Investigations on FBRSL1, a gene involved in a new complex dysmorphic retardation syndrome?

• Applicants: Professorin Dr. Annette Borchers; Professorin Dr. Silke Pauli
• Subject Area: Human Genetics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 418368305

In three children with an unknown mental retardation and malformation syndrome we detected by exome sequencing a nonsense variant (stop mutation and frameshift mutation leading to a premature stop codon) in the Fibrosin-like 1 gene (FBRSL1). The aim of the project is to characterize the molecular mechanisms by which the pathogenic FBRSL1 variants contribute to the clinical phenotype. The function of FBRSL1 is largely unknown, however, mutations in the FBRSL1 paralogue AUTS2 (old name FBRSL2) have already been associated with a mental retardation syndrome. In the first funding period we focused on the identification/validation of human FBRSL1 transcripts and the characterization of their expression as well as subcellular protein localization. In addition, we generated a Fbrsl1-knockdown animal model (Xenopus laevis) and validated the pathogenicity of a FBRSL1 stop mutation using rescue experiments. Furthermore, we identified FBRSL1 interaction partners and target genes suggesting that FBRSL1 functions in gene regulation of ribosomal proteins as well as cell division. In the renewal proposal we aim to analyze and characterize this dual function of FBRSL1. To clarify the observed dysregulation of genes encoding for ribosomal proteins, we will analyze if this is caused by malfunctioning of the FBRSL1-containing polycomb complex (using ChIP-Seq) or if this is a secondary effect of mitotic stress mechanisms and translational adaptation (analyzing the MDM2-p53, p21 and apoptosis pathways as well as the aneuploidy rate). To analyze the function of FBRSL1 during mitosis the expression and localization of different FBRSL1 isoforms will be characterized in synchronized cells at different timepoints of mitosis, and cell cycle-specific FBRSL1 interaction partners will be identified and further characterized, respectively. Complementing these studies, we will investigate the mode of action of Fbrsl1/FBRSL1 during neural and neural crest cell development, as well as during heart development using the generated Xenopus animal model and human cell culture systems. The investigation of the influence of patient-specific mutations on these systems will help to clarify the question how pathogenic FBRSL1-variants lead to the complex dysmorphic phenotype.

• DFG Programme: Research Grants