Mutations in the gonadotropin subunit genes appear to be extremely rare, perhaps because of their deleterious effects on reproduction (see Table 1; 26-39). Probably for the same reason, not a single patient with a germline mutation in the common a-sub-unit gene has been noted. Such mutations would lead to defective hCG, LH, FSH, and TSH function, a condition that would probably be lethal, particularly because of lack of hCG. Nevertheless, a-subunit Knockout mice are viable, though hypogondal and hypothyroid (26), but because this species does not produce chorionic gonadotropin, the gestation regulation may be under differential hormonal control. Only one somatic a-subunit point mutation (Glu56Ala) in the ectopically produced hCG of a carcinoma has been described in the literature (see Fig. 1) (26). Likewise, no hCGP mutations have been described. Besides several silent polymorphism in this gene, only one relatively common (4.5% allelic frequency) amino acid-altering point mutation, Val79Met, has been detected in heterozygous form (see Fig. 1) (33). This mutation hampers a/p dimer formation in vitro and, if present in homozygous form, could block sufficient bioactive hCG synthesis and be lethal. Milder forms of hCG mutations could, if they exist, cause intrauterine undermasculinization of male fetuses because of hCG's crucial role in the stimulation of fetal testicular testosterone synthesis.
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