SAT-704 - A Paradigm Shift: Partial Loss of Genes from Two Different Ciliopathies Cause Synergistic Ciliary Dysfunction

Primary cilia are signaling antenna that direct cellular signaling. Disruption of cilia function results in hyperphagia-associated obesity. Both Bardet-Biedl syndrome (caused by the loss of approximately 25 BBS genes) and Alström syndrome (caused by mutations in the ALMS1 gene) are examples of autosomal recessive disorders characterized by ciliary dysfunction associated with hyperphagia, and early-onset obesity. While heterozygous loss of ciliary genes does not typically cause disease, some children in our clinics with early-onset obesity carried heterozygous variants in both ALMS1 and a BBS gene. Whether heterozygous variants in both ALMS1 and BBS genes contribute to ciliary dysfunction and early-onset obesity remains unknown.
To address this, we systematically screened 20 BBS genes in human hTERT-RPE1 cells. To mimic heterozygous and homozygous gene loss in vitro, siRNA concentrations were optimized to achieve approximately 50 % (low dose) and over 70 % (high dose) knockdown of gene expression as confirmed by qPCR. The hTERT-RPE1 cells were transfected with low dose of siRNA targeting BBS genes and/or ALMS1, followed by ARL13B immunostaining to assess cilia length. By these measurements, siRNA partial reduction of eight BBS genes, BBS2, ARL6, BBS8, IFT172, BBS7, SDCCAG, IFT27, and BBS4, demonstrated a synergistic reduction in cilia length when cells were co-transfected with low dose siRNA targeting ALMS1. Further validation using the BBS4:ALMS1 combination in both human hTERT-RPE1 and mouse GT1-7 cells revealed that partial loss of BBS4 and ALMS1 led to decreased ciliogenesis, shortened cilia, and reduced Gli1 expression, indicating impaired ciliary structure and function.
These findings suggest that digenic interactions between ALMS1 and select BBS genes may contribute to ciliary dysfunction and early-onset obesity. This work provides new insight into the genetic basis of ciliopathies and may inform future diagnostic and therapeutic strategies.

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