RESEARCH
OUR FOCUS
The Page Lab’s long-term goal is to understand, at the molecular level, the differences and similarities between males and females throughout the body, in health and disease. This understanding of where male and female cells, tissues, and organs are essentially the same and where they are fundamentally different is a critical and overlooked facet of the teaching and practice of medicine and the conduct of biomedical research.
Many debilitating diseases, including heart failure, systemic lupus, autism spectrum disorder, and many cancers, show striking yet unexplained sex biases in prevalence and severity. Such biases have historically been attributed entirely to cell-extrinsic factors, such as sex hormones or environment. For a number of reasons, the medical and biomedical research community have long ignored the multifaceted function and impact of the X and Y chromosomes (the sex chromosomes). The resulting blind spot means that biomedicine has little knowledge of the regulatory capacities and specific effects of sex chromosomes, or of the potentially widespread molecular differences—and health implications—that result from being male (XY) or female (XX).
Recent advances by the Page Lab provide an intellectual framework for studying these fundamental questions. This framework is rooted in our intensive studies of the sex chromosomes – X and Y – over the past three decades. Despite their common origins dating back ~200 million years, the present-day human X and Y are highly differentiated. The Y chromosome retains only ~3% of the genes it once shared with the X. Meanwhile, the X chromosome exists in two distinct states: the “active X” or Xa, and the “inactive X” or Xi. The somatic cells of males have an Xa and a Y, and those of females have an Xa and an Xi. Females and males have 22 pairs of identical chromosomes (or autosomes). Xa is genetically identical and epigenetically indistinguishable in males and females, so it is functionally equivalent to an autosome. Thus, females and males differ by just the 46th chromosome: Xi vs Y.
We have discovered that, despite its name, Xi plays a substantial role in influencing gene expression across the genome and across the body. Our model offers a new view of the human X chromosome – historically the most intensely studied chromosome in all of human genetics because of its involvement in X-linked recessive traits (e.g., color blindness and hemophilia). We now understand that genes expressed from Xi, and their counterparts on the Y, have widespread regulatory effects that likely underlie all biologically based sex differences. Understanding these fundamental, global, and cell-autonomous sex differences is key to understanding health differences in women and men.
The Page Lab has embarked on a long-term, integrated program of scientific research, with the goal of understanding, at the genomic, epigenetic, molecular, and cellular levels, the contributions of the sex chromosomes to sex differences in health and disease. This program involves a network of collaborators, with the Page Lab serving as the hub, which expands their basic science investigations and pursues translational research and development.
OUR RECENT STUDIES
Our understanding of how Xi and Y influence biological sex differences has accelerated though the establishment of a research platform harnessing the natural variation in X and Y copy number found in humans. This platform is based on immortalized lymphoblastoid cell lines (LCLs) and primary fibroblast cultures from more than 100 individuals with sex chromosome aneuploidy (SCA). SCA is a catch-all term for sex chromosome constitutions other than XX or XY. Humans can carry as many as four or five X chromosomes, and as many as four Y chromosomes.
By folding the full gamut of human SCA into one linear analysis, we defined the impact of Xi and Y on gene expression, first on the X and Y chromosomes themselves and then across the genome. First, we showed that numerous genes on Xi, previously thought to be “silenced” through X inactivation, are actually expressed in a consistent and modular fashion (San Roman et al 2023). We found that each copy of Xi contributes an equal quantum of gene expression, but the quanta differ between genes. Xi genes also modulate the levels of Xa genes in trans. Altogether, Xi influences the expression of ~38% of genes on the X chromosome. Using this knowledge, we were able to identify the X chromosome genes that are most sensitive to under- or over-expression, which, together with the ubiquitously expressed genes on the Y, are the genes most likely to drive differences between XX and XY cells. Second, we found that ~1800 autosomal genes are affected by Xi and Y dosage, demonstrating that sex chromosome constitution broadly influences genome regulation and likely impacts many cellular processes (San Roman et al 2024). We identified a key sex-chromosome encoded transcription factor – ZFX/ZFY – that drives much of the autosomal response to Xi and Y dosage. Third, we found that these in vitro findings are recapitulated in vivo – demonstrating that Xi (and Y) expression is stable and robust in fibroblasts, LCLs, T cells, and monocytes; autosomal responders to Xi and Y expression are largely unique to each cell type (Blanton et al 2024). Altogether, these studies reveal that every cell in the body is likely affected by the cell-autonomous effects of Xi vs Y. Therefore, other factors affecting sex differences, including steroid hormones, act upon cells that are, a priori, biochemically distinct substrates.