By all appearances, Cassie Mehring looks and behaves like a normal 4-year-old girl.

She gets along well with her older sister, Autumn, though they get into the occasional spat. She’s looking forward to preschool in the fall. She likes to climb the stairs to the upper floor of her home, and she’s proud that her parents, Jeremy and Jessica, allow her to do it on her own.

“I can do it, day and night,” she said.

Cassie can do all these things despite the fact that she carries an extraordinarily rare genetic anomaly that nearly killed her two years ago.

The PPA2 gene produces an enzyme that generates energy at the cellular level, according to Children’s Hospital Colorado Genetic Counselor Janell Kierstein.

“Everybody has two copies of this gene,” Kierstein explained. “If there are variants in both copies – if you don’t have a normal, working copy – your cells can’t produce the enzyme, and therefore they can’t generate enough energy. That leads to the symptoms of the disorder.”

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Among those symptoms is the sudden cardiac arrest that fatally struck the Mehrings’ firstborn daughter, Scarlett, in 2012.

“They didn’t know about the PPA2 mutation until 2016,” said Jessica Mehring. “So for a long time, we didn’t have any answers.”

When Scarlett was born in 2010, she was a “perfectly normal, happy, healthy little girl,” Jessica said. In 2012, when she was 19 months old, Scarlett acquired a stomach bug.

“She was throwing up, but a stomach bug is a perfectly normal thing for a toddler,” Jessica said. “We gave her plenty of fluids and kept an eye on her, but we didn’t think it was anything more than a bug.”

She seemed to be recovering from her illness, but a couple of days later, Jessica woke up a little later than usual.

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“I didn’t set an alarm because, when you have a small child, they’re your alarm clock,” she said.

At first she thought Scarlett was sleeping in.

“But I went into her bedroom to check on her. And when I opened the door, I knew she was gone.”

What she didn’t know — and what no one could figure out — was why.

An autopsy failed to provide answers. Genetic testing was suggested, but the Mehrings were told that it might not provide the answers they were seeking, so they declined.

“Looking back now, that was the right decision,” Jeremy said. “Because no one knew anything about this mutation until 2016, we wouldn’t have learned anything.”

Scarlett’s cause of death was listed as an “unknown heart complication.” Stricken with an agony that was exacerbated by the lack of an explanation, the Merhings nonetheless had a second daughter, Autumn, in 2016. In 2019, they had Cassie.

In May 2021, Cassie developed a stomach bug.

“The days leading up to Scarlett’s death and the days leading up to Cassie’s cardiac arrest were almost identical,” Jessica said.

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Two days after Cassie started showing symptoms, she was sitting in a high chair as her father tried to get her to eat something.

“I was arguing with her, telling her she needed to eat, when she slumped over, and her body went completely stiff,” Jeremy said.

“Her lips were black,” Jessica said. “Not blue – they were black.”

Shortly after Jessica learned she was pregnant with Autumn, she and Jeremy took a course in child CPR. Jessica had retained much of what she learned, and she began working on Cassie while Jeremy called 911.

“We have an awesome firehouse nearby, and they got here really quickly.”

Revived in the ambulance, Cassie was initially taken to Memorial Hospital Central before being airlifted to Children’s Hospital in Aurora. After a battery of tests, doctors still couldn’t explain why her heart stopped.

“They did x-rays, MRI, and all kinds of testing, but they couldn’t figure out why a normal 2-year-old girl would go into cardiac arrest,” Jessica said.

To minimize the risk of another sudden cardiac arrest, the Heart Institute at Children’s implanted a cardioverter/defibrillator in Cassie’s chest.

At their wit’s end, willing to try anything to find out what had happened to their two daughters, Jeremy and Jessica finally submitted to genetic testing. Two weeks after Cassie got home from the hospital, they received the results.

Cassie has a genetic mutation in both copies of the PPA2 gene, which means she received one mutation from each parent. A person can have one defective copy of the gene and go through their entire life without knowing it, but having two mutated copies puts a person at high risk of cardiac arrest in certain situations, according to Kierstein.

Cassie’s condition is so rare, Kierstein said, that only about 20 people in the world are known to have it.

“Because this condition was reported for the first time in 2016, we only have a few years of information on it,” said Kierstein, who is in her 24^th year at Children’s Hospital. “Most of the initial cases that were reported were, unfortunately, postmortem.”

So little is known about the PPA2-related disorder that no one can accurately estimate Cassie’s life expectancy. The oldest known person with this genetic anomaly is about 40, Kierstein said.

The Mehrings — including 8-year-old Autumn — monitor Cassie’s diet closely. She can’t have any condiments on her food. She can’t ingest any form of alcohol, including vanilla extract, cough syrup, or vinegar. If she goes to a birthday party, she can’t eat the cake.

And each day, Jeremy and Jessica must navigate the delicate balance between keeping their daughter safe and healthy, and allowing her to acquire at least some of the scrapes and bruises that are a normal part of childhood.

“Life’s a lot different now,” Jessica said.

Cassie still has some left-side weakness, but she doesn’t let it deter her from her normal daily activities. The surgically-implanted device in Cassie’s chest sends daily data to Children’s Hospital so the cardiac team can monitor her health remotely.

And she can climb the stairs by herself. Day, and night.

“She’s so lively, and strong, and resilient,” Jessica said. “And doctors are learning more and more about her condition. They’re thinking this might explain a lot of SIDS (Sudden Infant Death Syndrome) cases. I absolutely believe that Cassie is going to save lives.”

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