Hypophosphatemic rickets caused by a novel splice donor site mutation and activation of two cryptic splice donor sites in the PHEX gene

Abstract

X-linked hypophosphatemic rickets (XLH) is the most common inherited form of rickets. XLH is caused by inactivating mutations in the PHEX gene and is transmitted as an X-linked dominant disorder. We investigated PHEX mutation in a sporadic Turkish girl with hypophosphatemic rickets. The patient was 2 years of age with a complaint of inability to walk. She had bowing of legs and growth retardation. Laboratory data showed normal calcium, low phosphate with markedly elevated ALP, and low phosphate renal tubular reabsorption. She was treated with Calcitriol 0.5 mg/kg/day and oral phosphate supplement with good response. The entire coding region of PHEX gene was sequenced from patient's peripheral leukocyte DNA and a novel 13 bp deletion at the donor splice site of exon5 was found (c.663+12del). Instead of using the donor splice site of intron 4 to splice out exon 5 and intron 5, the spliceosome utilized two nearby cryptic donor splice sites (5' splice site) to splice out intron 4, resulting in two smaller transcripts. Both of them could not translate into functional proteins due to frameshift. Her parents did not carry the mutation, indicating that this is a de novo PHEX mutation likely resulting from mutagenesis of X chromosome in paternal germ cells. We conclude that c.663+12del is a novel mutation that can activate nearby cryptic 5' splice sites. The selection of cryptic 5' splice sites adds the complexity of cell's splicing mechanisms. The current study extends the database of PHEX mutation and cryptic 5' splice sites.

Genetic Causes of Rickets

“…In the Turkish population, PHEX mutation is also the most common cause of HR, accounting for 87% cases ( 55 , 70 , 71 ). De novo mutations are frequent and more often occur in female patients, likely resulting from mutagenesis of the X chromosome in paternal germ cells ( 70 ).…”

“…Autosomal dominant HR (ADHR, MIM 193100) is caused by gain-of-function mutations in the proteolytic cleavage domain of FGF23 (R176XXR179, MIM 605380). Mutations that alter the arginine (R) residue at the position 176 or 179 would render the protein resistant to proteolytic cleavage and lead to increased serum levels of FGF23 and its activity, resulting in hypophosphatemia ( 61 , 71 , 72 ). It is less common than XLHR and 16 different mutations are reported in HGMD (accessed Nov 13, 2017).…”

Phosphorylation of Extracellular Bone Matrix Proteins and Its Contribution to Bone Fragility

“…Notably, hypophosphatemia is a genetically heterogeneous disease and several other genetic and acquired syndromes of hypophosphatemia other than Raine syndrome have also been described . For example, hypophosphatemic rickets known as X‐linked hypophosphatemia (XLH) is considered to be a systemic disorder in humans from mutation of the phosphate‐regulating gene homologous to endopeptidases on the X chromosome (PHEX) . Typical characteristics of XLH are impaired growth and rickets of the femur and tibia.…”

Clinical and genetic characteristics of 15 families with hereditary hypophosphatemia: Novel Mutations in PHEX and SLC34A3

“…De novo PHEX mutations were found in 6 out of 12 families. Together with our previous studies [ 30 – 32 ], we have investigated 50 patients from 31 unrelated families with hereditary hypophosphatemia, representing the largest series of patients from Turkish population. Among the 31 families, PHEX , FGF23 , CLCN5 , and SLC34A3 mutations was found in 26 (83.9%), 1 (3.2%), 1 (3.2%), and 1 (3.2%) families, respectively.…”

“…In our previous studies, 27 patients from 16 unrelated Turkish families were investigated [ 30 – 32 ]. In the present study, we analyzed clinical and genetic characteristics of additional patients from 15 unrelated families from a different region of Turkey.…”