A novel POMT2 mutation causes mild congenital muscular dystrophy with normal brain MRI

Long-term clinical and MRI follow-up in two POMT2-related limb girdle muscular dystrophy (LGMDR14) patients

INTRODUCTION

POMT2-related limb girdle muscular dystrophy (LGMDR14) is a rare muscular dystrophy caused by mutations in the POMT2 gene. Thus far only 26 LGMDR14 subjects have been reported and no longitudinal natural history data are available.

CASE REPORT

We describe two LGMDR14 patients followed for 20 years since infancy. Both patients presented a childhood-onset, slowly progressive pelvic girdle muscular weakness leading to loss of ambulation in the second decade in one patient, and cognitive impairment without detectable brain structural abnormalities. Glutei, paraspinal, and adductor muscles were the primarily involved muscles at MRI.

DISCUSSION

This report provides natural history data on LGMDR14 subjects, with a focus on longitudinal muscle MRI. We also reviewed the LGMDR14 literature data, providing information on the LGMDR14 disease progression. Considering the high prevalence of cognitive impairment in LGMDR14 patients, a reliable application of functional outcome measures can be challenging, therefore a muscle MRI follow-up to assess disease evolution is recommended.

Clinical exome sequencing identifies novel compound heterozygous mutations of the POMT2 gene in patients with limb-girdle muscular dystrophy

OBJECTIVE

Mutations in protein O-mannosyltransferase 2 (POMT2) (MIM#607439) have been identified in severe congenital muscular dystrophy such as Walker-Warburg syndrome (WWS) and milder limb-girdle muscular dystrophy type 2N (LGMD2N). The aim of this study is to investigate the genetic causes in patients with LGMD2N.

METHODS

Three patients diagnosed with mild limb-girdle muscular dystrophy were recruited. The genetically pathogenic variant was identified by clinical exome sequencing, and healthy controls were verified by Sanger sequencing.

RESULTS

Novel compound heterozygous mutations c.800A > G and c.1074_1075delinsAT of POMT2 were revealed in one affected individual by clinical exome sequencing. There was no report of these two variants and predicted to be highly damaging to the function of the POMT2.

CONCLUSION

The novel variants extend the spectrum of POMT2 mutations, which promotes the prognostic value of testing for POMT2 mutations in patients with LGMD2N.

Phenotype and Genotype Study of Chinese POMT2-Related α-Dystroglycanopathy

Objective

Alpha-dystroglycanopathy (α-DGP) is a subtype of muscular dystrophy caused by defects in the posttranslational glycosylation of α-dystroglycan (α-DG). Our study aimed to summarize the clinical and genetic features of POMT2-related α-DGP in a cohort of patients in China.

Methods

Pedigrees, clinical data, and laboratory tests of patients diagnosed with POMT2-related α-DGP were analyzed retrospectively. The pathogenicity of variants in POMT2 were predicted by bioinformatics software. The variants with uncertain significance were verified by further analysis.

Results

The 11 patients, comprising eight males and three females, were from nine non-consanguineous families. They exhibited different degrees of muscle weakness, ambulation, and intellectual impairment. Among them, three had a muscle-eye-brain disease (MEB)-like phenotype, five presented congenital muscular dystrophy with intellectual disability (CMD-ID), and three presented limb-girdle muscular dystrophy (LGMD). Overall, nine novel variants of POMT2, including two non-sense, one frameshift and six missense variants, were identified. The pathogenicity of two missense variants, c.1891G > C and c.874G > C, was uncertain based on bioinformatics software prediction. In vitro minigene analysis showed that c.1891G > C affects the splicing of POMT2. Immunofluorescence staining with the IIH6C4 antibody of muscle biopsy from the patient carrying the c.874G > C variant showed an apparent lack of expression.

Conclusion

This study summarizes the clinical and genetic characteristics of a cohort of POMT2-related α-DGP patients in China for the first time, expanding the mutational spectrum of the disease. Further study of the pathogenicity of some missense variants based on enzyme activity detection is needed.

