The Case of the Missing Y Chromosome
Loss of the Y chromosome in blood cells resulted in cardiac dysfunction
Background
Out of our 23 pairs of chromosomes, two in particular have a special role: the X and Y sex chromosomes. Females receive two X chromosomes while males receive an X and a Y.
The Y chromosome is one of the smallest human chromosomes, and for a long time, scientists didn’t think it did a lot. The SRY gene is found on the Y chromosome and codes for male characteristics, and was discovered in 1990. Outside of this gene, not much has been understood about the chromosome’s role.
In recent years, however, it’s been discovered that some males exhibit mosaic loss of chromosome Y (mLOY). This occurs when some of the cells in men’s blood do not have the Y chromosome. While you might assume that mLOY is a rare condition, it’s actually present in 40% of 70-year-old males and 57% of 93-year-old males.
It’s thought that mLOY could contribute to blood-related diseases and a shorter lifespan. But remember: correlation does not equal causation. While scientists have observed a correlational relationship between mLOY and these malignancies, a causal relationship had not been determined prior to this study.
Does the loss of the Y chromosome cause a higher mortality and disease risk? That’s what researchers at the University of Virginia School of Medicine set out to study.
Mouse Model
Researchers wanted to model the condition in mice to determine if and how it causes disease. In order to create a model of mLOY, researchers used CRISPR-Cas9 to target the Y chromosomes of mouse bone marrow cells. Genes on the centromere were targeted to cause the loss of the chromosome. The control was a guide RNA that did not have any targets, so that these cells retained their Y chromosomes.
Experimental mLOY and healthy control bone marrow cells were isolated and transferred to wild-type mice whose own bone marrow had been lethally irradiated, or destroyed. (All male mice, of course.)
To make sure their model worked, they used fluorescent in situ hybridization (FISH), which is a technique using a fluorescent probe that complements and binds to specific DNA sequences.
By looking through a special microscope to view the fluorescence, researchers can determine if that particular sequence is present in the genome. The researchers used this method to label the X and Y chromosomes, allowing them to visualize and count the cells based on their genotype.
Their results showed that their model worked! The results of the FISH showed that the ablation efficiency, or percentage of cells with successfully deleted Y chromosomes, was 80–90%.
They also performed karyotype analysis, where they imaged all 23 pairs of chromosomes and lined them up in order. This confirmed the effectiveness of their model, because they did not find the Y chromosome in the mLOY mice.
Researchers wanted to see if the loss of the Y chromosome would cause the same malignancies in mice that is seen in humans. The mice had shorter lifespans and more cardiomyopathies compared to the control, especially as they aged.
Additionally, the amount of fibrous tissue in the heart muscle increased. This is called fibrosis. Fibrosis of the heart muscle makes it harder for it to work properly, causing problems like diastolic dysfunction. There was additional fibrosis in the lungs and kidneys. The mLOY mice also had short-term memory deficits as they aged.
UK Biobank
The team then turned to information about human males with mLOY to see if the results they saw in their mouse model mirrored the clinical presentation.
They ran several statistical tests using data from the UK Biobank. This database provides a wealth of information for researchers, with biological and lifestyle information from over 500,000 participants. Using SNP data from 223,550 male participants, the team could determine the level of mLOY in participants’ blood cells.
A hazard ratio describes the probability that an event will occur in a treatment group compared to a control. A hazard ratio of 1 indicates that there’s no difference, but a ratio higher than 1 suggests that the treatment or condition did have an effect.
When looking at a subset of 6,222 men who had died as a result of cardiovascular disease, researchers found an association between death from cardiovascular disease and mLOY. This was evidenced by a hazard ratio of 1.0054 per 1% increase in blood cells missing the Y chromosome. This means that the hazard ratio increased as the percentage of leukocytes without Y chromosomes went up.
The causes of death associated with mLOY included diseases of the circulatory system, hypertensive heart disease, and other cardiovascular conditions.
Mechanism of Disease
Returning to their mouse model, researchers wanted to determine if mLOY would exacerbate heart failure. They performed transverse aortic constriction surgery (TAC) to mimic heart failure seen in elderly human patients. mLOY mice had worse cardiac function than the control after both groups underwent this procedure.
They saw that there was an elevated response of CCR2+ cardiac macrophages in the TAC-treated mLOY mice. CCR2+ macrophages cause inflammation, which makes it harder for the heart to repair itself. They found that mLOY encouraged the macrophages to develop this phenotype. When researchers used an antibody to block this macrophage response, the dysfunction reduced.
To determine what caused this inflammation, researchers performed single-cell RNA sequencing on immune cells from mLOY and control mice after TAC. This showed higher levels of RNA expression for proteins like TGF-β1, which is associated with fiber production. Neutralizing this protein prevented the fibroblast increase and reversed the dysfunction seen in mLOY mice.
It appears that fibrosis of heart muscle is a significant reason why men with mLOY experience cardiac pathologies. Future studies could utilize this finding to explore potential treatments for men with mLOY experiencing cardiac pathologies.
Grove City College students can related journals by simply searching the journal name in Discover on the Henry Buhl Library’s homepage. And don’t forget — if you’d like to find more related resources, the library maintains a list of A-Z Databases with an entire tab dedicated to biology!
