TRPV6 compound heterozygous variants result in impaired placental calcium transport and severe undermineralization and dysplasia of the fetal skeleton

Abstract

Transient receptor potential vanilloid 6 (TRPV6) functions in tetramer form for calcium transport. Until now, TRPV6 has not been linked with skeletal development disorders. An infant with antenatal onset thoracic insufficiency required significant ventilatory support. Skeletal survey showed generalized marked undermineralization, hypoplastic fractured ribs, metaphyseal fractures, and extensive periosteal reaction along femoral, tibial, and humeral diaphyses. Parathyroid hormone (PTH) elevation (53.4-101 pmol/L) initially suggested PTH signaling disorders. Progressively, biochemical normalization with radiological mineralization suggested recovery from in utero pathophysiology. Genomic testing was undertaken and in silico protein modeling of variants. No abnormalities in antenatal CGH array or UPD14 testing. Postnatal molecular genetic analysis found no causative variants in CASR, GNA11, APS21, or a 336 gene skeletal dysplasia panel investigated by whole exome sequencing. Trio exome analysis identified compound heterozygous TRPV6 likely pathogenic variants: novel maternally inherited missense variant, c.1978G > C p.(Gly660Arg), and paternally inherited nonsense variant, c.1528C > T p.(Arg510Ter), confirming recessive inheritance. p.(Gly660Arg) generates a large side chain protruding from the C-terminal hook into the interface with the adjacent TRPV6 subunit. In silico protein modeling suggests steric clashes between interface residues, decreased C-terminal hook, and TRPV6 tetramer stability. The p.(Gly660Arg) variant is predicted to result in profound loss of TRPV6 activity. This first case of a novel dysplasia features severe but improving perinatal abnormalities. The TRPV6 compound heterozygous variants appear likely to interfere with fetoplacental calcium transfer crucial for in utero skeletal development. Astute clinical interpretation of evolving perinatal abnormalities remains valuable in complex calcium and bone pathophysiology and informs exome sequencing interpretation.

Keywords: TRPV6 (transient receptor potential channel 6); hyperparathyroidism; placental calcium transfer; skeletal demineralization; skeletal dysplasia.

Figure 1

Clinical and radiological findings. (a) The bell‐shaped chest was associated with respiratory distress. (b–i) The skeletal survey at the age of 2 weeks showed generalized undermineralization, short, thin, and fractured ribs, absence of Wormian bones and normal vertebrae. The long bones showed a similar pattern of metaphyseal irregularities with corner fractures and periosteal reaction most obvious along the diaphyses of femora, tibiae and humeri. (j, k) Radiographs of chest and femur at the age of 10 weeks, showed broader, longer ribs, partial resolution of the metaphyseal lesions, and improved bone mineralization [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

Location of Gly660 and effect of p.(Gly660Arg) substitution. (a) Structure of the human TRPV6 tetramer in open form (PDB id 6bo8); chains A–D are colored cyan, yellow, gray, and pink respectively; atoms of Gly660 are shown in all subunits as spheres colored by atom type (white, carbon; blue, nitrogen; red, oxygen), and are labeled for subunits A and D. The complex is oriented to show view in the plane of the membrane, represented by the gray bar, with the cytoplasmic region at the top. (b) As A, but rotated to show the view from the cytoplasm; the ion channel lies at the center of the tetramer. (c) Detail of the interface between the C‐terminal hook of subunit A (cyan) and the N‐terminal helix of subunit B (yellow); the position of the Gly660 backbone is indicated, and side chains shown in stick format, colored by atom type, for other relevant residues; residues from subunits A and B are labeled in black or blue font, respectively. (d) As A, but showing detail of the p.(Gly660Arg) variant. Variant details: missense is c.1978G > C and nonsense is c.1528C > T. Genomic coordinates Chr7(GRCh37):g.142570162C > G and Chr7(GRCh37):g.142572288G > A) [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

The p.(Gly660Arg) variant disrupts the hydrophobic core of the C‐terminal hook. (a) Detail of the interface between the C‐terminal hook of subunit A and the N‐terminal helix of subunit B, as shown in Figure 2c except that chains are colored by hydrophobicity (red, most hydrophobic; white, most polar). (b) As A, except showing the p.(Gly660Arg) variant [Color figure can be viewed at wileyonlinelibrary.com]