Hematopoietic stem cells produce multiple blood cell types and replenish themselves through a self-renewal process. As with all cells, hematopoietic stem cells accumulate somatic mutations with age. While most mutations have little or no effect on cellular fitness, some mutations will occur in “driver” genes that enable their positive selection. Thus, the mutant cells can outcompete neighboring wild-type cells leading to a clonal expansion. Notably, these mutant hematopoietic stem cell clones give rise to progeny immune cells that harbor the same mutant allele, and it has been shown that these mutations can functionally corrupt the circulating immune cell pool. Clonal expansions in blood cells have been found to occur in relatively healthy individuals who lack overt signs of blood cancer. This pre-cancerous condition has historically been referred to as “clonal hematopoiesis” and more recently as “clonal hematopoiesis of indeterminate potential” or “age-related clonal hematopoiesis” to distinguish it from the clonal expansions that occur in malignant blood disorders.
Clonal hematopoiesis is particularly prevalent in the elderly where clone size can become relatively large (>20% of an individual’s white blood cells can be derived from a single clone). While clonal hematopoiesis had long been viewed as a benign feature of the aging process, recent studies have shown that it is associated with mortality due in large part to elevated cardiovascular disease risk. Studies in experimental systems have provided evidence that clonal hematopoiesis can contribute to the development of age-related diseases including atherosclerosis, heart failure and insulin resistance. Collective epidemiological and experimental studies indicate that clonal hematopoiesis is a newly recognized causal risk factor for cardiovascular disease that is as prevalent and consequential as the traditional risk factors (hyperlipidemia, hypertension, smoking and diabetes) that have been recognized for decades. Details of this new disease mechanism are being uncovered, and these findings could facilitate the development of precision medicine strategies that are tailored to account for the diverse clonal hematopoiesis patterns between individuals.
In addition to the age-related clonal hematopoiesis described above, there is a more aggressive form of this condition that is referred to as “therapy-associated clonal hematopoeisis”. This occurs in individuals who have been treated for cancer, and it is associated with clones that arise from mutations in DNA damage-response pathway genes that confer cellular resistance to the genotoxic stress of the cancer therapy. Recent experimental studies suggest that therapy-related clonal hematopoiesis can contribute to the medium- and long-term toxicity of anticancer agents on the heart, and this mechanism may contribute to the prevalent cardiovascular disorders that are observed in cancer survivors.
The field of clonal hematopoiesis is in its infancy. There is an incomplete understanding of the mechanisms that give rise to clonal expansions in hematopoietic cells and the health consequences of these events. The known driver genes comprise a diverse group of regulatory molecules, and it is likely that they play divergent roles in disease processes. It is also reasonable to speculate that driver genes will exhibit disease-specific actions and likely impact age-related diseases beyond the cardiovascular system. Given the large number of driver gene candidates, studies require methodologies that are far more expansive in scope than the typical “single gene/single disease” approach. In view of these considerations, it is increasingly appreciated that the currently known pool of driver genes can only account for a portion of the clonal hematopoiesis events observed in individuals. The mechanisms that give rise to these enigmatic clones are poorly understood, yet epidemiological studies indicate that they are also associated with increased mortality. Thus, there are major unaddressed challenges and ample opportunities for new investigators to make progress in this rapidly expanding area of medical research.
To extend our findings on somatic mosaicism, a new research direction has explored the consequences of Y chromosome loss in the hematopoietic system of men. Loss of the Y chromosome increases with age, and it is the most prevalent post-zygotic mutation in our population. This loss occurs in a fraction of cells and thus it is referred to as mosaic loss of Y (mLOY). This condition can be detected in >40% of men by 70 years of age. While the age-dependent Y chromosome loss was initially observed in 1963, it wasn’t until 2014 that our collaborator Lars Forsberg at Uppsala University made the association between mLOY and shorter lifespan [https://www.nature.com/articles/ng.2966]. Since then, many studies have associated mLOY with shorter lifespan and the increased incidence of age-associated diseases including cancer and Alzheimer’s disease. However, it has been unclear whether mLOY represents a benign biomarker of aging, such as gray hair, or whether it has a causal role in biological aging and the diseases of aging. Our study provides evidence, for the first time, that loss of the Y chromosome is a disease driver. Furthermore, the mechanism of the effect is surprising when viewed in the context of how clonal hematopoiesis/CHIP promotes disease.
In this study we employed CRISPR gene editing methodology to ablate the Y chromosome in a portion of bone marrow/blood cells of male mice. These mice showed no immediate effects following bone marrow transplantation; however, they aged poorly. They exhibited shorter lifespans and accelerated disease processes including cardiomyopathy, renal and lung fibrosis, and cognitive decline. In many ways the mice mimic the diseases associated with mLOY in men. In further analyses, we analyzed the effects of mLOY on non-ischemic heart failure, a condition that afflicts millions of individuals in the USA alone. To bolster the rationale for an experimental analysis of heart failure, we worked with Lars Forsberg to examine the consequences of mLOY on cardiovascular disease (CVD) mortality in the UK Biobank. Employing the most recent release of clinical phenotype data, his lab found that mLOY is associated with overall mortality and mortality associated with diseases of the circulatory system including heart failure. In a model of non-ischemic heart failure in young male mice, we found that the mLOY condition led to greater pathological remodeling. Mechanistic studies in this model revealed that the mLOY condition led to macrophage polarization toward an anti-inflammatory and pro-fibrotic state. In contrast, clonal hematopoiesis/CHIP leads to a pro-inflammatory state in macrophages; that can be viewed as an opposite phenotypic outcome from mLOY.
A copy of this paper and the supplement can be found here: Hematopoietic loss of Y chromosome leads to cardiac fibrosis and heart failure mortality
A Perspective on this study can be found here: https://www.science.org/doi/10.1126/science.add0839
A News Feature on this study can be found here: Men lose Y chromosomes as they age. It may be harming their hearts
A New York Times article on this study can be found here: As Y Chromosomes Vanish With Age, Heart Risks May Grow
For a partial list of Walsh Lab publications, please click here.