Mutant gene found to cause early ovarian failure in mice

May be factor in human infertility as well as cancer and aging

Researchers at Dana-Farber Cancer Institute and Brigham and Women’s Hospital (BWH) have discovered a gene mutation in mice that causes premature ovarian failure, a form of infertility affecting an estimated 250,000 women in the United States.
The investigators say the discovery will lead to unique animal models of premature ovarian failure (POF), or early menopause, useful for further studying the poorly understood condition.

POF, which is diagnosed in 1 in 100 women ages 30 to 39, results from the depletion of a woman’s supply of eggs early in her reproductive years.

A report on the discovery is being published in the July 11 issue of Science. Lead author is Diego H. Castrillon, MD, PhD, a postdoctoral fellow in the laboratory of Ronald A. DePinho, MD, at Dana-Farber, where the work was done. DePinho is the senior author, and three other Dana-Farber researchers contributed to the paper.

Castrillon is a staff pathologist at BWH and will move to the University of Texas-Southwestern Medical School in September.

“This provides a molecular foothold into a process that we knew little about – that is, the mechanism that constrains or triggers the activation and maturation of the egg,” says DePinho.

The findings grew out of an experiment in which the researchers created mice lacking both copies of the FOXO3a gene, which belongs to the forkhead gene family. As transcription regulators, or switches that turn other genes on and off, forkhead genes are believed to control processes related to aging, cancer and diabetes. In this experiment, the researchers “knocked out” the FOXO3a gene, effectively mutating it, in mice to observe the consequences.

As the gene-altered mice aged, the females were observed to have fewer and smaller litters, and by 15 weeks of age – comparable to early adulthood in a woman – they were sterile. Further study revealed that within the ovaries of mice lacking the FOXO3a gene, the follicles that contain eggs had been activated earlier and much more widely than in the females with normal FOXO3a genes. When a follicle is activated, it moves from the “resting pool” – a female’s entire repository of eggs – to the “growing pool” and begins maturation, a process that is necessary before the egg can be released (ovulation) prior to fertilization. Once activated, a follicle has a finite lifespan, however, so the premature activation of follicles resulted in the early death of most eggs in mice that lacked the FOXO3a gene.

Castrillon made the comparison to an hourglass: “There is a finite number of grains of sand that are released in a metered way. Similarly, a woman’s eggs are gradually released over her reproductive lifespan. Menopause occurs when all the grains of sand have fallen. In mice lacking the FOXO3a gene, all of the grains of sand fall out very quickly, resulting in early menopause. Our findings raise the possibility that increased activation of eggs over a woman’s lifespan could result in premature ovarian failure and early menopause.”

From the experimental results, it appears that abnormal FOXO3a gene function leads to misregulation of follicle activation, causing POF in mice. Further research is needed to show that increased follicle activation occurs in women with POF.

On the basis of these findings, the researchers suggest that it might someday be possible to develop a contraceptive that would delay follicular activation, keeping follicles in the resting pool until a woman wants to become fertile – perhaps at a later-than-usual age. In contrast, current oral contraceptives prevent ovulation (the release of eggs) but do not slow the rate of activation, thereby permitting the depletion of woman’s reserve of eggs.

DePinho, who studies the relationship between biological aging and cancer, noted the FOXO3a gene is a player in a network of signals inside cells that has been implicated in cancer and aging. The network, known as the PI3 kinase pathway, is frequently commandeered to spur abnormal growth in cancer cells, said DePinho. Moreover, the same pathway has a role in regulating longevity in several species. Premature ovarian failure itself results in the early onset of several age-related conditions in women, and the affected mice showed some other subtle signs that might indicate early aging.

“Analyzing this pathway in great detail will have a fundamental impact on understanding cancer and aging,” said DePinho, who is also a professor of medicine at Harvard Medical School.

The research was supported by the National Institutes of Health.

Dana-Farber Cancer Institute (www.dana-farber.org) is a principal teaching affiliate of the Harvard Medical School and is among the leading cancer research and care centers in the United States. It is a founding member of the Dana-Farber/Harvard Cancer Center (DF/HCC), a designated comprehensive cancer center by the National Cancer Institute.

BWH is a 725-bed nonprofit teaching affiliate of Harvard Medical School and a founding member of Partners HealthCare System, an integrated health care delivery network. Internationally recognized as a leading academic health care institution, BWH is committed to excellence in patient care, medical research, and the training and education of health care professionals. The hospital’s preeminence in all aspects of clinical care is coupled with its strength in medical research. A leading recipient of research grants from the National Institutes of Health, BWH conducts internationally acclaimed clinical, basic and epidemiological studies.