Evolutionarily conserved genetic interactions between nphp-4 and bbs-5 mutations exacerbate ciliopathy phenotypes

Abstract

Primary cilia are sensory and signaling hubs with a protein composition that is distinct from the rest of the cell due to the barrier function of the transition zone (TZ) at the base of the cilium. Protein transport across the TZ is mediated in part by the BBSome, and mutations disrupting TZ and BBSome proteins cause human ciliopathy syndromes. Ciliopathies have phenotypic variability even among patients with identical genetic variants, suggesting a role for modifier loci. To identify potential ciliopathy modifiers, we performed a mutagenesis screen on nphp-4 mutant C. elegans and uncovered a novel allele of bbs-5. Nphp-4;bbs-5 double mutant worms have phenotypes not observed in either individual mutant strain. To test whether this genetic interaction is conserved, we also analyzed zebrafish and mice mutants. While Nphp4 mutant zebrafish appeared overtly normal, Bbs5 mutants exhibited scoliosis. When combined, Nphp4;Bbs5 double mutant zebrafish did not exhibit synergistic effects, but the lack of a phenotype in Nphp4 mutants makes interpreting these data difficult. In contrast, viable Nphp4;Bbs5 double mutant mice were not obtained and there were fewer mice than expected carrying three mutant alleles. Additionally, postnatal loss of Bbs5 in mice using a conditional allele compromised survival when combined with a Nphp4 allele. As cilia are formed in the double mutant mice, the exacerbated phenotype is likely a consequence of disrupted ciliary signaling. Collectively, these data support an evolutionarily conserved genetic interaction between Bbs5 and Nphp4 alleles that may contribute to the variability in ciliopathy phenotypes.

Composition, organization and mechanisms of the transition zone, a gate for the cilium

“…This may help explain why some IFT proteins, as well as IFT cargo, notably the Hedgehog signaling protein Smoothened, are observed to stall within the TZ region (Shi et al , 2017). Consistent with a functional interaction between TZ components and the IFT/BBSome complexes, genetic interactions between the two were uncovered in both C. elegans and mice (Yee et al , 2015; Goetz et al , 2017; Bentley‐Ford et al , 2021). However, it is unclear whether these interactions represent a conserved functional feature of the TZ, suggested by Shi et al (2017) to be a waypoint (or hub) for ciliary protein trafficking (Shi et al , 2017).…”

Astrogliosis and Neuroinflammation Underlie Scoliosis Upon Cilia Dysfunction

“…Zebrafish mutants in genes involved in cilia motility or in intraflagellar transport display severe embryonic axis curvature and are usually not viable beyond larval stages (4,5). In contrast, mutants in genes implicated in ciliary gating or ciliary trafficking encoding respectively components of the transition zone (TZ) or the BBsome complex survive to adult stage and develop scoliosis with variable penetrance (3,(6)(7)(8). The molecular basis for these differences is still poorly understood, even if progress has been made in our understanding of zebrafish scoliosis.…”

Astrogliosis and Neuroinflammation Underlie Scoliosis Upon Cilia Dysfunction

“…Zebrafish mutants in genes involved in cilia motility or in intraflagellar transport display severe embryonic axis curvature and are usually not viable beyond larval stages (4,5). In contrast, mutants in genes implicated in ciliary gating or ciliary trafficking encoding respectively components of the transition zone (TZ) or the BBsome complex survive to adult stage and develop scoliosis with variable penetrance (3,(6)(7)(8). The molecular basis for these differences is still poorly understood, even if progress has been made in our understanding of zebrafish scoliosis.…”