The paradoxical coexistence of hypophosphatemic rickets and increased bone density in spine of a subject carrying a novel splice site mutation in PHEX.

To the Editor: We reported a 46-year-old man presented with progressive pain in his left lower extremities for more than 5 years. Physical examination showed short stature, “O”-type bending in his lower extremities, and “penguin” gait. The family tree of the proband's pedigree is shown in Figure 1A, and the X-rays of his skull are shown in Figure 1B. X-rays found increased bone mass in his spine [Figure 1C], and left femoral fracture with sclerosis at the fracture edge [Figure 1D and 1E]. Dual-energy X-ray absorptiometry (DEXA) scan measured in 2018 revealed increased bone mass in his spine [Supplementary Table 1]. Notably, the second DEXA scan performed in July, 2019 showed that the bone mass in his spine was decreased and the bone mass in his femoral neck was increased. His mother (I-2), brother (II-2), and one of his sisters (II-5) also presented with short stature, abnormal lower extremity shape and gait. The X-rays of II-2 and II-5 are shown in Supplementary Figure 1. The DEXA scan of II-2 also found increased bone mass in his spine, while with decreased bone mass in his femoral neck; II-5 had normal bone mass in her spine, and the bone mass in her femoral neck showed a tendency to decrease; the proband (II-7), III-3, and III-9 had hypophosphatemia and elevated parathyroid hormone levels [Supplementary Table 1]. Their serum calcium levels were in the lower limit of normal range. The serum phosphate levels of II-2 and II-5 were unavailable. The serum levels of fibroblast growth factor 23 (FGF23) in II-2, II-5, II-7, III-3, and III-9 were increased. II-7 and III-3 had lower tubular maximum re-absorption threshold of phosphate per glomerular filtration rate [Supplementary Table 1].

Figure 1

Genetic analysis and X-rays of the proband. (A) Family tree of the proband's pedigree. The arrow denotes the proband (II-7), solid symbols represent the affected individuals (I-2, II-2, II-5, II-7, III-3, III-9, and IV-6), and symbols with a diagonal line across denote deceased individuals (I-1 and I-2). (B) X-rays of the proband's skull. (C) Increased bone mass was evident in the proband's spine. (D, E) Left femoral fracture with sclerosis at the fracture edge. (F) Genetic analysis of the affected individuals. A novel splice-site mutation (c.1483-1G>C) in intron 13 of the PHEX gene was found in the proband, II-2, II-5, III-3, III-9, and IV-6. II-2 and II-7 were hemizygous for this mutation, while II-5, III-3, III-9, and IV-6 were heterozygous carriers.

Whole-exome sequencing (WES) was performed for the proband, II-2, and II-5. A novel splice site mutation (c.1483-1G>C) in intron 13 of the phosphate-regulating endopeptidase (PHEX) gene was found in the proband, II-2, and II-5 by WES [Figure 1F]. This is a splice acceptor mutation that has not previously been reported in the Online Mendelian Inheritance in Man (OMIM), Human Gene Mutation Database or Clinvar databases, nor has it been published in population databases such as 1000 Genomes, dbSNP, Exome Variant Server, or the ExAC Browser. According to the American College of Medical Genetics and Genomics guidelines (2015), this mutation is predicted to be pathogenic (PVS1 + PM2 + PP1 + PP3).[ This mutation was also found in III-3, III-9, and IV-6. II-2 and II-7 were hemizygous for this mutation, while II-5, III-3, III-9, and IV-6 were heterozygous carriers of this mutation [Figure 1F].

This is a classic X-linked hypophosphatemic rickets (XLH, OMIM 307800) family, and the mutant allele was speculated to be inherited from I-2. In this pedigree, a novel splice-site mutation was identified and predicted to be the causative mutation based on the following findings: it is a novel mutation that is absent from multiple public databases; it locates in the splice acceptor site; it is predicted to be deleterious by different in silico prediction programs; and in this family, this mutation co-segregates with elevated FGF23 levels and hypophosphatemia.

In this pedigree, it is notable that the proband and his brother (II-2) had increased bone mass in their vertebral body, despite the existence of clinical manifestations of rickets. Previous studies have suggested a site-dependent influence of XLH on bone mass. Differential effects of XLH on trabecular bone vs. cortical bone have been found in affected children and adults.[ In patients with XLH, axial bone mass tends to be increased, while peripheral bone mass tends to be decreased.[ Consistent with previous studies, the proband and his brother showed increased T and Z values of the spine and decreased T and Z values in the femoral neck when compared with the reference values.[ The increased axial bone mass observed in patients with XLH is partially attributable to hyperosteoidosis.[ Treatments including calcitriol and phosphate supplementation may play a role in the increased trabecular bone mass observed; however, increased trabecular bone volume has also been found in several untreated subjects, indicating that it may be an intrinsic change caused by the disease process of XLH.[

In conclusion, we reported an XLH pedigree carrying a novel splice-site mutation (c.1483-1G>C) in intron 13 of PHEX, and the proband and his brother showed significantly elevated bone mass in their spine and the clinical features of hypophosphatemic rickets.

Declaration of patient consent

All participants provided their written informed consent before participating in the study. In the forms, all patients have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Funding

This work was supported by grants from the National Natural Science Foundation of China (Nos. 81770875 and 81572639), the Science and Technology Department of Sichuan Province (No. 2018SZ0142), Sichuan University (No. 2018SCUH0093), and the National Clinical Research Center for Geriatrics of West China Hospital (No. Z2018B05).

Conflicts of interest

None.