A homozygous mutation in the POMT2 gene in four siblings with limb-girdle muscular dystrophy 2N

Mutations in protein O-mannosyltransferase 2 can cause a wide spectrum of clinical phenotypes from severe congenital muscular dystrophy such as Walker-Warburg syndrome to milder limb-girdle muscular dystrophy 2N. We aimed to describe the clinical and paraclinical features, laboratory tests, and molecular findings of four siblings with a homozygous mutation in the protein O-mannosyltransferase 2 gene. There were two sisters and two brothers, aged 4 to 17 years, with an age of onset symptoms at 3 to 12 years. The main neurologic findings were mild intellectual disability, hypoactive deep tendon reflexes, symmetrical weakness of the proximal lower and/or upper limbs, and difficulties in walking on heels and/or toes. The scoliosis found in two siblings has not been associated with protein O-mannosyltransferase 2 gene mutations related to limb-girdle muscular dystrophy 2N in previous reports. This report expands the phenotypic spectrum of protein O-mannosyltransferase 2 gene mutation-related limb-girdle muscular dystrophy 2N.

[A Japanese family with POMT2-related limb girdle muscular dystrophy]

Mutations in the gene encoding the protein O-mannosyl-transferase 2 (POMT2) are known to cause autosomal recessive limb girdle muscular dystrophy type 14 (LGMDR14). No Japanese patient with LGMDR14 has been reported previously. Here, we report three patients with LGMDR14 in one family. The first and second patients harbored a novel homozygous mutation of c.1568A>G, while the third harbored a compound heterozygous mutation of c.1568A>G and c.869C>T. The novel c.1568A>G mutation is classified as likely pathogenic by the guideline of the American College of Medical Genetics and Genomics. Similar to previous cases, all three patients presented difficulty walking and cognitive impairment, and the hamstring muscles were severely affected. Although eye abnormality has only been reported in one previous case, two our patients showed eye abnormalities. As POMT2 enzymatic activity has been demonstrated in the mammalian retina, an eye abnormality may represent a phenotype associated with POMT2 mutation.

Structure of the eukaryotic protein O-mannosyltransferase Pmt1-Pmt2 complex

In eukaryotes, a nascent peptide entering the endoplasmic reticulum (ER) is scanned by two Sec61-translocon-associated large membrane machines for protein N-glycosylation and protein O-mannosylation, respectively. While the structure of the eight-protein oligosaccharyltransferase complex has been determined recently, the structures of mannosyltransferases of the PMT family, which are an integral part of ER protein homeostasis, are still unknown. Here we report cryo-EM structures of the S. cerevisiae Pmt1–Pmt2 complex bound to a donor and an acceptor peptide at 3.2-Å resolution, showing that each subunit contains 11 transmembrane helices and a lumenal β-trefoil fold termed the MIR domain. The structures reveal the substrate recognition model and confirm an inverting mannosyl-transferring reaction mechanism by the enzyme complex. Furthermore, we found that the transmembrane domains of Pmt1 and Pmt2 share a structural fold with the catalytic subunits of oligosaccharyltransferases, confirming a previously proposed evolutionary relationship between protein O-mannosylation and protein N-glycosylation.

Limb girdle muscular dystrophy due to mutations in POMT2

BACKGROUND

Mutations in the gene coding for protein O-mannosyl-transferase 2 (POMT2) are known to cause severe congenital muscular dystrophy, and recently, mutations in POMT2 have also been linked to a milder limb-girdle muscular dystrophy (LGMD) phenotype, named LGMD type 2N (LGMD2N). Only four cases have been reported so far.ClinicalTrials.gov ID: NCT02759302 METHODS: We report 12 new cases of LGMD2N, aged 18-63 years. Muscle involvement was assessed by MRI, muscle strength testing and muscle biopsy analysis. Other clinical features were also recorded.

