ESPE Abstracts (2019) 92 FC5.5

Identification of TRPC4AP as a Novel Candidate Gene Causing Thyroid Dysgenesis

Birgit Eberle1, Daniela Choukair2, Philipp Vick3, Pia Hermanns4, Birgit Weiß1, Nagarajan Paramasivam5, Matthias Schlesner6, Stefan Wiemann7, Ralph Roeth1, Carina Klutmann4, Georg F. Hoffmann2, Joachim Pohlenz4, Gudrun A. Rappold1, Markus Bettendorf2, Eberle, Chouklair, Rappold, Bettendorf should be indicated as shared authors2

1Department of Human Molecular Genetics, University of Heidelberg, Heidelberg, Germany. 2Division of Paediatric Endocrinology, Children's Hospital, University of Heidelberg, Heidelberg, Germany. 3Institution of Zoology, University of Hohenheim, Hohenheim, Germany. 4Division of Paediatric Endocrinology, Children's Hospital, University of Mainz, Mainz, Germany. 5Theoretical Bioinformatics Division, German Cancer Research Center (DKFZ), Heidelberg, Germany. 6Bioinformatics and Omics Data Analytics (B240), German Cancer Research Center (DKFZ), Heidelberg, Germany. 7Genomics and Proteomics Core Facility, German Cancer Research Center (DKFZ), Heidelberg, Germany

Background: Congenital hypothyroidism (CH) is the most common endocrine disorder in neonates and is predominantly caused by developmental abnormalities known as thyroid dysgenesis (TD). Several transcription factors have been described in its aetiology, but defects in the known genes only account for a small proportion of cases.

Methods: To identify novel genes involved in TD, we performed exome sequencing in 7 unrelated patients with CH due to TD and their respective parents. After filtering the raw data, the functional effects of the most relevant variants were predicted using the in silico prediction tools MutationTaster, SIFT, PolyPhen2, PROVEAN, and CADD. The most promising candidate gene, TRPC4AP, was sequenced in a cohort of 200 unrelated Caucasian patients with TD, diagnosed in the German newborn screening program. Expression of TRPC4AP in thyroid tissue was performed using RT-PCR. Trpc4ap- functional analysis was performed in Xenopus laevis using morpholinoantisense oligomeres.

Results: TRPC4AP was considered as a promising candidate gene. Targeted sequencing of TRPC4AP in a cohort of 200 patients with TD demonstrated gene variants with predicted damaging potential: one mutation leading to a de novo stop codon p.Q552*, and four additional mutations resulting in an amino acid exchange (p.P706S, p.F729L, p.S777C, and p.N229S). Three of them were transmitted from a phenotypically unaffected parent. We could also demonstrate that TRPC4AP is expressed in human thyroid gland tissue. Using Xenopus laevis as an animal model, we could show that the volume of the tadpole thyroid gland was reduced by 20% in Trpc4ap morpholino knockdowns compared to controls. One of the 7 patients carried two known variants with predicted functional relevance in the PAX8 gene (p.I34T, V35I); both were transmitted by the affected mother.

Discussion: TRPC4AP encodes a 797-amino acid protein, which is expressed in various tissues including thyroid. A putative interaction of TRPC4AP and the NF-kappa-B- essential-modulator (NEMO) encoded by the IKBKG gene was identified by IPA analysis. IKBKG plays a role in the activation of the NF-κB-signaling pathway and regulates various genes involved in thyroid development.

Conclusion: TRPC4AP was identified as a novel candidate gene in TD, but further functional studies are needed for validation