Polymorphism of gonadotropin action: clinical implications

It has recently became apparent that the structural heterogeneity of gonadotropin molecules can contribute to variations of their action in different physiological and pathophysiological conditions. One reason for the structural variations of circulating gonadotropin molecules is the microheterogeneity caused by the variability of glycosylation of individual gonadotropin molecules. The carbohydrate moieties of gonadotropins are important for their intrinsic bioactivity, as reflected by measurement of their bioactivity to immunoreactivity (B/I) ratios. We have reassessed this phenomenon by improved in vitro bioassay and immunoassay methods, and it appears that the intrinsic bioactivity of gonadotropins, in particular of LH, is more constant than previously assumed. Many of the previously documented differences, some even considered diagnostic for certain clinical conditions, have turned out to be methodological artifacts. The first part of this review summarizes our recent findings on the B/I ratios of LH, with special reference to the male. The second part of this review describes a common polymorphism that was recently discovered in the gene of the LH beta-subunit. The variant LH beta allele contains two point mutations, which introduce to LH two amino acid changes and an extra glycosylation site. The LH variant is common world-wide, with carrier frequency varying from 0 to 52% in various ethnic groups. The LH variant differs functionally from wild-type LH, and it seems to predispose its carriers, both men and women, to mild aberrations of reproductive function. It is important for the clinician to be aware of this variant LH form, not detected by all immunoassays, because it may explain some aberrant results of LH measurements in patient samples.