A case of severe x-linked hypophosphatemia caused by a novel phex mutation

CASE REPORTS

A case of severe X-linked hypophosphatemia caused by a novel PHEX mutation

Madalena Almeida Borges¹, Maria Costa¹, Rute Baeta Baptista¹, Ana Laura Fitas², Telma Francisco¹, Margarida Abranches¹.

¹Pediatric Nephrology Unit, Hospital Dona Estefânia, Centro Hospitalar Universitário de Lisboa Central, Lisbon, Portugal,
²Pediatric Endocrinology Unit, Hospital Dona Estefânia, Centro Hospitalar Universitário de Lisboa Central, Lisbon, Portugal.

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Abstract
X-linked hypophosphatemia (XLH) is the most common form of genetic rickets and it is caused by mutations in the phosphate regulating endopeptidase homolog X-linked (PHEX).
We present a case of a five-year-old girl with progressive limb deformities since the age of two. Upon observation the patient presented with enamel defects, absent superior incisive teeth, genu valgum, severe tibial curvature, widening of metaphyseal ends and height-for-age z-score below -3. Blood analysis revealed elevated alkaline phosphatase, hypophosphatemia, borderline low serum calcium levels, low 25-hydroxyvitamin-D, high 1,25-dihydroxycholecalciferol and parathormone; low tubular reabsorption of phosphate and a low ratio of tubular maximum reabsorption of phosphate to glomerular filtration rate. Radiographs showed metaphyseal widening, diaphyseal trabeculation, rarefaction and pseudofractures. She started support treatment and underwent orthopedic surgery with partial improvement in growth. At the age of eight, a novel heterozygous PHEX mutation (c.156_174del) was identified and she started burosumab with improvement.
In this case, there was a significant delay in the genetic diagnosis and appropriate treatment, therefore prompt referral is essential to improve the prognosis. The type of mutation may be associated with the clinical presentation, however, other factors, such as coexisting vitamin D deficiency, can explain this case severity.

Why this article important?

In our manuscript, we report a severe case of X-Linked Hypophosphatemia (XLH) due to a novel PHEX mutation. We believe that this manuscript is important because XLH is a rare disease that has a heavy effect on children’s health and growth. It is important to be aware of this condition to allow an early diagnosis and therefore the appropriate and target treatment initiation, thereby improving clinical outcomes. We also raise awareness for the importance of genetic testing to clarify the diagnosis, allowing the beginning of targeted therapy. In our case a novel heterozygous truncating mutation in PHEX gene (c.156_174del) was identified, raising the question if this truncating mutation may have played a role in this case severity.

Summary of article

Dear Editor, We wish to submit a manuscript entitled “A case of severe X-linked hypophosphatemia caused by a novel PHEX mutation” for consideration in the Pediatric Oncall Journal as a case report. We confirm that this work is original and has not been published elsewhere, nor is it currently under consideration for publication elsewhere. In this paper, we report a severe case of X-Linked Hypophosphatemia (XLH) due to a novel PHEX mutation. We believe that this manuscript is appropriate for publication in the Pediatric Oncall Journal because XLH is a rare disease that has a heavy effect on children’s health and growth. It is important to be aware of this condition to allow a prompt referral, an early diagnosis, and therefore the appropriate and target treatment initiation, improving clinical outcomes. Thank you for your consideration of this manuscript. Sincerely, Madalena Almeida Borges

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