Modeling an Unpredictable Disease: A New Window Into Heritable Pulmonary Hypertension
Some people inherit a genetic mutation that can cause an incurable lung disease and, ultimately, heart failure. Others carry the exact same mutation and stay healthy their whole lives — and no one can tell a family which future their child will have.
Now, a team led by UCSF pediatricians has created the first large-animal model of pulmonary arterial hypertension (PAH), giving researchers a way to finally watch it unfold and test how to treat it.
Published in JCI Insight and featured on the journal's cover, the research used gene editing to create sheep carrying mutations in BMPR2, the most common genetic cause of PAH.
Led by Sanjeev Datar, MD, PhD, of the UCSF Division of Pediatric Critical Care, the study shows that the sheep develop key features of human PAH while reproducing some of the variability seen among people who carry BMPR2 mutations. The work brought together researchers at UCSF, UC Davis, the University of Washington, and Vanderbilt University.
Why Mice Weren't Enough
Rodent studies have revealed much of what researchers know about PAH. But while researchers can measure pressures in the hearts of mice and rats, a mouse heart is roughly the size of a pencil eraser, making it difficult to follow the disease over time or test devices. Sheep, long used to study newborn circulation and congenital heart defects, have hearts and lungs that work much more like a child's, letting researchers track the disease more closely than doctors can in a patient.
“With a large-animal model, we can directly measure how the heart and pulmonary arteries respond as disease develops and eventually test treatments in ways that simply can't be done in a mouse,” says Datar.
Genetics play a large role in pediatric PAH, accounting for roughly half of cases in children compared with about 20% in adults. But even among people carrying a BMPR2 mutation, disease is unpredictable: some remain healthy while others develop PAH, even with careful monitoring. Researchers still don't know what determines who gets sick or when.
Editing Around a Lethal Gene
Creating the model required researchers to reproduce a specific genetic combination. Completely disabling BMPR2 is lethal before birth, so to mirror human carriers, the sheep needed one functioning copy of the gene and one disrupted copy.
To do this, Datar sought out critical expertise at UC Davis, from Alison Van Eenennaam, PhD, and her group in the Department of Animal Science and Bret McNabb, DVM, at the School of Veterinary Medicine. The team used a modified CRISPR/Cas9 approach to produce the first reported live-born sheep with an intentional heterozygous gene edit.
A Model Like the Disease
Four lambs were born carrying BMPR2 mutations. Like human carriers, their disease development didn’t follow a uniform course.
The three females developed evidence of pulmonary vascular disease within two months, ranging from subtle changes visible only in lung tissue to severe PAH detectable through cardiac imaging and direct pressure measurements.
The one live-born male was different. Despite carrying disrupted BMPR2, he showed no signs of disease. This pattern echoes human PAH, where female carriers are about three times as likely as males to develop the disease.
What This Opens Up
The unaffected male carries the edited gene in half his sperm and produced viable embryos in the lab. Future generations of BMPR2 heterozygous sheep are now being bred without having to repeat the original editing process each time, creating a lasting research platform.
“Now we can study how normal development, including the transition to sexual maturity, affects disease progression — and whether and how second hits like pregnancy, high altitude, or congenital heart disease can trigger disease,” says Datar. “Down the line, it could also help us test new devices, biomarkers, and therapies.”
The sheep model doesn't yet explain why some people with a BMPR2 mutation develop PAH while others remain healthy — or what determines when the disease emerges. But for the first time, researchers have a way to investigate those questions.
Authors: Other UCSF authors include Rachel Hutchings, Elena Amin, MD, Hythem Nawaytou, MBBCH, Jeffrey Fineman, MD. For all authors, see the paper.
Funding: This work was supported by the National Institutes of Health (grants R01HL133034 and R01HL61284) and a Catalyst Grant from the UCSF Pediatric Heart Center. For all funding, see the paper.