Behind the paper: Untangling Zebrafish Genetic Annotation of TCOF1 and NOLC1

Our research journey began with an intriguing paradox: two genes,  TCOF1 and NOLC1, critical for ribosomal RNA processing and nucleolar function, were confusingly annotated in zebrafish databases....
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Untangling Zebrafish Genetic Annotation: Addressing Complexities and Nomenclature Issues in Orthologous Evaluation of TCOF1 and NOLC1 - Journal of Molecular Evolution

Treacher Collins syndrome (TCS) is a genetic disorder affecting facial development, primarily caused by mutations in the TCOF1 gene. TCOF1, along with NOLC1, play important roles in ribosomal RNA transcription and processing. Previously, a zebrafish model of TCS successfully recapitulated the main characteristics of the syndrome by knocking down the expression of a gene on chromosome 13 (coding for Uniprot ID B8JIY2), which was identified as the TCOF1 orthologue. However, database updates renamed this gene as nolc1 and the zebrafish database (ZFIN) identified a different gene on chromosome 14 as the TCOF1 orthologue (coding for Uniprot ID E7F9D9). NOLC1 and TCOF1 are large proteins with unstructured regions and repetitive sequences that complicate alignments and comparisons. Also, the additional whole genome duplication of teleosts sets further difficulty. In this study, we present evidence that endorses that NOLC1 and TCOF1 are paralogs, and that the zebrafish gene on chromosome 14 is a low-complexity LisH domain-containing factor that displays homology to NOLC1 but lacks essential sequence features to accomplish TCOF1 nucleolar functions. Our analysis also supports the idea that zebrafish, as has been suggested for other non-tetrapod vertebrates, lack the TCOF1 gene that is associated with tripartite nucleolus. Using BLAST searches in a group of teleost genomes, we identified fish-specific sequences similar to E7F9D9 zebrafish protein. We propose naming them “LisH-containing Low Complexity Proteins” (LLCP). Interestingly, the gene on chromosome 13 (nolc1) displays the sequence features, developmental expression patterns, and phenotypic impact of depletion that are characteristic of TCOF1 functions. These findings suggest that in teleost fish, the nucleolar functions described for both NOLC1 and TCOF1 mediated by their repeated motifs, are carried out by a single gene, nolc1. Our study, which is mainly based on computational tools available as free web-based algorithms, could help to solve similar conflicts regarding gene orthology in zebrafish.

 TCOF1 is well-known for its role in craniofacial development, where mutations lead to Treacher Collins Syndrome (TCS), a disorder characterized by facial malformations caused by disruptions in neural crest cells. Yet, in zebrafish—a widely used model for genetic studies—the gene previously identified as tcof1 was suddenly renamed nolc1. To complicate things further, a different gene on chromosome 14 was proposed as the zebrafish TCOF1 orthologue.

This led us to question: What is the true relationship between TCOF1 and NOLC1 in zebrafish? And, how does this relationship play out across vertebrate evolution?

Using bioinformatics tools, we discovered that the zebrafish gene previously labeled as tcof1 actually serves as nolc1, with the functions of both genes seemingly carried out by a single gene. To resolve the confusion surrounding the gene on chromosome 14, we identified a new group of genes across fish species and proposed naming them LisH-containing Low Complexity Proteins (LLCPs). These genes resemble TCOF1 but lack essential features for full TCOF1 functionality.

One of the most exciting aspects of our work was building a phylogenetic tree of TCOF1 and NOLC1 across vertebrates. This revealed that TCOF1 orthologues are primarily restricted to tetrapods, including amphibians and coelacanths, suggesting a significant evolutionary event in the shift from aquatic to terrestrial life. The evolution of TCOF1 may have been key to the development of complex craniofacial structures in tetrapods, a feature absent in zebrafish and other non-tetrapods.

Beyond the technical challenge of reconciling gene annotation, our work also speaks to the importance of accurate database entries. Gene misannotation in model organisms like zebrafish can lead to far-reaching consequences in biomedical research. By leveraging computational tools available as free web-based resources, we hope our findings will inspire others to address similar orthology issues in other species.

Ultimately, our study untangles the complex relationship between TCOF1 and NOLC1 in zebrafish and highlights the evolutionary paths that shape the genetic architectures of vertebrates.

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Molecular Evolution
Life Sciences > Biological Sciences > Evolutionary Biology > Molecular Evolution
Ribosome
Life Sciences > Biological Sciences > Molecular Biology > Protein Synthesis and Translation > Ribosome
Protein Structure
Life Sciences > Biological Sciences > Chemical Biology > Biochemistry > Protein Biochemistry > Proteins > Protein Structure
Evolutionary Developmental Biology
Life Sciences > Biological Sciences > Evolutionary Biology > Evolutionary Developmental Biology
Zebrafish
Life Sciences > Biological Sciences > Biological Techniques > Experimental Organisms > Model Vertebrates > Zebrafish
Animal Disease Models
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