New study reveals previously overlooked regulator in the body’s oxygen-sensing system
Every cell in the human body is equipped with a molecular system designed to sense oxygen. When functioning properly, it allows us to adapt to high altitude, intense exercise and changing environments. When disrupted, it can contribute to rare blood disorders, even cancer.
New research from the lab of Dr. Michael Ohh in the Department of Laboratory Medicine and Pathobiology (LMP) at the Temerty Faculty of Medicine sheds light on how this system goes awry. In a study published in eLife, PhD candidate Cassandra Taber demonstrates how subtle mutations in a key oxygen-sensing enzyme can derail its regulatory function.
Taber’s work focuses on PHD2, an enzyme that plays a central role in regulating hypoxia-inducible factors (HIFs) - proteins that control the body’s response to low oxygen. Under normal oxygen conditions, PHD2 chemically modifies HIF, marking it for destruction. When oxygen levels drop, PHD2 activity slows, HIF stabilizes and activates genes that help cells adapt. These include genes responsible for producing erythropoietin (EPO), the hormone that stimulates red blood cell production.
“In cancer, this system is often hijacked. Solid tumours not only contain pockets of low oxygen, cancer cells exploit the hypoxic response even when oxygen is plentiful to survive and continue growing,” says Ohh.
This oxygen-sensing pathway is highly conserved across animals and essential for maintaining balance in the body. When mutations disrupt its regulation, the result can be a group of rare conditions known as “pseudohypoxic” diseases, disorders in which the hypoxic response is triggered inappropriately despite normal oxygen levels.
One such condition is PHD2-driven erythrocytosis, a rare inherited disorder caused by mutations in the EGLN1 gene, which encodes PHD2. Since the first reported case in 2006, more than 150 cases have been documented worldwide. Patients can develop excessive red blood cells and, in rare cases, neuroendocrine tumours.
Understanding which mutations matter
For clinicians, identifying a mutation is often only the beginning. Many genetic changes are classified as “variants of uncertain significance,” meaning it is unclear whether they will cause disease.
“When a doctor finds a mutation, the question is: Is it harmful? Does it actually do anything?” says Ohh. “If you can’t interpret it, it’s very difficult to guide patient care.”
Taber’s research addresses exactly that challenge. In her paper, she and her colleagues examined seven disease-associated PHD2 mutations using a combination of structural biology, biophysical analysis and cellular assays.
They demonstrated that all seven mutants showed structural and/or catalytic defects that impair the enzyme’s ability to properly regulate HIF, reinforcing the long-standing theory that dysregulation of the HIF pathway underpins these disorders.
One mutation in particular, known as P317R, yielded an unexpected insight. The oxygen-dependent degradation domain of HIF contains two key regulatory sites which are often described as molecular “on/off” switches. Previously, researchers believed that modification of one site, the C-terminal site, was sufficient to ensure proper regulation, and that the second site - N-terminal site - played a minor or redundant role.
Taber’s research challenges the assumption that the N-terminal site is biologically redundant. Instead, her data suggest it plays a meaningful role in maintaining proper oxygen regulation.
“The prevailing idea was that one site was sufficient, but what we observed indicates that the second site is not dispensable. Its loss can contribute to disease,” Taber explains.
By establishing measurable biochemical differences between disease-causing and less disruptive variants, the work strengthens the ability to predict which mutations are likely to have clinical consequences, an important step toward earlier diagnosis and more informed patient monitoring.
“If you understand the mechanism,” says Ohh, “you’re much more likely to solve the problem.”
A question of evolution
Taber’s curiosity did not stop at molecular structure. As her experiments revealed the importance of the N-terminal regulatory site, she began to ask a broader question: why did it evolve and if it was redundant, why was it still there?
To investigate, she constructed an evolutionary analysis tracing the emergence of the oxygen-dependent degradation domains across early animal lineages. Her work suggests that the N-terminal regulatory site appeared in the last common ancestor of bilaterians, likely during a period of fluctuating atmospheric oxygen, hinting that it may have evolved as a biological “backup” system as oxygen sensing became increasingly critical.
The project even led her beyond the lab. “I went to the Royal Ontario Museum to look at early animal evolution and think about how atmospheric oxygen shaped development,” she says. “It was fascinating to connect molecular biology to deep evolutionary history.”
That interdisciplinary curiosity reflects the intellectual environment of the Ohh lab, which studies fundamental mechanisms common across many cancers and hypoxic diseases. Rather than focusing on one tumour type, the lab investigates shared biological features - knowledge that may ultimately inform therapeutic strategies across multiple conditions.
“There are two approaches to finding a treatment for diseases: one is large-scale screening, hoping to find something that works, often without knowing how it works. The other is to understand how the system functions at a fundamental level. Like a mechanic, if you understand how a car works, you can fix any car. We take that second approach,” explains Ohh.
Powered by community support
Taber’s research was made possible through a combination of federal research funding and sustained grassroots support from the community.
For the past decade, employees, families and friends connected to the Canadian company Colorworks Express Autobody (whose CEO Kevin Carter is a University of Toronto alumnus) have organized fundraising events, including barbecues and car washes, to support cancer research in the Ohh lab. All donated funds have gone directly to research activities. “Every cent has gone to the science,” says Ohh. “Basic research doesn’t always produce immediate clinical outcomes, but it builds the foundation for everything that follows.”
As Taber prepares to complete her PhD, she is finalizing a second manuscript that builds on these findings and further explores how specific mutations alter oxygen regulation.
Reflecting on her doctoral journey, she points to the satisfaction of bringing clarity to uncertainty.
“It’s incredibly rewarding to take something that’s a question mark in the clinic - a mutation no one fully understands - and provide evidence about what it does,” she says.
Read the paper in elife: Erythrocytosis-inducing PHD2 mutations implicate biological role for N-terminal prolyl-hydroxylation in HIF1α oxygen-dependent degradation domain
This story showcases the following pillars of the LMP strategic plan: Impactful Research (pillar 3), Disruptive Innovation (pillar 4) and Agile Education (pillar 5).
