When Eliana was born, her mother, Sally, knew to be on the alert for signs of X-linked hypophosphatemia, a rare genetic disorder that can affect bone development and strength. That’s because Sally has the condition herself. She also knew where to turn for diagnosis and treatment. Michael Levine, MD, the endocrinologist who had treated her as a child in Ohio, was now at Children’s Hospital of Philadelphia (CHOP) and was still treating her as an adult.

Eliana is Sally and Mark’s second child. When their first child, William, was born, they’d had him tested for X-linked hypophosphatemia, a rare condition that Sally has lived with and managed with medication since childhood. Untreated, the condition can lead to malformed bones, especially in the legs, difficulty walking and vulnerability to bone fractures. With treatment, starting early in life, those problems can be avoided.

The “X-linked” in the condition’s name refers to its pattern of genetic inheritance. A son or daughter born to a mother with the condition has a 50% chance of inheriting it. (When a father has an X-linked condition, all his daughters and none of his sons will inherit it.)

Early connection with a specialist in rare diseases

William didn’t have the condition, fortunately, but the experience of having him tested turned out to be a useful practice run for the parents. When Eliana (Ellie) was born two years later, they knew when and how to have her tested. Where and by whom were never in question. “I just called and made an appointment with Dr. Levine,” remembers Sally.

Dr. Levine has extensive experience in working with children and adults with rare endocrine disorders. He has special expertise in diagnosing and treating disorders that affect bone development, including hypophosphatemia. When CHOP created the Center for Bone Health in 2006, specifically to care for children with a broad spectrum of bone conditions, he was named its Medical Director.

Not only had Dr. Levine treated Sally as a child — he’d treated her mother, too. When Sally moved to central Pennsylvania as an adult, she was pleased to find that Dr. Levine was practicing at CHOP and that he could again be her specialist. Specialists in rare conditions, especially those that begin in childhood, see adult patients at CHOP. The four-hour drive to the appointments was well worth the extra effort.

The value of specialist experience

“When you’re dealing with a rare condition,” Sally explains, “you want to find a specialist who has experience with it — someone who’s treated multiple patients. Treatments that might be effective for other conditions with similar symptoms can cause permanent harm in people with X-linked hypophosphatemia.

“At one point, I tried seeing a specialist who was nearer. She was a good endocrinologist, but she didn’t have expertise in this condition. I immediately switched back to Dr. Levine after one appointment.”

Diagnosis in infancy

Even before he saw Ellie as a patient, Dr. Levine coordinated with the family in ordering a genetic test to look for the gene change that would confirm a diagnosis of X-linked hypophosphatemia. Results of the test came back when Ellie was just 6 weeks old: She had inherited the condition.

Ellie had her first appointment with Dr. Levine when she was 2 months old. Blood work and urine tests at 4 months showed the characteristic signs of X-linked hypophosphatemia: low levels of phosphate in the blood and high levels of phosphate in the urine. The combination indicates “phosphate wasting,” in which excessive amounts of phosphate are filtered out of the blood by the kidneys and excreted in the urine, depriving the bones and teeth of the minerals they need to grow and harden.

X-linked hypophosphatemia

An early start to treatment

Dr. Levine started Ellie on phosphate and vitamin D supplements, both in liquid form. Luckily, she liked the taste — she needed seven doses a day. The supplements helped to raise the phosphate concentration in her blood, reducing the risk of problems with her bones and teeth.

When she was 1 year old, Ellie was put on burosumab (Crysvita^®), a monoclonal antibody treatment approved by the Food and Drug Administration in 2018 for X-linked hypophosphatemia. Burosumab works by suppressing the hormone FGF23, which is responsible for the over-filtration of phosphate by the kidneys. The medication gets at the root cause of the disorder, so phosphate levels in the bloodstream are maintained with a normal diet, without the need for supplements.

Here, too, Sally was a well-informed parent. She’d been taking burosumab herself since it was approved. Knowing it was available and effective had been a factor in the couple’s decision to have children.

“She walks everywhere now.”

Almost 2 now, Ellie is doing well. She gets an injection of burosumab every two weeks. She also gets regular tests to monitor the levels of phosphate and other minerals in her blood.

Where children with X-linked hypophosphatemia are typically diagnosed after they start walking, when weakness and malformations of the legs become noticeable, Ellie’s early diagnosis and treatment have meant that she didn’t experience those effects. Her legs are straight and strong, and she started walking at a year old.

“She walks everywhere now,” says Sally. “She climbs the stairs, climbs the banisters on the stairs, and loves to jump. She goes to her older brother’s gymnastics class and thinks that she’s part of the class, too. She does almost everything they do: the forward roll, jumping on the trampoline, the obstacle course. She doesn’t have any idea that she has a disorder. And that makes me very happy.

“Dr. Levine is the best doctor I ever had. While I never wanted Ellie to inherit my condition, it is wonderful that she’s getting care from Dr. Levine. She’s in great hands.”