In vivo karakterisering van TFIIIA deficiëntie in zebravissen
(Severe) Combined Immunodeficiency ((S)CID) comprises a heterogeneous group of inborn errors of immunity (IEI) characterized by impaired T cell development and/or function. Recently, my host lab identified bi-allelic variants in GTF3A, encoding transcription factor IIIA (TFIIIA), as a novel cause of (S)CID. TFIIIA is a unique transcription factor of 5S ribosomal RNA (rRNA). TFIIIA deficiency results in reduced nascent 5S rRNA transcription, suggesting that disturbed ribosomal function may underlie the disease pathogenesis, potentially linking TFIIIA deficiency to the broader spectrum of ribosomopathies. However, the in vivo consequences of TFIIIA deficiency and the functional impact of some patient-derived variants remain incompletely understood.
In this thesis, a zebrafish (Danio rerio) model was used to functionally characterize TFIIIA deficiency in vivo. First, an in vivo complementation assay was established by injecting CRISPR-Cas9-mediated Tg(lck:EGFP) gtf3aa crispants with wild-type (WT) human GTF3A mRNA. This complementation restores the thymic EGFP area and rescues severe developmental abnormalities in gtf3aa crispants, demonstrating a strong conservation between the human and zebrafish protein. These findings establish the in vivo complementation assay as a platform for functional variant analysis.
Next, the functional impact of multiple patient-derived TFIIIA variants was assessed in vivo. Complementation with known loss-of-function variants H120Y and ZF7KO completely fails to restore thymic EGFP area, confirming the robustness of the assay. The H64D variant similarly fails to rescue the thymic fluorescence, demonstrating its loss-of-function in an in vivo context. Interestingly, developmental phenotypes such as craniofacial abnormalities are more readily rescued than the T cell phenotype, indicating that T cell development might be particularly sensitive to reduced TFIIIA activity.
Overall, this thesis establishes a robust in vivo zebrafish complementation platform for functional assessment of TFIIIA variants and provides novel mechanistic insights into TFIIIA deficiency. The findings further support the hypothesis that TFIIIA deficiency can be classified as a ribosomopathy with prominent immunological manifestations and highlight a differential tissue sensitivity to TFIIIA deficiency, with T cell development being particularly vulnerable.
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