A novel homozygous truncating mutation of the SFRP4 gene in Pyle's disease.

To the Editor: Pyle’s disease (OMIM: %265900) is a rare autosomal recessive skeletal dysplasia, characterized by a massive expansion of metaphyseal trabecular bone with significant cortical thinning (1). These changes are particularly obvious in the distal part of the femora, showing an Erlenmeyer flask deformity. Mild cranial sclerosis and mild platyspondyly can also be observed (1, 2). Clinical phenotype includes genu valgum of variable severity, metaphyseal fractures because of cortical thinning, and teeth problems (prolonged retention, delayed eruption, misalignment, caries) (1, 2). The genetic cause of Pyle’s disease has remained elusive until the very recent report of mutations in SFRP4 in four affected patients (3). We report here the molecular characterization by whole genome sequencing (WGS) of a two-generation family harboring a novel SFRP4 mutation, confirming the causative role of this gene in Pyle’s disease. Patient 1 (VI4, Fig. 1) was born to Algerian healthy consanguineous parents. She presented at the age of 18months with genu valgum. Her occipitofrontal circumference (OFC), length and weight were normal (respectively, 47 cm, 0 SD; 84 cm, +1.5 SD; 10.4Kg, 0 SD). She had four normal teeth. Skeletal survey was consistent with Pyle’s disease (Fig. 1). Abdominal ultrasound scan, blood levels of calcium, phosphate, alkaline phosphatase, parathyroid hormone, and 25-hydroxy-vitamin D, acid beta-glucosidase enzymatic assay, and chromosomal microarray were all normal. Patient 2 (V3) presented at 17 years of age with patella instability and mild genu valgum. Her height and body mass index were, respectively, 157 cm (−1 SD) and 32 (+3.5 SD). She had prolonged retention of deciduous teeth requiring multiple extractions. Lower limbs radiographs showed striking similarities with those of her niece (Fig. 1). WGS was performed on a NextSeq500 (Illumina) after TruSeq Nano Library preparation and analyzed with BWA and GATK HaplotypeCaller. Over the 246,768 single nucleotide variants (SNVs) compatible with the recessive transmission hypothesis, 17,819 were rare (<1% in ExAC and 1000G). Of these, 38 were exonic or splice variants including only 3 truncating variations, absent from public databases, in 3 different genes. ZNF717 and LINC01535 are poorly known. SFRP4 is part of the family of secreted frizzled-related proteins which act as soluble decoy receptors for the Wnt ligand, playing a significant role in bone development (4). The recently described Sfrp4-deficient mice show striking similarities to patients affected by Pyle’s disease, such as expanded metaphyseal trabecular bone and significant cortical thinning (4). Therefore, the variant found in exon 1 of SFRP4 (Chr7:37955823TCG>T [hg19]; NM_003014.3: c.315_316delCG; p.Asp106Argfs*26) was considered to be a good candidate. Sanger sequencing confirmed the homozygous status in the two patients; all tested unaffected individuals (IV1, IV2, V1, V2, V4, V5, VI3) were heterozygous. While preparing this manuscript, we read with interest the article by Simsek Kiper et al. (3) reporting three mutations in exons 2 and 4 of SFRP4 in Pyle’s disease. Their experiments on Sfrp4-null mice showed that this gene inhibits endocortical osteoclastogenesis and bone resorption by inhibiting noncanonical Wnt and BMP signaling pathways in cortical bone; these effects were compartment-specific as they were not observed in trabecular bone (3). The three mutations previously described in two Turkish siblings and two unrelated patients, respectively, of Japanese and Indian origin (3), were all truncating and affecting the SFRP4 netrin-like domain. We report, in a family of Algerian origin, a two-nucleotide deletion affecting the SFRP4 frizzled (cysteine-rich) domain. Our study shows that Pyle’s disease is so far caused by SFRP4 truncatingmutations only. The predicted pathogenic effect can be caused by either nonsense-mediated mRNA decay or by the synthesis of a protein lacking the netrin-like domain. In conclusion, we present here the second report of biallelic SFRP4 truncating mutations in Pyle’s disease, which favors the hypothesis of genetic homogeneity in this condition and emphasizes the major role of SFRP4 in the development and modeling of the trabecular and cortical bone.