Could the ENPP1 p.D85H Mutation be Associated with Hypophosphatemic Rickets?

Objective: A 35-year-old Turkish male patient was referred to us with a year-long history of joint paint and congenital hearing loss. Family history revealed more family members with hearing loss without paraneoplastic syndrome. These findings led us to investigate the genetic alterations associated with familial hypophosphatemia, which revealed an ENPP1 mutation. Methods: Serum samples were obtained after 12-hour fasting. The mutation analysis was performed using previously described primers. Total RNA was isolated from blood samples using Qiagen Total RNA extraction mini kit. cDNA samples were amplified using polymerase chain reaction (PCR), and these PCR products were purified using commercial kits. Following amplification and purification, the PCR products were sequenced. Results: The patient was found to have hypophosphatemia, a high level of PTH, and elevated plasma alkaline phosphatase. Sequencing results revealed an ENPP1 p.D85H mutation. Conclusion: We present the identification of an inactivating mutation in the ectonucleotide pyrophosphatase/phosphodiesterase-1. The substituted amino acid residue is highly conserved in ENPP1. At present, we have no further explanation, but our results suggest that ENPP1 p.D85H mutation may be associated with hypophosphatemic rickets accompanied by hearing loss.