POGLUT1 biallelic mutations cause myopathy with reduced satellite cells, α-dystroglycan hypoglycosylation and a distinctive radiological pattern

Abstract

Protein O-glucosyltransferase 1 (POGLUT1) activity is critical for the Notch signaling pathway, being one of the main enzymes responsible for the glycosylation of the extracellular domain of Notch receptors. A biallelic mutation in the POGLUT1 gene has been reported in one family as cause of an adult-onset limb-girdle muscular dystrophy (LGMD R21; OMIM# 617232). As the result of a collaborative international effort, we have identified the first cohort of 15 patients with LGMD R21, from nine unrelated families coming from different countries, providing a reliable phenotype-genotype and mechanistic insight. Patients carrying novel mutations in POGLUT1 all displayed a clinical picture of limb-girdle muscle weakness. However, the age at onset was broadened from adult to congenital and infantile onset. Moreover, we now report that the unique muscle imaging pattern of "inside-to-outside" fatty degeneration observed in the original cases is indeed a defining feature of POGLUT1 muscular dystrophy. Experiments on muscle biopsies from patients revealed a remarkable and consistent decrease in the level of the NOTCH1 intracellular domain, reduction of the pool of satellite cells (SC), and evidence of α-dystroglycan hypoglycosylation. In vitro biochemical and cell-based assays suggested a pathogenic role of the novel POGLUT1 mutations, leading to reduced enzymatic activity and/or protein stability. The association between the POGLUT1 variants and the muscular phenotype was established by in vivo experiments analyzing the indirect flight muscle development in transgenic Drosophila, showing that the human POGLUT1 mutations reduced its myogenic activity. In line with the wellknown role of the Notch pathway in the homeostasis of SC and muscle regeneration, SC-derived myoblasts from patient's muscle samples showed decreased proliferation and facilitated differentiation. Together, these observations suggest that alterations in SC biology caused by Servián-Morilla et al.

Xylosyl Extension of O-Glucose Glycans on the Extracellular Domain of NOTCH1 and NOTCH2 Regulates Notch Cell Surface Trafficking

“…In this study, we only showed that the xylosyl extension of O-Glc glycans may be involved in the regulation of Notch receptor trafficking using an overexpression system with room for improvement, and it should be further investigated how a xylosyl extension is involved in the above-mentioned, and possibly other, biological processes in vivo. Taken together, our findings suggest the importance of protein folding and O-glucosylation on multiple cysteine-rich EGF repeats of Notch receptors and provide insights into the pathological mechanisms in POGLUT1-related human diseases [51,52,[55][56][57].…”

“…It is also known that Notch signaling is involved in the maintenance of stem cells in many adult tissues in mammals, and O-Glc glycosylation is likely to be involved in these events. Indeed, an abnormal somite formation has been observed in Poglut1 knockout mice [21], and we have recently shown that a reduced enzymatic activity of POGLUT1 in satellite cells, the stem cells of muscles, leads to a reduced Notch signaling, resulting in the aberrant maintenance and differentiation of satellite cells, which induces muscular dystrophy [51,52]. Multiple NOTCH receptors, particularly NOTCH2, are important for satellite cell function in muscles, and their expression levels have been reported to decrease with differentiation [53,54].…”

Sequential targeted exome sequencing of 1001 patients affected by unexplained limb-girdle weakness

“…The participating centers provided consent for data sharing facilitated by the European Genome-Phenome Archive (EGA) and RD-Connect (https://platform.rd-connect.eu/), and we advocate that adopting such an approach will enable future matchmaking between extremely rare cases, such as BVESrelated myopathy 34 or LGMD R21 POGLUT1-related. 35 In addition, thanks to the large cohort size and standardized deep phenotypic data, we were able to expand the clinical and mutational spectrum of known causative genes, such as TRIM32, 36 POMK, 37 DPM3, 38 POMT2, 39 and other dystroglycanopathies. 40 Based on our findings from this large-scale international collaboration, we suggest a new diagnostic approach in the clinic and/or private health providers.…”

Megf10 deficiency impairs skeletal muscle stem cell migration and muscle regeneration

“…Overall, more precise information regarding the relationship between MEGF10 and the Notch pathway could be drawn out by exploring potential interactions with other Notch pathway components, as well as with protein O-glucosyltransferase 1 (encoded by POGLUT1 and previously known as Rumi); the latter is associated with muscular dystrophy and is also known to be a regulator of the Notch pathway [53,54]. POGLUT1 mutations have been associated with a reduction in muscle satellite cells [55]. Detailed fluorescence activated cell sort (FACS) experiments could also elucidate how MEGF10 deficiency could alter early stages of the satellite cell cycle.…”