improbable molecular results and the need to reevaluate a priori genetic risks: A report of 2 neuromuscular disorders

Case 1: A 27-year-old woman whose brother and deceased father were diagnosed with myotonic dystrophy presented for preconceptional counseling. Three other first and second degree relatives were judged clinically affected on the basis of myotonia by positive electromyelograms (EMGs). The proband appeared to be clinically affected with mild percussion myotonia and a positive EMG. Initial studies by linkage analysis required postulation that the at-risk chromosome 19 inherited by the patient and her brother had undergone recombination in the brother between 2 very closely linked RFLP markers. Conclusion: Molecular testing did not support a unifying diagnosis for myotonia in this family. In addition to one classic example of myotonic dystrophy, it is possible a milder form of myotonia is also segregating in this pedigree. Case 2: The maternal aunt of a young man with Becker`s muscular dystrophy (BMD) presented for reproductive counseling. Serum CK values were elevated in her mother, who shared an X chromosome haplotype with the affected grandson and was judged an obligate carrier. The aunt was judged a likely carrier because of elevated serum CK values. Three maternal uncles, including ones with the same X chromosome haplotype as the affected nephew, were clinically normal. Finally, dosage analysis formore » dystrophin exons deleted in the grandson showed the grandmother and aunt to be non-carriers, but the mother of the affected boy to be a somatic carrier. The grandmother and aunt are not BMD carriers. Since the elevated CK levels do not segregate with the affected X chromosome, they may reflect autosomal inheritance or nongenetic influences; in either case their clinical significance is uncertain. These cases reinforce the importance of periodic reevaluation of initial genetic assignments, particularly when rare events must be invoked to explain molecular studies.« less