be expressed, but it is known that some genes from the "silent" X chromosome escape this silencing. "These genes are known, and it is the same set of genes that escape X-inactivation, including in the XX males," Sandberg says. "We think there may be something special about these genes that escape X-inactivation and which are only expressed in XX cells."
The other possible explanation is conflict between cells that are expressing different parental X chromosomes. While each cell in a female expresses only one X, that X could come from either the male or female parent. Conflict can arise when a cell expressing a maternal X is adjacent to a cell expressing a paternal X. "That can drive an immune response because the two X's may recognize their own molecular self differently," she says.
The other theory is that the Y chromosome contains a gene or genes that protects against hypertension, and that "since postmenopausal women don't have that added benefit from those Y genes, once the beneficial effects of estrogen are gone, blood pressure rises."
"These very exciting findings deserve much more research into what is encoded within the sex chromosomes that affects blood pressure control," Sandberg says.
She adds that if researchers can zero in on that particular mechanism, it may be possible to design a therapy to stop intransigent postmenopausal rise in blood pressure, Sandberg says. "There is a real jump in blood pressure and incidence of hypertension in menopausal women, and while the condition is treatable, blood pressure in many of these women is not fully under control making them far more susceptible to cardiovascular and kidney disease and stroke. Therefore, it would be wonderful to have specific therapies that target the root cause of this hypertension."
- Hong Ji, Wei Zheng, Xie Wu, Jun Liu, Carolyn M. Ecelbarger, Rebecca Watkins, Arthur P. Arnold, and Kathryn Sandberg. Sex Chromosome Effects Unmasked in Angiotensin II%u2013Induced Hypertension. Hypertension, 2010; DOI: 10.1161/HYPERTENSIONAHA.109.144949
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