Kallmann syndrome caused by mutations in the PROK2 and PROKR2 genes: pathophysiology and genotype-phenotype...

Mutations in the prokineticin 2 peptide (PROK2) and its seven-transmembrane domain type 2 receptor PROKR2 are newly identified molecular culprits in autosomal Kallmann syndrome (KS). Prok2 and prokr2 gene knockout mice both have agenesis or hypoplasia of the olfactory bulbs, associated with hypogonadotropic hypogonadism linked to abnormal GnRH neuron migration. Prok2-/- and prokr2-/- mice are the first murine models of this human disease. KS patients of both sexes have a variety of point mutations, missense mutations, frameshifts and nonsense mutations in the PROK2 and PROKR2 genes that lead to a loss of peptide or receptor function. When only one allele is affected, penetrance of the two main clinical features of KS may be incomplete: subjects with only one mutant allele may have (1) no symptoms, with normal olfaction and complete pubertal development, (2) congenital isolated (idiopathic) hypogonadotropic hypogonadism (IHH) but normal olfaction, (3) anosmia/hyposmia but normal pubertal development and gonadal function or (4) the two cardinal clinical KS signs, anosmia and IHH. These phenotypic dissociations can be seen in family members with the same PROK2/PROKR2 mutations. By contrast, patients with two mutant alleles almost always have the cardinal signs of KS. Even when monoallelic PROK2/PROKR2 mutations are associated with full-blown KS, the reproductive phenotype in males is less severe than in KS associated with biallelic mutations, evidenced by significantly lower frequency of cryptorchidism and micropenis, greater testicular volume, and higher serum levels of LH, FSH and testosterone. Moreover, at least some of these monoallelic cases are in fact digenic, in that they also carry mutations of other KS/IHH genes. Overall, these observations point towards a combination of mendelian autosomal recessive transmission, with more complex oligogenic transmission. Patients with this genetic form of KS have been reported to have a possible increased prevalence of obesity and sleep disorders, which may be related to the role of PROK2 and PROKR2 in food intake and circadian rhythms. However, diurnal variation of serum cortisol levels appears to be physiologically maintained.