Expanding the Prenatal Phenotypic Spectrum of TRPV6 Variants With Ocular Anomalies

Summary

• What is known?

  • ◦Genetic disorders of calcium/phosphate metabolism can cause prenatal bone disease, often leading to postnatal respiratory distress and with patients exhibiting bone improvement with calcium supplementation.

• What's new?

  • ◦We broaden the TRPV6 variant fetal phenotype with bilateral ocular involvement, mirroring postnatal hypocalcemia features.
  • ◦Prenatal bone anomalies in constitutional bone diseases may overlap with calcium/phosphate disorders; thus, gene screening for these disorders should be performed in all unexplained fetal skeletal dysplasia cases.

1. Fetal Phenotype

We report a 33‐year‐old primigravida with a female fetus born to non‐consanguineous parents. The mother's history included partial dental agenesis, which was also present in her relatives. The pregnancy was spontaneous, with normal first‐trimester screening and ultrasound. At 23.1 weeks of gestational age (weeks’GA), ultrasound revealed intrauterine growth restriction driven by markedly shortened femora and humeri (< 1%ile) associated with normal range abdominal and head circumference, fibular bowing, an irregular rib cage, and bilateral cataracts. Amniocentesis performed also at 23.1 weeks’GA confirmed a normal female karyotype (46, XX) and an unremarkable chromosome microarray (CMA) (Table 1).

TABLE 1.

Clinical data EFW: Estimated fetal weight.

Case	Parental details			Gestational age at diagnosis	Phenotypes	Obstetric history	Family history	Outcome
1	Maternal	Age	33	23.1 GA	–IUGR (EFW< 1%ile) –Short long bones (< 1%ile) –Abnormal bone shape –Abnormal rib cage morphology –Bilateral cataract	–Primigravida –Spontaneous pregnancy	– Maternal family history: Partial dental agenesis	TOP
		Ethnicity	Caucasian
	Paternal	Age	36
		Ethnicity	Caucasian

Abbreviations: GA: Gestational Age, IUGR: Intra‐Uterine Growth Restriction, TOP: Termination of Pregnancy.

2. Diagnostic Method

Genomic analysis included prenatal CMA on amniotic fluid, and post‐TOP trio‐Exome Sequencing (ES) and Genome Sequencing (GS) on fetal lung tissue and parental blood samples. Selection of fetal lung tissue biopsies for genomic studies was to ensure high DNA quality as defined by our molecular fetal pathology protocol. Sequencing and interpretation were performed in different laboratories using Illumina NovaSeq, achieving > 98% coverage (≥ 20x) for ES and > 96% for GS (GRCh38). Only ACMG‐classified pathogenic/similar pathogenic variants were reported. Familial segregation and variants were confirmed by Sanger sequencing (Supporting Information S1).

3. Diagnostic Results and Interpretation

Trio‐ES was performed first, as standard practice at the time, but was inconclusive. Trio GS identified compound heterozygous loss‐of‐function and likely pathogenic variants in TRPV6 (Table 2).

• Paternal variant: Absent from gnomAD v4.1.0, classified as likely pathogenic (ACMG: PM1, PM2, PM3, PP3, PP4), affecting a conserved α‐helix in the TRPV6 transmembrane segment.

• Maternal variant: Rare (gnomAD v4.1.0, allele frequency = 2.63e‐05), classified as likely pathogenic (ACMG: PM1, PM2, PM3, PP4), impacting the S3‐S4 extracellular loop of TRPV6.

TABLE 2.

Genetic findings.

Procedure	Direct/culture?	Performed test	Secondary confirmatory test	Gene	Known disease (OMIM)	Variant	ACMG classify‐cation	Criteria applied	Inheritance & zygosity	Interpret‐ation
Post‐natal (fetal lung)	Direct	Trio GS	Sanger sequencing	TRPV6	Hyperparathyroidism, transient neonatal MIM #618188	c.1465G > A: p. (Gly489Arg)	Likely pathogenic	–PM1 –PM2 –PM3 –PP4	–Inherited from the mother –Heterozygous (VAF: 55.6%)	2 likely pathogenic missense variants responsible for the clinical presentation
			Sanger sequencing	TRPV6	Hyperparathyroidism, transient neonatal MIM #618188	c.1286A > G: p.(Glu429Gly)	Likely pathogenic	–PM1 –PM2 –PM3 –PP3 –PP4	–Inherited from the father –Heterozygous (VAF: 40.9%)

Abbreviations: Arg: Arginine, Glu: Glutamic Acid, Gly: Glycine, Trio GS: Trio Genome Sequencing, VAF: Variant Allele Frequency.

