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Biallelic pathogenic variants in SLC26A2 are conclusively associated with achondrogenesis type IB, a prenatally lethal autosomal‑recessive chondrodysplasia within the SLC26A2‑related skeletal dysplasia spectrum. Multiple unrelated fetuses and neonates with classic radiographic and histopathological features of achondrogenesis type IB (ACG1B) harbor either homozygous or compound‑heterozygous SLC26A2 loss‑of‑function or severe missense variants, with unaffected parents being obligate heterozygous carriers and occasionally confirmed heterozygous siblings.PMID:8571951, PMID:9637425, PMID:11241838, PMID:18708426, PMID:31880411, PMID:36007841 Experimental data show that these variants abolish or severely reduce sulfate transport, leading to undersulfated cartilage proteoglycans and recapitulation of severe chondrodysplasia in cellular and animal models.PMID:8931695, PMID:15294877, PMID:11457925, PMID:15703192, PMID:17120769 Collectively, the gene–disease relationship meets ClinGen “Definitive” criteria, with genetic and functional evidence at or above standard scoring caps and consistent replication over decades.
Mode of inheritance and severity. Achondrogenesis type IB is inherited in an autosomal recessive manner. All clearly described ACG1B cases carry biallelic SLC26A2 variants, while heterozygous carriers are clinically normal.PMID:8571951, PMID:9637425, PMID:11241838, PMID:31880411 The phenotype is prenatally or perinatally lethal, characterized by severe micromelia, markedly shortened ribs, poor vertebral ossification, and often hydrops fetalis.PMID:8571951, PMID:9637425, PMID:11241838
Case reports and segregation. A Japanese fetus with classic ACG1B had homozygous c.1011TGT[3] (p.Val341del) with both parents and a healthy brother heterozygous, supporting autosomal‑recessive segregation.PMID:9637425 Two unrelated Japanese pedigrees with ACG1B showed homozygous c.1987G>A (p.Gly663Arg) in affected fetuses and heterozygosity in parents, and linkage analysis demonstrated a 75‑kb homozygous haplotype compatible with a founder effect.PMID:31880411 Historical cohorts of lethal SLC26A2‑related chondrodysplasias include multiple ACG1B individuals with biallelic truncating or severe missense variants; in large mutation reviews, c.1011_1013delGTT (p.Val341del) and c.532C>T (p.Arg178Ter) are repeatedly associated with lethal phenotypes.PMID:8571951, PMID:11241838, PMID:15294877, PMID:36007841 An Indian prenatal series of lethal skeletal dysplasias identified homozygous frameshifts c.796dup (p.Thr266AsnfsTer16) and c.1724del (p.Thr266AsnfsTer16) as ACG1B‑causing, plus compound heterozygosity for c.532C>T (p.Arg178Ter) with c.1382C>T (p.Ala461Val) in severe phenotypes at the lethal end of the spectrum.PMID:36007841
Given the multiple independent families, consistent biallelic segregation, and absence of disease in heterozygotes, the cumulative genetic evidence is “Strong” and likely exceeds ClinGen’s maximum when fully tallied.
ACG1B is primarily caused by variants that are predicted or demonstrated to confer near‑null SLC26A2 function. These include numerous frameshift and nonsense variants, severe splice alterations, and specific missense changes with absent transport activity. Representative variants directly associated with ACG1B or the lethal end of the SLC26A2 spectrum include:
Frameshift / truncating variants
c.796dup (p.Thr266AsnfsTer16) – homozygous in lethal ACG1B.PMID:36007841c.1724del (p.Thr266AsnfsTer16) – homozygous in lethal ACG1B.PMID:36007841c.15_19del (p.Ser5fs), c.69del (p.Pro24fs), c.78_88dup (p.Glu30fs), c.100del (p.Glu34fs), c.255del (p.Asn87fs), multiple exonic deletions/duplications such as c.705_711del (p.Met236fs), c.705_711del (p.Met236fs), c.1155del (p.Asp385fs), c.1242_1245del (p.Asn415fs), and diverse distal frameshifts (for example c.1714del (p.Leu572fs), c.2017_2018del (p.Asp673fs), c.2124_2125dup (p.Phe709fs)) are recurrent in lethal ACG1B/AO2 series and typically act as null alleles.PMID:8571951, PMID:11241838, PMID:15294877c.532C>T (p.Arg178Ter) – strongly associated with severe phenotypes, including lethal forms when combined with other severe alleles.PMID:11241838, PMID:36007841In‑frame and missense variants with severe effect
c.1011TGT[3] (p.Val341del) – homozygous in a Japanese ACG1B fetus and recurring in severe cases; functional data indicate absent sulfate transport.PMID:9637425, PMID:15294877c.1987G>A (p.Gly663Arg) – homozygous cause of ACG1B in two Japanese families, with founder haplotype; loss of membrane localization and transport shown in vitro.PMID:31880411, PMID:16642506c.1382C>T (p.Ala461Val) and similar) are predicted and modeled to destabilize transmembrane helices and the STAS domain in lethal phenotypes.PMID:36007841, PMID:28941661Splice‑site variants
c.-26+2T>C (“Finnish mutation”) is common in Finland and, when paired with severe alleles such as c.1535C>A (p.Thr512Lys) or truncating variants, shifts the phenotype toward severe DTD or de la Chapelle dysplasia, bordering the lethal ACG1B/AO2 end.PMID:18708426, PMID:11241838Across cohorts, more than 50 distinct pathogenic or likely‑pathogenic SLC26A2 variants have been compiled, with genotype–phenotype correlations showing that ACG1B arises when both alleles are functionally null or extremely hypomorphic.PMID:11241838, PMID:15294877, PMID:36007841
