Dec. 8, 1994
Vol. 14, No. 8

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    Researchers discover how molecule determines sex

    Finding also explains mutation that results in females with male chromosomes The key determinant of human maleness is a tiny molecular lever that reaches into a strand of DNA and pries a kink into it, setting in motion all the changes necessary to convert a sexless 35-day-old embryo into a boy, researchers at Chicago, Harvard and Massachusetts General Hospital have shown.

    The finding, reported in the Dec. 1 issue of the journal Science, shows in atomic detail how SRY, the molecule produced by the maleness gene on the Y chromosome, initiates the cascade of molecular events that leads to the production of testes and blocks the formation of female reproductive organs. The finding also explains how an undetected pinpoint mutation in the Y chromosome can result in the birth of a girl to a mother whose amniocentesis test -- which taps into the chromosomal makeup of the fetus -- has indicated that she is carrying a male fetus.

    "For researchers, what's valuable about studying sexual development is that it's the first thing parents and doctors look at, so subtle genetic defects come to light right away," said Michael Weiss, Professor in Biochemistry & Molecular Biology, Director of the Center for Molecular Oncology and senior author of the study. "In the case of mutations in the SRY gene, the effects are not subtle and result in complete sex reversal of the child."

    The mutation in the SRY molecule produces females who are chromosomally male and have defective gonads, but are otherwise physically normal females. The condition -- called "pure gonadal dysgenesis," meaning that only the gonads are affected -- can be treated with hormones. With the aid of in-vitro fertilization, such women can even carry a pregnancy.

    The reason these patients are so nearly normal is that all embryos begin life more female than male. During the first 35 days of gestation, embryos of either sex develop along classic female lines and have the beginnings of a uterus, fallopian tubes and the upper part of the vagina, Weiss said.

    "If it's to be a boy, consider the biological challenges at 35 days post-conception -- male organs have to be developed and the nascent female organs have to be instructed to regress. The key event is in the development of a testis," Weiss said. One set of cells then makes testosterone, which governs male sexual development, and another set of cells makes Muellerian Inhibiting Substance, or MIS, which instructs the female structures to regress.

    The researchers' report in Science shows how both processes depend on the action of SRY.

    The scientists used nuclear magnetic resonance, the same technique doctors use to make MRI images of the body, to show that a normal SRY protein, which contains the long amino acid isoleucine at a crucial position, inserts that molecular appendage between two specific base pairs in DNA, forcing them apart and bending the DNA helix. The scientists observed that SRY lacking this isoleucine -- the same specific pinpoint mutation found in some patients with pure gonadal dysgenesis -- fails to interact with DNA in this manner and cannot induce production of MIS in cultured cells.

    Weiss said other key events in fetal development are likely to be governed by very similar mechanisms. "The process of embryogenesis is full of molecular switches as cells determine their fates," he said. "If we can understand the general switches involved in sex determination, then we can understand other molecular processes, such as how other organs develop and how cancer arises."

    The master switch for maleness, he said, is related to switches in immune cells that turn on to battle infection or to cause rejection of a transplanted organ. "It's as if nature hit upon a solution for how to regulate a set of genes and has used this means over and over," Weiss said.

    SRY, named for the sex-determining region of the Y chromosome, was discovered in 1990 by British researchers. It makes up less than one-half of one percent of the DNA on the short Y chromosome. A defective SRY gene is estimated to account for a quarter of the cases of sex reversal in females, a condition that occurs once in every 50,000 births.

    The Science report also demonstrates that one or more factors intervene between the action of SRY and the production of MIS. "Part of our agenda for the next five years will be to trace the pathway from SRY to MIS," Weiss said. "We expect to find new genes that will solve the mystery of unsolved cases of human sex reversal."

    The research was funded in part by the National Institutes of Health and the March of Dimes Foundation.