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X-linked hypophosphataemia: genetics, diagnosis and management

Published on July 31, 2023   44 min

A selection of talks on Metabolism & Nutrition

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Hello, this is Tom Carpenter. I'm Professor of Pediatrics at the Yale University School of Medicine. I've had a career long interest in metabolic bone diseases in children, and I'm grateful today to the Henry Stewart group for asking me to talk about one of my favorite topics in this area, and that is the genetic disorder of X-linked type of phosphatemia, the most common form of inherited rickets that clinicians encounter.
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X-linked hypophosphatemia is the most common cause of inherited rickets. It's predominantly explained by excessive renal phosphate losses. It presents usually in young childhood with rickets and leg deformities. Osteomalacia also occurs, but usually not visible to the clinician. The skeletal deformity can be progressive, and short stature is also a common manifestation of this disorder. Other features that occur that are less well recognized as features of this disorder are dental abscesses, which can be frequent and devastating to overall lifelong dentition. And craniosynostosis may occur to variable extend so that the head shape may be abnormal and that rarely surgical intervention may be required. This photograph shows sisters, actually twins, one of which has this disorder. And you can see despite the same age, the affected girl has both legs, and it's much shorter. Biochemical findings seen in the disorder include low blood phosphate levels and decreased indices of renal phosphate retention. That is a decreased tubular reabsorption of phosphate and a reduced threshold maximum for phosphate as corrected by glomerular filtration rate or TMP/GFR. These low indices indicate that for a given serum phosphorus level, excessive amounts of phosphate are showing up in the urine. Also interestingly, there is a secondary biochemical abnormality that for many years has been unexplained, that is until recently. And that is that the circulating 1,25 dehydroxy vitamin D level, the activated metabolite of vitamin D tends to be low or normal but inappropriately so given the low blood phosphate level, which is a general stimulus for the production of this active metabolite.
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X-linked hypophosphataemia: genetics, diagnosis and management

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