Loss-of-function mutation of the gene encoding makorin ring finger protein 3 (MKRN3) prematurely activates the hypothalamic–pituitary–gonadal (HPG) axis. It is the most common genetic cause of central precocious puberty.
Based on findings from a novel mouse model, researchers at Brigham and Women’s Hospital now suggest the reverse may also be true: genetic variants resulting in increased MKRN3 expression may be a previously unrecognized cause of delayed puberty. Ursula B. Kaiser, MD, chief of the Division of Endocrinology, Diabetes, and Hypertension at the Brigham, and colleagues explain the rationale for their study and expand on the results in Endocrinology.
The onset of puberty occurs when secretion of gonadotropin-releasing hormone (GnRH) is activated after quiescence in childhood. MKRN3 is thought to be an inhibitor of GnRH.
Makorin proteins have been detected in invertebrate and vertebrate animals, indicating a high degree of evolutionary conservation. Even the roundworm Caenorhabditis elegans has a makorin protein, lep-2, and lep-2 deficiency results in delayed maturation from juvenile to adult.
A previous study published in ELife showed that, similarly, overexpression of human MKRN3 in C. elegans led to delayed sexual maturation. That suggested strong conservation of MKRN3 function and hinted that overexpression of MKRN3 or gain-of-function mutations may have a role in delayed puberty. The current study investigated that possibility.
Methods and Results
On postnatal day 1, wild-type mice received bilateral intracerebroventricular injections of a recombinant adeno-associated virus expressing Mkrn3, and they were monitored for pubertal changes beginning on postnatal day 21. The principal findings were:
- There was a sex difference in response to Mkrn3 overexpression: female mice exhibited a significant delay in vaginal opening and first estrus, but male mice showed no change in pubertal timing as measured by preputial separation, an indicator of androgen exposure
- The delayed puberty in female mice could not be attributed to lower body weight, as Mkrn3 overexpression had no effect on body weight at the ages and metabolic conditions tested
- Fertility was normal in female mice (it was not evaluated in male mice)
- Mkrn3 overexpression resulted in reduced levels of kisspeptin and neurokinin B protein in the hypothalamus, compared with controls
Kisspeptin and its receptor are important regulators of GnRH secretion in the HPG axis. Human and mouse models have shown that gain-of-function mutations or deletions in the genes encoding kisspeptin and its receptor (KISS1/KISS1R) are associated with central precocious puberty.
Conversely, loss-of-function mutations in KISS1 and its receptor are associated with congenital hypogonadotropic hypogonadism, which can present with delayed puberty. Similarly, loss-of-function mutations in genes that encode neurokinin B and its receptor have also been associated with congenital hypogonadotropic hypogonadism.
In this study, the decreased hypothalamic levels of kisspeptin and neurokinin B imply a potential new mechanism of action for Mkrn3. It’s unclear whether reductions of kisspeptin and neurokinin B are direct or indirect effects of Mkrn3 action.
Still, the findings suggest the central response to Mkrn3 is not restricted by developmental windows. MKRN3 may be a therapeutic target for central precocious puberty and other pubertal disorders.