The discrepancy of results between ES and GS is most likely due to a difference in classification of the maternal variant of TRPV6 (from VUS to likely pathogenic according to ACMG criteria).

4. Fetopathological Findings

The parents elected termination of pregnancy at 26.1 weeks’GA. Autopsy confirmed gestational age–appropriate biometry. External examination disclosed bilateral corneal opacities (Figure 1A–B) and low‐set ears. Internally, a prenatally undetected 5‐mm ostium secundum atrial septal defect and moderate right‐ventricular hypertrophy (4 vs. 2 mm in controls) were observed. Radiography showed global bone hypomineralization with shortened, bowed long bones, thin ribs, and tortuous clavicles (Figure 1D,‐E‐G). Ocular histopathology confirmed bilateral cataracts with a centrally hyalinized lens zone (Figure 1C). Femoral histology revealed a regular chondro‐osseous junction line but increased chondrocyte columns and downstream disorganized, thin trabeculae with excess osteoclasts (Figure 1F). The placenta was moderately hypotrophic, otherwise normal for gestational age both on gross and histological examination.

FIGURE 1.

Images from fetal ultrasound and fetopathological examination. (A) Transverse ultrasound section of the fetal cephalic pole passing through the eyes, showing homogeneous opacity and loss of the normal hypoechoic center of the lens, suggestive of congenital cataract. (B) Macroscopic appearance of the fetal right eye showing central corneal opacity. (C) Histology of the right eye confirming cataract with a central lens hyalinized/eosinophilic mass. No retinal or optic nerve anomalies were associated (HES, x0,5). (D) Longitudinal ultrasound section of the femur showing short biometrics without abnormal angulation or echogenicity. (E) Longitudinal ultrasound section of the leg bones demonstrating abnormal angulation in the frontal plane of the external leg bone (fibula). (F) Histology of femoral meta/diaphysis showing regular chondro‐osseous junction line, high chondrocyte colonnes, and slim trabeculae in the osseous zone. (HES, x2). (G) Whole‐body radiograph demonstrating global bone hypomineralization, bowing of the long bones, extremely thin ribs, and tortuous clavicles.

5. Discussion

TRPV6 encodes a calcium‐selective ion channel essential for placental and intestinal calcium transport [1]. During pregnancy, calcium fetal transfer peaks in the second and third trimesters, which explains the detection of skeletal anomalies in affected fetuses [2]. TRPV6 variants result in a prenatal phenotype resembling more frequent constitutional bone diseases such as osteogenesis imperfecta (OI). Affected patients present with global hypomineralization, long bone bowing, and polyhydramnios [3] (no cases of dental anomalies or agenesis have been reported). A key novelty in this case is the first description of bilateral cataract associated with biallelic loss‐of‐function TRPV6 variants, a finding usually linked to hypocalcemia yet with an unclear pathophysiological mechanism [4].

TRPV6 biallelic defects differ from OI postnatally: affected newborns present with transient hyperparathyroidism, which normalizes with calcium supplementation, and often show resolution of skeletal anomalies (observed in 6 of 8 surviving cases reported [3, 5, 6]). Unlike severe OI, the prognosis of TRPV6 biallelic defects appears more favorable, as most of live‐born cases in the literature survived with respiratory support at birth, except one death from apparently unrelated complications (volvulus with multiorgan decompensation [5]). However, early‐onset fetal disease with previously unreported phenotypic features, combined with the limited number of cases in the literature, warrants caution when discussing postnatal prognosis with parents. Postnatal follow‐up of severe prenatal cases receiving early neonatal calcium supplementation should be further explored.

Other uncertainties persist. The mechanism by which placental TRPV6 dysfunction causes fetal hypocalcemia, and yet allows postnatal recovery through oral calcium supplementation, despite presumed intestinal TRPV6 dysfunction, remains unexplained. Hypotheses include compensatory intestinal paracellular absorption. Additionally, the link between hypocalcemia and cataract formation in TRPV6 biallelic defects remains poorly understood.

6. Conclusion

This case further expands the phenotype of TRPV6 biallelic defects. Adding calcium and phosphate metabolic disrupting genes to prenatal skeletal dysplasia anomaly exploration is essential. An accurate differentiation between these pathologies is critical to provide precise prenatal counseling, prognosis, and postnatal management, as well as organization for future pregnancy genetic testing.

Funding

The authors have nothing to report.

Ethics Statement

In accordance with French law, single case reports are not considered research involving human participants and thus do not require review by an ethics committee.

Consent

Written informed consent was obtained from the parents.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting Information S1

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.