Immunoglobulin concentration and Gm allotypes in a family with thirty‐three cases of myotonic dystrophy

Serum IgG, IgA, and IgM concentrations were measured in 120 members of a family with 33 cases of Dystrophia myotonica (Dm) and 27 members who were “possibly affected”. The Dm individuals had significantly lower serum concentrations of IgG and IgA (P<0.01), while the “possibly affected” did not differ from the matched pair controls. IgG subclass concentrations were measured and Gm and Am types determined. The lower concentration of IgA in the affected individuals was not associated with a particular Am type. The concentration of IgG3 was barely lower in the affected than in the controls (P=0.05), but there were no differences for IgGl. When IgG3 concentration was compared according to Gm haplotype, only the two affected individuals who were Gm gg had a statistically significant lower concentration than the 12 controls (P<0.02). Thus, there is no evidence that a particular subclass of IgG is being hypercatabolized in our Dm patients. A rare Gm haplotype, Gm(-, n, b) had entered the family with two brothers; it is not known whether this codes for an IgGl-IgG3 hybrid molecule or a normal IgGl molecule with an unknown Gm allele or a yl deleted Gm haplotype.