Unveiling the molecular mechanisms of compensation: MBNL paralogs and their role in modifying disease severity

Living organisms possess a remarkable ability to endure and adapt in the face of challenges. When genetic malfunctions occur, organisms often employ compensatory mechanisms to overcome these deficiencies. One such example involves the activation of redundant genes with similar functions, known as paralogs.

A fascinating illustration of compensation can be observed in the case of two genes belonging to the muscleblind-like (MBNL) family of RNA-binding proteins, which lose their function in Myotonic Dystrophy Type 1, the most prevalent cause of adult-onset muscular dystrophy. When MBNL1 loses its function, the levels of its paralog, MBNL2, increase in tissues where protein expression is typically low. This augmented expression of MBNL2 allows it to effectively compensate for the loss of MBNL1. In animal models, the loss of one paralog generally yields minimal functional consequences, while the loss of both paralogs leads to severe conditions and even lethality.

A research paper elucidating this phenomenon has been published in the journal Nucleic Acids Research. Dr. Larissa Nitschke, a postdoctoral associate of pathology and immunology in Dr. Thomas A. Cooper’s lab at Baylor College of Medicine, serves as the first author of the study. Dr. Cooper, who holds the R. Clarence and Irene H. Fulbright Chair as well as the S. Donald Greenberg Chair in Pathology, is the corresponding author of the paper.

MBNL proteins play a vital role in RNA processing, particularly in alternative splicing, which allows cells to generate a multitude of proteins from a limited number of genes.

Dr. Cooper explains, “During the process of protein synthesis, genes within the DNA are transcribed into RNA, which is subsequently translated into proteins. Before RNA is translated, it undergoes processing, including splicing, where fragments are joined together in different ways.” He further notes that alternative splicing is a prevalent occurrence, enabling a single gene to produce various proteins. To illustrate, it is akin to creating distinct bracelets by combining a finite number of beads in unique arrangements.

By studying the compensation mechanisms triggered by the loss of MBNL1 and the subsequent increase of MBNL2 protein, researchers aim to unravel the underlying molecular mechanisms and comprehend their roles in preventing or modifying disease severity.

Loss of MBNL1 increases MBNL2 protein but not RNA levels in skeletal muscle, heart and brain. Representative western blot and quantification of MBNL1 and MBNL2 protein and RNA levels in (A) skeletal muscle, (B) heart and (C) brain of Mbnl1+/+ (WT) and Mbnl1−/− mice. For the western blot quantifications, MBNL1 and MBNL2 protein levels were normalized to vinculin. Note that we found that the MBNL1 antibody used cross reacts with MBNL2 (data not shown), which we believe is the residual band in Mbnl1−/− tissues. For the RT-qPCR quantifications, Mbnl1 and Mbnl2 RNA levels were normalized to Gapdh. For each assay, a minimum of four replicates were performed. Simple comparisons used Student’s t-test. In each case, **, ***, **** and ns denote P < 0.01, P < 0.001, P < 0.0001 and P > 0.05, respectively. All data are represented as means ± SEM. Credit: Nucleic Acids Research (2023). DOI: 10.1093/nar/gkac1219

Nitschke, Cooper, and their research team have made an intriguing revelation regarding the loss of MBNL1. They have identified that this loss leads to the production of a modified form of MBNL2, which incorporates two additional segments known as exon 6 and exon 9.

Dr. Nitschke explains, “The inclusion of these exons imparts new properties to MBNL2, enabling it to compensate for the absence of MBNL1 function.” She further clarifies that the inclusion of exon 6 facilitates more efficient transportation of MBNL2 into the cell nucleus, where the protein carries out its essential functions. Additionally, the addition of exon 9 renders the protein less susceptible to degradation, enhancing its stability. This increased stability enables MBNL2 to effectively compensate for the loss of MBNL1 function.

Dr. Cooper, who is affiliated with the Dan L Duncan Comprehensive Cancer Center at Baylor, suggests that these findings may also shed light on the wide spectrum of disease variability observed in Myotonic Dystrophy Type 1, where symptoms can range from severe to mild, even within the same family. Individual variations in the ability to engage these compensatory mechanisms could potentially account for the differences in disease severity.

Furthermore, the discovery of this compensatory mechanism opens up possibilities for future investigations. Dr. Nitschke highlights the potential for exploring therapeutic applications, similar to the splicing-enhancing drugs utilized in the treatment of Spinal Muscular Atrophy. Leveraging this compensatory mechanism for therapeutic purposes in Myotonic Dystrophy Type 1 warrants further exploration.

Source: Baylor College of Medicine

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