A groundbreaking study from the University of Modena and Reggio Emilia (UNIMORE) is changing our understanding of facioscapulohumeral muscular dystrophy (FSHD), a prevalent hereditary muscle disorder. Published in the esteemed journal Nucleic Acids Research, the research identifies a novel molecular mechanism involving a long non-coding RNA (lncRNA) called FRG2A-t, shedding new light on how the disease progresses. 

Led by Professor Rossella Tupler at UNIMORE’s MIOGEN laboratory, the study, conducted by Dr. Valentina Salsi and Dr. Francesca Losi, reveals that FRG2A-t, derived from segmental genomic duplications, accumulates in a crucial cell structure called the nucleolus. Within the nucleolus, this RNA forms unique molecular condensates, termed “Froggy2 bodies.” The formation of these bodies disrupts the cell’s ability to produce ribosomes and produce muscle proteins, ultimately contributing to muscle weakness and vulnerability in FSHD patients. 

At the core of FSHD-leading mechanisms

Normally, we have repetitive DNA sequences called D4Z4 macrosatellites on chromosome 4 (specifically a part called 4q35). In FSHD, there are fewer copies (deletions) of these D4Z4 repeats. A key diagnostic feature for FSHD is this primary genetic defect.

If you image the DNA as a very long instruction manual for building and running our body, these D4Z4 deletions represent missing instruction in the manual, impacting local chromatin structure. Think of chromatin as the way DNA is packaged inside our cells. It can be tightly packed (heterochromatin) or more open (euchromatin). These changes due to D4Z4 deletions make the chromatin in that region less condensed.

This altered chromatin packaging then leads to an abnormal and increased expression of nearby genes, specifically a long non-coding RNA (lncRNA) called FRG2A-t. Normally, FRG2A is barely detectable, but its levels are abnormally elevated in FSHD muscle samples.

The UNIMORE study found that this overexpressed FRG2A-t specifically localizes in the nucleolus. The nucleolus is a structure within the cell nucleus that’s like a factory for ribosomes, which are essential for making proteins.

Elevated FRG2A-t expression in FSHD cells alters the three-dimensional architecture of heterochromatin at the periphery of the nucleolus. This alteration in nucleolar architecture, driven by FRG2A-t, directly impacts the production of ribosomes. The study shows a reduction in ribosomal DNA (rDNA) transcription and translation rates. Ribosomal DNA is the genetic blueprint for ribosomal RNA (rRNA), which is a major component of ribosomes. So, less rDNA transcription means less rRNA, and consequently, fewer ribosomes.

Since ribosomes are crucial for making proteins, the decreased rDNA transcription and translation rates lead to a reduction in the overall synthesis of skeletal muscle proteins. This impaired protein synthesis is a key factor contributing to muscle wasting, a hallmark of FSHD.

The research employed a sophisticated, multidisciplinary approach, integrating genomics, advanced imaging techniques, and 3D nuclear architecture analysis. Essential support for the study came from UNIMORE’s Interdepartmental Center for Advanced Instrumentation (CIGS). These findings highlight a previously unrecognised role for non-coding RNAs and repetitive DNA elements in regulating nuclear structure and maintaining cellular balance. 

A critical aspect of this research was the access to patient-derived samples from the Italian FSHD registry, a unique clinical and genetic resource within Europe. The project received significant funding from Cariplo-Telethon and the HEAL Italia initiative (PNRR), underscoring the collaborative and globally impactful nature of UNIMORE’s research in muscular dystrophies, also evidenced by the coordination of the European project CoMPaSS-NMD, funded by the European Union within the framework of the Horizon Europe program.

The CoMPaSS-NMD project aims to revolutionize the diagnosis and treatment of Hereditary neuromuscular diseases (HNMDs) by leveraging artificial intelligence. By analysing vast amounts of patient data, including genetic information, clinical data, MRI scans, and muscle biopsies, AI algorithms can identify patterns and correlations that may be missed by human experts, supporting the medical specialists to find a more accurate diagnosis. By integrating AI into the diagnosis and treatment of HNMDs, CoMPaSS-NMD has the potential to significantly improve the lives of patients and their families.

This study marks a significant step forward in understanding FSHD, proposing a disease model where the accumulation of D4Z4-driven lncRNA in the nucleolus impairs protein synthesis, directly contributing to muscle wasting. This new paradigm opens avenues for future research and potential therapeutic strategies targeting these newly identified molecular mechanisms.

Link to the scientific publication: https://academic.oup.com/nar/article/53/13/gkaf643/8196084