therefore an affected parent has a 50% chance of passing on the defective copy to their child as illustrated in the diagram below. There are exceptions, however, to this pattern of inheritance. In up to 30% of cases of individuals with FSHD, no signs of FSHD are detected in either parent and genetic testing shows no defect on chromosome 4 in either. Such a situation, known as sporadic FSHD, means that the FSHD genetic defect occurred spontaneously in that individual. Such individuals can, in turn, pass this defect to their offspring.
About 5% of individuals with symptoms of FSHD have FSHD2. The inheritance pattern in FSHD2 is more complicated to predict because it is the result of two separate genetic changes one on chromosome 4 and the other, in most patients with FSHD2 on chromosome 18. Depending on the genetic makeup of the unaffected spouse, the chances of someone with FSHD2 passing the disease on to their children varies from 25-50%.
For more information, please contact the Fields Center FieldsCenter@urmc.rochester.edu