Decades later, a chromosomal breakthrough

Jell-O-like substance could be key to treating Fragile X and Rett syndromes

The X chromosome creates a challenge for human cells. Unlike most chromosomes, which are present in duplicate regardless of a person’s sex, females have two copies of X while males have only one. Females don’t need twice as many of the genes encoded on the X chromosome as males, however, so they must inactivate one of their two copies.

How this inactivation occurs has been a long-standing question in cell biology — one Jeannie Lee’s lab at Mass General has been central to answering. In a study published last month, Lee and her colleagues describe how cells orchestrate this chromosomal silencing. The findings could lead to relief for the many thousands of people living with diseases caused by mutations on the X chromosome.

Inactivation depends on a gelatinous substance that coats all chromosomes, creating discrete bubbles that work as separators. “It’s like Jell-O. And if chromosomes weren’t surrounded by this Jell-O, they’d get tangled up like spaghetti,” says Lee, who is vice chair of the Department of Genetics at Harvard Medical School.

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