Researchers from Indiana University School of Medicine have discovered a molecular “clamping” mechanism within a male-specific protein-DNA complex whose mutation causes sex reversal: children with XY chromosomes but whose bodies are female, a condition called Swyer Syndrome. The pincer utilizes a water molecule that bridges the male factor (designated SRY) at the weak beginnings of male development and the DNA control regions.
The study focuses on the subtle substitution of a conserved aromatic residue in SRY (tyrosine) with a closely related aromatic residue (phenylalanine). The clinical mutation shared by a fertile XY father and his sterile XY daughter positions the embryonic male switch at the margin of genetic function. The two aromatic rings are apparently interchangeable in the protein’s structure, but differ in their ability to anchor the water molecule forming a bridge in the protein-DNA complex.
Head of the Department of Biochemistry and Molecular Biology, Dr. “Normally, the father has an XY chromosome and a daughter has an XX chromosome, but in some families, daughters may have an XY chromosome as there is a mutation in SRY. The sex chromosomes can degenerate over evolutionary time scales, leading to the recruitment of new upstream switches to male determinant. “The pathways grow backwards. Such first steps can be biochemically weak.”
In the study, the researchers focused on the 72nd position of SRY in the DNA binding domain, which had not previously been considered an area of special interest. However, the researchers discovered that tyrosine at this location enables the operation of a water-mediated kinetic clamp that extends the lifespan of the protein-DNA complex. This mechanism is conserved in all mammalian SRY factors and is widely observed in a related family of key factors in multicellular (and some unicellular) animals. The second family, designated “SOX” (the SRY-related HMG box), is fundamental for modeling and development in metazoans.
Researchers have published two new papers about their work in Frontiers in Endocrinology. The first describes his findings on focusing on box 72, and the second explains how the water-mediated gripper mechanism works. Weiss said they call this the “humpty-dumpty” model because of the rapid breakdown of the male-determining protein-DNA complex in the absence of water-mediated clamping.
“Because the normal and mutant version of SRY are so similar in standard experimental assays,” said Joseph D. Racca, PhD, Associate Research Professor in the Department of Biochemistry and Molecular Biology and first author of the new study. “It took several years to uncover the water-mediated mechanism. Molecular dynamics (MD) simulations of boundary water molecules in this system provided critical information.”
“In MD simulations, a distinctive water molecule is attached by tyrosine to DNA as a bridge: this particular hydration site is occupied for thousands of picoseconds and then it will separate,” Weiss said. “But then another water molecule in the bulk solvent will bounce back into place almost instantly, restoring the bridge.”
The subtle change from tyrosine to phenylalanine alters such hydration; this is a perturbation radiating from position 72, which is predicted to destabilize successive protein-DNA contacts in the tail of the domain. Tail uncoupling will accelerate dissociation of the protein-DNA complex and possibly male-specific gene regulatory assemblies in target genes.
XY girls with sexual differentiation differences due to Swyer Syndrome lack functional ovaries and are at risk for rare forms of early-onset gonadal cancer. Recognition of this syndrome is important to allow for surgical removal of the gonads before the cancer starts. Otherwise, the affected woman has a normal womb and birth canal and can therefore have children following fertilization of a donated egg in IVF.
Similar mutations can occur in SOX genes and cause various birth defects or diseases, Weiss said.
“Swyer mutations provide clues to help us understand a wide variety of SOX diseases and could lead to much more advanced protocols for different fields of medicine, such as regenerative medicine or cancer,” Weiss said. Said. “This discovery thus goes far beyond gender reassignment because SRY is a prototype key.”
In addition to Weiss, other study authors from IU include Yen-Shan Chen, PhD, Joseph Racca, PhD, Deepak Chatterjee, PhD, Ratan Rai, PhD, Yanwu Yang, PhD, and Millie Georgiadis, PhD. Elisha Haas, PhD, from Bar Ilan University in Israel, was also a co-author.