New study reveals previously overlooked regulator in the body’s oxygen-sensing system

New research from the University of Toronto’s Temerty Faculty of Medicine is shedding light on how the body’s oxygen sensors go awry.

In a study published in eLife, Michael Ohh, a professor of laboratory medicine and pathobiology at Temerty Medicine, and PhD student Cassandra Taber demonstrated how subtle mutations in a key oxygen-sensing enzyme can derail its regulatory function, potentially leading to rare blood disorders and cancer.

Every cell in the human body is equipped with a molecular system that can sense oxygen and help us to adapt to high altitude, intense exercise and changing environments. One component of this system — and the focus of the new study — is PHD2, an enzyme that plays a central role in regulating a group of proteins called hypoxia-inducible factors (HIFs). HIF proteins control the body’s response when oxygen levels are low, a condition also known as hypoxia.

This oxygen-sensing pathway is highly conserved across animals and essential for maintaining balance in the body. When mutations in PHD2 disrupt the regulation of HIF, the result can be so-called “pseudohypoxic” diseases — disorders in which the low-oxygen, or hypoxic, response is triggered inappropriately despite normal oxygen levels.

One such condition is PHD2-driven erythrocytosis, a rare inherited disorder where patients can develop excessive red blood cells and uncommonly, neuroendocrine tumours. Since the first reported case in 2006, more than 150 cases have been documented worldwide.

For clinicians treating patients with genetic conditions like this one, 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 that challenge by examining seven disease-associated PHD2 mutations using a combination of structural biology, biophysical analysis and cellular assays.

The researchers demonstrated that all seven mutants showed defects that impair the enzyme’s ability to properly regulate HIF, reinforcing the long-standing theory that dysregulation of the HIF pathway underpins PHD2-driven erythrocytosis.

One mutation known as P317R yielded an unexpected insight into HIF’s structure.

HIF contains two key regulatory sites in its oxygen-dependent degradation domain where modification by PHD2 can trigger the protein’s destruction. Researchers previously believed that modification of the so-called C-terminal site was sufficient to ensure proper regulation, and that the second N-terminal site only played a minor or redundant role.

“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 says.

Her findings suggest that the N-terminal site plays a meaningful role in maintaining proper oxygen regulation.

Taber’s research also described measurable biochemical differences between disease-causing and less disruptive genetic variants in PHD2, which strengthens our ability to predict which mutations are likely to have clinical consequences.

“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.

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 HIF’s oxygen-dependent degradation domains across early animal lineages. Her work suggests that the N-terminal site appeared in the last common ancestor of bilaterians — the vast group of animals, including humans, that have a left-right symmetric body plan. Taber says the trait likely evolved during a period of fluctuating atmospheric oxygen, suggesting that it may have served as a biological “backup” system as oxygen sensing became increasingly critical.

Taber’s 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 with the goal of generating knowledge that may inform therapeutic strategies across multiple conditions.

“There are two approaches to finding a treatment for diseases,” says Ohh.

“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.”

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 have organized fundraising events like barbecues and car washes to support cancer research in the Ohh lab.

“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.”