Kisspeptin and Clinical Disorders - Kisspeptin Signaling in Reproductive Biology - page 184

presentation, with variation in the presence or absence of olfactory defects (and other
somatic anomalies), severity of the hypogonadism, and neuroendocrine patterns.
Thus, patients with isolated GnRH defi ciency represent a unique opportunity to iden-
tify genes that awaken the reproductive cascade at the time of sexual maturation and
maintain normal reproductive function throughout life.
In 2003, homozygosity mapping and candidate gene analysis of two large consan-
guineous pedigrees with isolated GnRH defi ciency led to the identifi cation of loss-of-
function mutations in a then little-known G protein-coupled receptor, GPR54 (later
to be renamed KISS1R =kisspeptin receptor), by two investigative groups [ 3 , 4 ].
The identifi cation of mutations in multiple families, as well as unrelated probands,
coupled with a parallel reproductive phenotype in a Kiss1r mutant mouse, cata-
pulted kisspeptin into the spotlight as a key regulator of GnRH secretion.
Initial Reports
In searching for novel gene defects associated with isolated GnRH defi ciency, one
group employed homozygosity mapping of a large consanguineous family with fi ve
affected siblings. Chromosome localization and candidate gene sequence analysis led
to the identifi cation of a homozygous deletion of 155 nucleotides in the KISS1R [ 3 ].
This deletion encompassed the splicing acceptor site of intron 4-exon 5 junction and
part of exon 5. In the unlikely event that this abnormal transcript was translated, the
deleted receptor would be truncated within the third intracellular loop, lacking
transmembrane domains 6 and 7. The proband of the index family was a 20-year-old
male who presented with abnormal pubertal development, 4 mL testes, and a normal
sense of smell. While his affected brothers all had similar clinical features, his
affected sister had partial breast development and had experienced a single episode
of uterine bleeding. His mother, a heterozygote carrier of the deletion, was noted to
have experienced menarche at age 16.
Homozygosity mapping of a different consanguineous family with GnRH defi -
ciency, this time from the Middle East, was the focus of a second investigative group [ 4 ].
Candidate gene sequencing of KISS1R led to the discovery of a homozygous mis-
sense mutation, p.L148S, within the second intracellular loop of the receptor.
Transfection of COS-7 cells with a mutant construct representing L148S revealed
signifi cantly decreased accumulation of inositol phosphate in vitro compared to
wild type [ 4 ]. Further sequencing of KISS1R led to the identifi cation of compound
heterozygote mutations, p.R331X and p.X399R, in an African American male [ 4 ].
As mRNAs with premature termination codons are known to be subject to nonsense-
mediated decay [ 5 ], and mRNAs without an in-frame termination codon had
recently been appreciated to be subject to nonstop decay [ 6 , 7 ], it was hypothesized
that the combination of nonstop and nonsense mutations in a single individual
would result in the absence of a functional receptor. Quantitative RT-PCR confi rmed
a signifi cant reduction of KISS1R mRNA in immortalized white blood cells from
the p.R331X/p.X399R proband. Should a protein have been produced by either the