IncreasedTBX6gene dosages induce congenital cervical vertebral malformations in humans and mice

Abstract

BackgroundCongenital vertebral malformations (CVMs) manifest with abnormal vertebral morphology. Genetic factors have been implicated in CVM pathogenesis, but the underlying pathogenic mechanisms remain unclear in most subjects. We previously reported that the human 16p11.2 BP4-BP5 deletion and its associated TBX6 dosage reduction caused CVMs. We aim to investigate the reciprocal 16p11.2 BP4-BP5 duplication and its potential genetic contributions to CVMs.Methods and resultsPatients who were found to carry the 16p11.2 BP4-BP5 duplication by chromosomal microarray analysis were retrospectively analysed for their vertebral phenotypes. The spinal assessments in seven duplication carriers showed that four (57%) presented characteristics of CVMs, supporting the contention that increased TBX6 dosage could induce CVMs. For further in vivo functional investigation in a model organism, we conducted genome editing of the upstream regulatory region of mouse Tbx6 using CRISPR-Cas9 and obtained three mouse mutant alleles (Tbx6up1 to Tbx6up3) with elevated expression levels of Tbx6. Luciferase reporter assays showed that the Tbx6up3 allele presented with the 160% expression level of that observed in the reference (+) allele. Therefore, the homozygous Tbx6up3/up3 mice could functionally mimic the TBX6 dosage of heterozygous carriers of 16p11.2 BP4-BP5 duplication (approximately 150%, ie, 3/2 gene dosage of the normal level). Remarkably, 60% of the Tbx6up3/up3 mice manifested with CVMs. Consistent with our observations in humans, the CVMs induced by increased Tbx6 dosage in mice mainly affected the cervical vertebrae.ConclusionOur findings in humans and mice consistently support that an increased TBX6 dosage contributes to the risk of developing cervical CVMs.

Clinical characterization of individuals with the distal 1q21.1 microdeletion

“…These findings were revealed by clinical ES or genome‐wide microarray analysis. Two unrelated subjects had duplications in the 16p11.2 region, which have been linked to developmental delay, congenital anomalies, microcephaly, and seizures (Ren et al, 2020; Shinawi et al, 2010). Two subjects (full siblings) had 11q13.4 deletions, which include exons 8–10 of SHANK2 , an autism susceptibility gene (Berkel et al, 2010).…”

Clan genomics: From OMIM phenotypic traits to genes and biology

“…This unique combination of rare variants located (Liu, Wu, et al, 2019). Interestingly, when a noncoding 'up-expression' allele, that is, a hypermorph, is paired with a null allele the vertebral anomalies localize to the cervical spine (Ren et al, 2020). (d) Penetrance of CAKUT, congenital anomalies of the kidney and urinary tract and unilateral renal agenesis, can be related to gene action and gene dosage/expression at "the TBX6" locus La Reunion paradox (Beckmann, 1996;Zlotogora et al, 1996) in human genetics.…”

Chiari 1 malformation and exome sequencing in 51 trios: the emerging role of rare missense variants in chromatin-remodeling genes

“…It is, therefore, logical to hypothesize that the situation of compound heterozygosity for KMT2E missense variants, a low frequent and a rare one, caused alterations only in a limited embryogenetic field, in this specific case the cranio-spinal junction. Indeed, there are numerous examples which demonstrate that the quantity of residual protein shapes the final phenotype, with rare biallelic LoF variants causing early lethality with aberration in numerous embryonic fields, and the combination of a rare and a low-frequent variant causing malformations limited to specific organs (Ren 2020).…”