SLC26A2 encodes a sulfate/chloride antiporter of the plasma membrane essential for inorganic sulfate uptake required for proteoglycan sulfation in cartilage. Primary fibroblasts and chondrocytes from patients with SLC26A2‑related chondrodysplasias, including lethal phenotypes, show markedly reduced uptake of inorganic sulfate and undersulfation of proteoglycans, directly linking transporter dysfunction to extracellular matrix abnormalities.PMID:8931695, PMID:11241838
In HEK‑293 and CHO cell expression systems, wild‑type SLC26A2 localizes to the plasma membrane and mediates robust sulfate transport, whereas many ACG1B‑associated variants behave as loss‑of‑function alleles: truncating variants abolish expression or surface localization; p.Val341del and p.Gly663Arg result in severely reduced or absent sulfate uptake and retention in intracellular compartments.PMID:15294877, PMID:16642506 N‑glycosylation studies show that pathogenic variants are often retained in the endoplasmic reticulum via calnexin and fail to reach the cell surface, further supporting a mechanism of loss of correctly folded, trafficked transporter.PMID:30462520 Cryo‑EM structures of SLC26A2 delineate the anion‑binding site and demonstrate how many lethal‑spectrum substitutions affect substrate coordination or structural stability, providing a structural framework consistent with null or near‑null transport activity.PMID:38684689
Animal models reinforce the human data. A knock‑in mouse with partial Slc26a2 loss‑of‑function develops generalized skeletal dysplasia with growth retardation, delayed ossification, chondrocyte disorganization, and cartilage proteoglycan undersulfation, mirroring human SLC26A2‑related disease and demonstrating that SLC26A2 deficiency is sufficient to cause a chondrodysplasia phenotype.PMID:15703192, PMID:17120769 More recent conditional mouse models show that SLC26A2 deficiency leads to growth‑plate abnormalities that can be partially ameliorated by pharmacologic FGFR3 inhibition, indicating that aberrant FGFR3 signaling lies downstream of sulfate‑transport deficiency and underscored as a potential therapeutic axis for non‑lethal forms.PMID:38282752
Collectively, the experimental evidence—comprising expression, transport assays, structural modeling, and in vivo models—clearly supports loss‑of‑function (functional null/hypomorphic) as the mechanism for ACG1B and fulfills a “Strong” tier of functional support under ClinGen criteria.
No studies directly refute the association between biallelic SLC26A2 variants and ACG1B. Instead, data emphasize a continuous phenotypic spectrum: milder mutations (for example, c.835C>T (p.Arg279Trp), c.1957T>A (p.Cys653Ser)) cause recessive multiple epiphyseal dysplasia or diastrophic dysplasia, whereas combinations of one null and one partial‑function allele yield intermediate phenotypes such as atelosteogenesis type II or classic DTD.PMID:8931695, PMID:11241838, PMID:15294877, PMID:21155763, PMID:36140680 This reinforces rather than contradicts the causal role of SLC26A2. A separate autosomal‑dominant association with dysplastic spondylolysis has been proposed for heterozygous SLC26A2 missense variants, but this pertains to a different phenotype and does not conflict with the recessive, lethal mechanism underlying ACG1B.PMID:26077908
The combination of extensive human genetic data, clear autosomal‑recessive segregation, robust functional loss‑of‑function evidence, and concordant animal models establishes a Definitive association between biallelic SLC26A2 variants and achondrogenesis type IB. For prenatal or perinatal diagnostic work‑ups of lethal micromelic skeletal dysplasia with characteristic radiologic features, targeted SLC26A2 analysis is strongly warranted, especially in populations with known founder alleles such as p.Val341del (Japan) or haplotypes involving severe truncating variants.
Key take‑home sentence: Biallelic loss‑of‑function or equivalently severe SLC26A2 variants cause autosomal‑recessive, prenatally lethal achondrogenesis type IB through failure of sulfate transport and consequent cartilage proteoglycan undersulfation, a relationship that is now clinically and experimentally definitive.
Gene–Disease AssociationDefinitiveMultiple unrelated fetuses and neonates with classic achondrogenesis type IB harbor biallelic SLC26A2 variants; segregation in several families with unaffected heterozygous relatives; strong and consistent functional evidence, animal models, and structural data over >25 years. Genetic EvidenceStrongBiallelic pathogenic or likely-pathogenic variants (frameshift, nonsense, splice, severe missense, in-frame deletions) reported in numerous independent ACG1B or lethal-spectrum probands across multiple populations, with clear autosomal-recessive segregation and consistent genotype–phenotype correlations showing ACG1B when both alleles are functionally null or extremely hypomorphic. Functional EvidenceStrongPatient cells demonstrate markedly reduced sulfate uptake and undersulfated proteoglycans; expression studies show ACG1B-associated variants abolish or severely reduce sulfate transport and/or membrane localization; transgenic and knock-in mouse models with Slc26a2 deficiency recapitulate chondrodysplasia and cartilage undersulfation; cryo-EM and modeling data mechanistically explain how lethal-spectrum variants disrupt transporter function. |