Browsing by Subject "Nemaline myopathy"

Nemaline myopathy (NM) is a rare congenital disorder, the most common of congenital myopathies. It affects primarily the skeletal muscles and it is recognised by nemaline bodies in muscle tissue samples and muscle weakness. Mutation of eleven genes are known to lead to NM and the most frequent disease-causing variants are either recessive NEB variants or dominant ACTA1 variants. Variants in NEB are thought to be well tolerated and only 7% of them are hypothesized to be pathogenic. Over 200 pathogenic NEB-variants have been identified in Helsinki and the majority occurred in patients as a combination of two different variants. The missense variants were speculated to have a modifying effect on pathogenicity by affecting nebulin-actin or nebulin-tropomyosin interactions. Nebulin is a gigantic protein coded by NEB and is one of the largest proteins in vertebrates. It is located in the thin filament of the skeletal muscle sarcomere. Enclosed by terminal regions, nebulin has an extensive repetitive modular region that covers over 90% of the protein. The repetitive zone comprises of 26 modules called super repeats (SR). SRs consist of seven simple repeats. There are seven conserved SDXXYK actin-binding sites at each super repeat, one per simple repeat, and one conserved WLKGIGW tropomyosin-binding site. Due to its enormous size and highly repetitive sequence, nebulin is one of the least studied proteins in vivo, in vitro or in silico. In the NM patient database used for this study, there are 70 families with verified pathogenic mutations and in 30 of them, there were additional missense variants in NEB. These missense variants can be pathogenic modifying factors or have no impact on the phenotype. Seven missense variants were selected to study the effect of these mutations on actin-binding capacity compared to wild-type nebulin using the SR panel constructed previously by Laitila and Lehtonen. Also, due to the differences in actin-binding capacity of SRs compared to each other, one of the aims was to determine whether corresponding mutations in different SRs would have a similar or different effect on actin-binding capacity. For this aim, one missense mutation in the strongly actin-binding SR 1, and one in the weakly actin-binding SR 7 were selected from the NM database, and corresponding variants were created. Also, an in-frame deletion in SR7 found in the ExAC database and the corresponding mutation in SR1 were constructed for this study. The actin-binding strength was determined using actin co-sedimentation assay and actin affinity assay. The results for co-sedimentation assay indicate that missense variants can have an effect on nebulin-actin interactions and, therefore, can be a possible cause for NM. The corresponding mutations had no correlation in their effect on actin-binding strength, just the opposite. S1-m-2 decreased actin-binding strength of SR1 and S7-m-2 had no effect on SR7. Likewise, S7-m-1 and S7-del-1 decreased actin-binding strength of SR7 and corresponding mutations had no effect on SR1. The selected missense mutations found in NM patients in SRs 2 and 4 decreased actin-binding strength, if located at the actin-binding sites and in SR 10 increased the actin-binding strength, if located at the actin-binding site. The change in actin binding strength was defined as significant if the P-value was below 0.005. The more accurate affinity assay was performed as a trial only for S16 and S16-m-1, a variant at a tropomyosin-binding site close to an actin-binding site. It indicated a difference in actin-binding affinity missed by the actin co-sedimentation assay. The results are preliminary, but show big promise and should be optimized and implemented in the future missense mutation affinity studies. In an attempt to understand if the effect missense mutations have on nebulin-actin interaction is based on the change in nebulin structure, the 3D-structure of each produced fusion protein was predicted in silico. Considering that the variants were produced as GST-fusion proteins, the position and effect of GST in them is also a point of interest. In order to predict the structure of these large proteins, a combined approach was implemented using I-TASSER (Iterative Threading ASSEmbly Refinement) software. The software uses ab initio modeling, threading methods and atomic-level structure refinement to build an accurate 3D-model of a protein from sequence. According to the predicted 3D models of the fusion proteins, the GST-part of the proteins folds into a globular structure and acts as a core around which the nebulin fragments fold. The GST does not bind to actin and is positioned on the inside, which indicates minimal effect on nebulin-actin interaction, but may be a reason for an alternative nebulin fragment folding. The accuracy of the default set of programs in software does not give the definitive answer of the possible effect missense mutations can have on structural changes. However, I-TASSER approach for 3D-modeling is promising with further software optimization and can possibly serve as an effective bioinformatic tool in the future.