Why a condition that frequently occurs in aging men, in which an increasing number of hematopoietic cells display a loss of the Y chromosome, is associated with increased risk of mortality and age-related diseases is now a little clearer, thanks to work in mice.

This condition – known as mosaic loss of Y chromosome, or mLOY – is a major risk factor for heart failure and cardiac fibrosis in men growing older, according to the study, which includes prospective data from the UK Biobank.

The study also revealed that a neutralizing antibody could reverse some cardiac impacts caused by mLOY. "[M]en with mLOY could represent a patient subpopulation that exhibits a superior response to this class of therapeutic agents," the study's authors say. 

The Y chromosome has been long considered a “genetic wasteland,” and beyond biological sex determination, there is little understanding of its functional role. Nevertheless, mLOY in blood cells has been linked to increased risk for mortality, cardiovascular disease, and other age-related disorders.

In human somatic cells, mLOY is the most commonly acquired mutation in the male’s genome. However, a relationship between mLOY and pathogenesis has not yet been established.

Using CRISPR-Cas9, the researchers developed a mouse model of hematopoietic mLOY by reconstituting their bone marrow with cells lacking the Y chromosome. The researchers discovered that these mice displayed increased mortality and were more prone to age-related cardiac fibrosis and decreased cardiac function. According to the findings, bone marrow-derived mLOY macrophages that infiltrate the heart trigger high transforming growth factor β₁ (TGF-β₁) activity, which leads to fibroblast proliferation and accelerated cardiac tissue fibrosis.

Treatment with a TGF-β₁ neutralizing antibody was shown to ameliorate these harmful effects. What’s more, a prospective study in human patients showed that those with mLOY in blood were also at a greater risk for cardiovascular dysfunction and associated mortality, suggesting the potential clinical relevance of the findings in mice.

“Indeed, several unexpected links between the Y chromosome, immune system, and complex polygenic traits have been discovered, suggesting an influence of the Y chromosome on immune and inflammatory responses in men,” write authors  in a related Perspective. “The study  reinforces this view and uncovers a crucial function of the Y chromosome in maintaining a healthy innate immune system, but further research is required to elucidate the mechanisms.”

https://www.science.org/doi/10.1126/science.abn3100