New model simplifies the understanding of vascular malformations

Our bodies are made up of 60,000 miles of complex pipes that play a vital role in transporting nutrients throughout our bodies, performing waste disposal, and supplying our organs with fresh oxygen and blood.

Several things can go wrong with this complex system, including vascular malformations (VMs), a group of rare genetic disorders that causes an abnormal formation of veins, arteries, capillaries, or lymphatic vessels at birth. VMs can interfere with the duties of our precious pipes by causing blockages, poor drainage, and the formation of cysts and tangles.

To address a need for further study, William Polacheck, PhD, an assistant professor at the UNC-NCSU Joint Department of Biomedical Engineering and the Department of Cell Biology and Physiology, and his team spanning across the UNC School of Medicine, have developed a model that mimics VMs that are specifically caused by a mutation of PIK3CA -- a gene that has been implicated in multiple types of lymphatic, capillary, and venous malformations.

Their work was published in Science Advances, an open access multidisciplinary journal from the American Association for the Advancement of Science (AAAS).

There are number of 'chicken and the egg problems' of the PIK3CA mutation. Is it causing something else to go wrong? Or is there something else in the environment causing the mutation to have more severe effects? Working in a much more controlled environment, such as a microfluidic model, allows us to isolate and figure out how the genetics of the disease relate to what's happening in the cells."

William Polacheck PhD, Assistant Professor, Department of Biomedical Engineering, University of North Carolina

VMs are caused by mutations in the genes that direct the development of the vasculature throughout the body. Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) is one of those genes. Activating mutations in PIK3CA commonly contribute to malformations of the smaller blood vessels, causing blood to pool underneath the skin.

This specific type of vascular malformation is usually discovered at birth. These diseases start as the baby is developing. Since there are a multitude of changes happening at this point in the child's development, it can be difficult condition for researchers to study.

Julie Blatt, MD, professor of pediatric hematology-oncology in the UNC Department of Pediatrics, saw the need for a new approach to model the disease, which affects a majority of her patients. She has had a long-standing interest in clinical management of patients with vascular malformations, as well as an interest in repurposing cancer drugs for the disease. Impressed with his prior manuscripts, Dr. Blatt picked up the phone and cold-called Polacheck, who is a biomedical engineer by trade, to ask if he could create a microfluidic model of PIK3CA-specific vascular malformations.

"I think the transdisciplinary aspect keeps the possibility of application to patients at the forefront, said Dr. Blatt. "The Polacheck lab has prioritized introduction of genetic mutations that are relevant to patients and to studying drugs which we know or think will have benefit."

Around the same time, Wen Yih Aw, PhD, was working as a postdoctoral researcher at UNC Catalyst, a research group focused on understanding rare diseases in the Eshelman School of Pharmacy. Aw was collaborating with the Polacheck lab on a vascular Ehlers Danlos Syndrome project. Eventually, Aw joined the Polacheck lab and used her molecular biology expertise to help develop the VMs model.

In addition to Dr. Blatt and Aw, the lab has an on-going collaboration with Boyce Griffith, PhD in the Department of Mathematics and the Computational Medicine Program at the UNC College of Arts and Sciences, who is helping with analyzing the structures of the networks.

"All those pieces were necessary to complete the work," said Polacheck. "It does say something about UNC-Chapel Hill because there were multiple departments across campus involved. There were no barriers whatsoever to working together on this project."

Microfluidic models are incredibly small – about the size of a millimeter – three-dimensional devices that can be used to control or simulate the environment within the body. In this case, a small piece of blood vessel composed of healthy human endothelial cells or endothelial cells expressing the PIK3CA mutation is centered inside of the device. From there, the researchers can look into the process of vascular formation, and introduce specific chemicals and mechanical forces to the model to simulate the conditions of the body. They observed formation of enlarged and irregular vasculature with the introduction of PIK3CA mutation.

To confirm whether or not their model accurately portrays the manifestation of the disease, the team next conducted a drug efficacy study.

There are two drugs currently used for the treatment of vascular malformations: rapamycin and alpelisib. The latter is a newly discovered PIK3CA-specific inhibitor recently approved by the FDA to treat certain types of breast cancer and PIK3CA-related overgrowth spectrum. So far, pre-clinical studies in mouse models and in patients have shown that alpelisib is more effective in reversing vascular malformation defects.

After selecting the two drugs, Polacheck and Aw applied the treatment to their devices. The study was a success.

"The blood vessels used to be really dilated and large," said Aw, first author of the study. "By imaging the vessels before and after treating with drugs, we observed the vessels shrink and, more or less, revert it back to a normal shape and function. We were very excited to be able to replicate some of the results in vitro with the model we built."

Moving forward, Aw and Polacheck are looking to replicate the finding in tissues from vascular malformation patients, especially those who don't have the PIK3CA mutation or don't have clear genetic information. Their model can now be used to evaluate new medications or to perform synergistic drug studies.

Now that they know that their model works, Aw and Polacheck plan to use it to study the behavior of the mutated cells overtime, as well as how the mutation affects malformations of the lymphatic tissue.

The disease initially begins with an individual cell that acquires the PIK3CA mutation. Then, much like a chain reaction, the effects of the mutation in that one cell spreads to the surrounding cells until the malformation is fully formed. As their model currently stands, the lab cannot mimic that natural process.

Aw is currently working on a new and different approach for a microfluidic model. She aims to create a platform that will allow them to start with cells that are healthy, and then "flip on" the mutation, and watch it progress across the tissue of interest. Ultimately, it will help them understand how the mutation is able to affect other cells and move throughout space.

Vascular malformations can also occur in lymphatic tissue. As opposed to blood vessels, lymphatic vessels have a duty to recycle excess fluid throughout the body and acts as a superhighway for immune cells to get to sites of infection. Very little is known about the basic cell biology of lymphatic endothelial cells, so Polacheck is hoping to do a study that is similar to his most recent one.

"The outputs are slightly different because the function of the lymphatics is different from blood vessels," said Polacheck. "By comparing and contrasting what happens on the blood side and the lymphatic side, we will also be able to learn something about the basic biology of those two types of tissues."