Up to 8% of all live-born children are affected with a congenital disorder. Some are Mendelian disorders of known etiology, but many are of undetermined genetic cause and mechanism, limiting diagnosis and treatment. This project aims to investigate the underlying causes of unresolved Mendelian disorders, and especially syndromes associated with intellectual disability, by using cutting-edge sequencing techniques and molecular tools in a translational setting that intends to directly benefit affected families. In Paper I, we report the first keratitis-ichthyosis-deafness syndrome patient presenting with reversion of disease phenotype, a phenomenon known as revertant mosaicism. Third-generation sequencing and a cell assay were used to pin-point the mechanism of the somatic variants giving rise to healthy looking skin in the patient. In Paper II, we describe a novel approach to investigate parental origin, gonadal mosaicism, and estimate recurrence risk of disease in two families. Third-generation sequencing was used for haplotype phasing and detection of low-frequency variants in paternal sperm. The recurrence risk in future offspring in the families affected with Noonan syndrome and Treacher Collins syndrome was determined to be 40% and <0.1% respectively. In Paper III, we describe a novel variant in a patient affected with Cornelia de Lange Syndrome, primarily associated with intellectual disability. The affected gene is linked to an extremely rare form of the syndrome, with limited cases described in the literature, usually associated with mild symptoms. Investigation of rare intellectual disability syndromes was continued in Paper IV, by clinical and genetic characterization of six affected males with a likely pathogenic variant in the TAF1 gene. By creating the first TAF1 orthologue knockout we revealed that taf1 is essential for life and that lack of functional taf1 during embryonic development in zebrafish primarily impacts expression of genes in pathways associated with neurodevelopment. By progressive translational research, using state-of-the-art methodology, this project has illuminated the implication of revertant and gonadal mosaicism in disease (Papers I-II), as well as two extremely rare intellectual disability syndromes (Papers III-IV). In total, five families affected with five different disorders have gained clinical and genetic diagnosis and/or further understanding of prognosis and recurrence risk. The study has led to improved understanding of disease etiology and basic developmental processes, enabling development of new therapies and improved care of future patients
