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Functional analysis of two novel TBX5 variants present in individuals with Holt–Oram syndrome with different clinical manifestations

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Abstract

Holt–Oram syndrome (HOS) is a rare disorder characterized by cardiac and upper-limb defects. Pathogenic variants in TBX5—a gene encoding a transcription factor important for heart and skeletal development—are the only known cause of HOS. Here, we present the identification and functional analysis of two novel TBX5 pathogenic variants found in two individuals with HOS presenting distinct phenotypes. The individual with the c.905delA variant has a severe cardiac phenotype but mild skeletal defects, unlike the individual with the c.246_249delGATG variant who has no cardiac problems but severe upper limbs malformations, including phocomelia. Both frameshift variants, c.246_249delGATG and c.905delA, generate mRNAs harbouring premature stop codons which, if not degraded by nonsense mediated decay, will lead to the production of shorter TBX5 proteins, p.Gln302Argfs*92 and p.Met83Phefs*6, respectively. Immunocytochemistry results suggest that both mutated proteins are produced and furthermore, like the wild-type protein, p.Gln302Argfs*92 mutant appears to be mainly localized in the nucleus, in contrast with p.Met83Phefs*6 mutant that displays a higher level of cytoplasmic localization. In addition, luciferase activity analysis revealed that none of the TBX5 mutants are capable of transactivating the NPPA promoter. In conclusion, our results provide evidence that both pathogenic variants cause a severe TBX5 loss-of-function, dramatically reducing its biological activity. The absence of cardiac problems in the individual with the p.Met83Phefs*6 variant supports the existence of other mechanisms/genes underlying the pathogenesis of HOS and/or the existence of an age-related delay in the development of a more serious cardiac phenotype. Further studies are required to understand the differential effects observed in the phenotypes of both individuals.

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Acknowledgements

This study received national funds from Foundation for Science and Technology (FCT) through the project UIDB/04326/2020 (CCMAR). DV and TV are supported by FCT doctoral fellowships (Grant number: SFRH/BD/141918/2018 and SFRH/BD/144230/2019, respectively). The authors are grateful to the individuals enrolled in this study and to Dr. Qing Wang for providing the pcDNA3.1-His6-TBX5 construct.

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Correspondence to Natércia Conceição or M. Leonor Cancela.

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The authors declare no competing interests.

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All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

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Informed consent was obtained from all individual participants included in the study.

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Communicated by Stefan Hohmann.

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Varela, D., Varela, T., Conceição, N. et al. Functional analysis of two novel TBX5 variants present in individuals with Holt–Oram syndrome with different clinical manifestations. Mol Genet Genomics 296, 809–821 (2021). https://doi.org/10.1007/s00438-021-01781-2

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  • DOI: https://doi.org/10.1007/s00438-021-01781-2

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