In frame exon skipping of COL A
In frame exon 41 skipping of COL6A3 has been suspected to be responsible for isolated dystonia in several human kindreds without muscular defects . In order to validate their hypothesis, the authors used zebrafish to selectively knock down several exons in col6a3 and found a negative impact on motor neuron pathfinding only after exon 41 ortholog deletion. This observation suggested that the C3-C4 domains encoded by exon 41 are not important for muscle physiology but in peripheral nervous system in humans as well as in zebrafish. This paper nicely underscored the usefulness of zebrafish model in linking gene mutations to human disease. However, some mutants do not recapitulate the corresponding human disease. This is the case of the COL4A5 gene. Mutations in COL4A5 were reported to be responsible for the Alport Syndrome that mainly affects kidney function whereas the dragnet/col4a5 mutant does not manifest renal defects. Further investigation is clearly needed to clarify such discrepancies.
Outlook Beside the fundamental point of view, understanding the function of collagens and their mechanisms of action has significant and broad impact on human health issues such as what he said disease treatment, wound repair, tissue regeneration and tissue ageing prevention. Zebrafish has proven to be a relevant animal model to interrogate collagen gene functions. The generation in zebrafish of reporter gene lines and transgenic lines expressing fluorescent fusion collagen will allow to further develop an imaging approach unmatched in its ability to image in real-time the dynamics of collagen synthesis, secretion, extracellular assembly and remodeling. Mutations in 34 human collagen genes result in disease . Zebrafish models for the various human diseases associated with collagen deficiency are also needed to enable a clear understanding of the pathophysiological mechanisms and to provide a suitable preclinical model for drug screening. We have no doubt that this will be rapidly achieved. The generation of zebrafish stable transgenic lines that represented a long and difficult task until a very short time ago is now largely facilitated thanks to the recent advances in genome editing technologies.
Acknowledgements We acknowledge the contribution of SFR Biosciences (UMS3444/CNRS, ENS de Lyon, University of Lyon) zebrafish facility, specifically Laure Bernard and Robert Renard for their technical assistance and of the “Centre Technologique des Microstructures” (University of Lyon, France). This work was supported by the“Agence Nationale de la Recherche” (ANR-16-CE18-0023-03) and the AFM-Téléthon (#21064). PN is a recipient of a “Fondation l’Oréal” 2016 French Fellowship L’Oréal-UNESCO For Women in Science, the French government (NMRT) and the “Fondation pour la Recherche Médicale” (FDT20160435169).
Introduction Collagens are widespread proteins in the extracellular matrix within the animal kingdom, with 28 distinct types having been identified within the human genome . In all cases, the collagen tertiary structure is a triple helix in which three polypeptide chains, each in a left-handed polyproline II-like helix, are wound together to form a right-handed coiled, rope-like triple helical structure . The amino acid sequences of the collagens are characterised by a large amount of Gly, about one third of the composition, and Pro which comprises about one fifth of the composition. The Gly residues are essential and occur every third residue as Gly is the only amino acid that is small enough to fit within the centre of the triple helix. This gives collagens a characteristic (Gly-Xaa-Yaa)n repeating sequence. There are frequent proline residues in the Xaa and Yaa positions that are important to assist in the folding, stability and interactions of the triple helical structure . An interesting feature of most, if not all the Pro residues in the Yaa position of mammalian collagens is that they are modified to 4-hydroxyproline (Hyp) residues. This occurs during intracellular biosynthesis, prior to the three individual chains folding to give a triple helical structure . This secondary modification occurs through the action of the specific enzyme, prolyl-4-hydroxylase (P4H). Generally, most Pro residues in the Xaa position are not modified, although rarely, about one per chain, a 3-hydroxproline is found resulting from a separate specific prolyl-3-hydroxylase enzyme activity during biosynthesis .