RESULTS

Presenting symptoms were difficulties in walking, pain during exercise, delayed motor milestones and learning disabilities at school. All had some degree of cognitive impairment. Brain MRIs were abnormal in 3 of 10 patients, showing ventricular enlargement in one, periventricular hyperintensities in another and frontal atrophy of the left hemisphere in a third patient. Most affected muscle groups were hip and knee flexors and extensors on strength testing. On MRI, most affected muscles were hamstrings followed by paraspinal and gluteal muscles. The 12 patients in our cohort carried 11 alleles with known mutations, whereas 11 novel mutations accounted for the remaining 13 alleles.

CONCLUSION

We describe the first cohort of patients with LGMD2N and show that unlike other LGMD types, all patients had cognitive impairment. Primary muscle involvement was found in hamstring, paraspinal and gluteal muscles on MRI, which correlated well with reduced muscle strength in hip and knee flexors and extensors. The study expands the mutational spectrum for LGMD2N, with the description of 11 novel POMT2 mutations in the association with LGMD2N.

CLINICAL TRIAL REGISTRATION

NCT02759302.

Uniparental disomy unveils a novel recessive mutation in POMT2

Mutations in POMT2 are most commonly associated with Walker-Warburg syndrome and Muscle-Eye-Brain disease, but can also cause limb girdle muscular dystrophy (LGMD2N). We report a case of LGMD due to a novel mutation in POMT2 unmasked by uniparental isodisomy. The patient experienced proximal muscle weakness from three years of age with minimal progression. She developed progressive contractures and underwent unilateral Achilles tenotomy. By age 11, she had borderline low left ventricular ejection fraction and mild restrictive lung disease. Muscle biopsy showed mild dystrophic changes with selective reduction in α-dystroglycan immunostaining. Sequencing of POMT2 showed a novel homozygous c.1502A>C variant that was predicted to be probably pathogenic. Fibroblast complementation studies showed lack of functional glycosylation rescued by wild-type POMT2 expression. Chromosomal microarray showed a single 15 Mb copy number neutral loss of heterozygosity on chromosome 14 encompassing POMT2. RNAseq verified homozygosity at this locus. Together, our findings indicate maternal uniparental isodisomy causing LGMD2N.

A fourth case of POMT2-related limb girdle muscle dystrophy with mild reduction of α-dystroglycan glycosylation

BACKGROUND

POMT2 mutations have been identified in Walker-Warburg syndrome or muscle-eye-brain-like, but rarely in limb girdle muscular dystrophy (LGMD).

RESULTS

Two POMT2 mutations, one null and one missense, were found in a patient with LGMD and mild mental impairment, no brain or ocular involvement, minor histopathological features, and slight reduction of α-dystroglycan (α-DG) glycosylation and α-DG laminin binding.

CONCLUSIONS

Our case, the fourth LGMD POMT2-mutated reported to date, provides further evidence of correlation between level of α-DG glycosylation and phenotype severity.

Biochemical correlation of activity of the α-dystroglycan-modifying glycosyltransferase POMGnT1 with mutations in muscle-eye-brain disease

Congenital muscular dystrophies have a broad spectrum of genotypes and phenotypes and there is a need for a better biochemical understanding of this group of diseases in order to aid diagnosis and treatment. Several mutations resulting in these diseases cause reduced O-mannosyl glycosylation of glycoproteins, including α-dystroglycan. The enzyme POMGnT1 (protein-O-mannose N-acetylglucosaminyltransferase 1; EC 2.4.1.-) catalyses the transfer of N-acetylglucosamine to O-linked mannose of α-dystroglycan. In the present paper we describe the biochemical characterization of 14 clinical mutants of the glycosyltransferase POMGnT1, which have been linked to muscle-eye-brain disease or similar conditions. Truncated mutant variants of the human enzyme (recombinant POMGnT1) were expressed in Escherichia coli and screened for catalytic activity. We find that three mutants show some activity towards mannosylated peptide substrates mimicking α-dystroglycan; the residues affected by these mutants are predicted by homology modelling to be on the periphery of the POMGnT1 surface. Only in part does the location of a previously described mutated residue on the periphery of the protein structure correlate with a less severe disease mutant.