IgM anti-GM2 antibodies in patients with multifocal motor neuropathy target Schwann cells and are associated with early onset.

BACKGROUND: Multifocal motor neuropathy (MMN) is a rare, chronic immune-mediated polyneuropathy characterized by asymmetric distal limb weakness. An important feature of MMN is the presence of IgM antibodies against gangliosides, in particular GM1 and less often GM2. Antibodies against GM1 bind to motor neurons (MNs) and cause damage through complement activation. The involvement of Schwann cells (SCs), expressing GM1 and GM2, in the pathogenesis of MMN is unknown. METHODS: Combining the data of our 2007 and 2015 combined cross-sectional and follow-up studies in Dutch patients with MMN, we evaluated the presence of IgM antibodies against GM1 and GM2 in serum from 124 patients with MMN and investigated their binding to SCs and complement-activating properties. We also assessed the relation of IgM binding and complement deposition with clinical characteristics. RESULTS: Thirteen out of 124 patients (10%) had a positive ELISA titer for IgM anti-GM2. Age at onset of symptoms was significantly lower in MMN patients with anti-GM2 IgM. IgM binding to SCs correlated with IgM anti-GM2 titers. We found no correlation between IgM anti-GM2 titers and MN binding or with IgM anti-GM1 titers. IgM binding to SCs decreased upon pre-incubation of serum with soluble GM2, but not with soluble GM1. IgM anti-GM2 binding to SCs correlated with complement activation, as reflected by increased C3 fixation on SCs and C5a formation in the supernatant. CONCLUSION: Circulating IgM anti-GM2 antibodies define a subgroup of patients with MMN that has an earlier onset of disease. These antibodies probably target SCs specifically and activate complement, similarly as IgM anti-GM1 on MNs. Our data indicate that complement activation by IgM antibodies bound to SCs and MNs underlies MMN pathology.

A 21-bp deletion in the complement regulator CD55 promotor region is associated with multifocal motor neuropathy and its disease course.

BACKGROUND AND AIMS: To further substantiate the role of antibody-mediated complement activation in multifocal motor neuropathy (MMN) immunopathology, we investigated the distribution of promotor polymorphisms of genes encoding the membrane-bound complement regulators CD46, CD55, and CD59 in patients with MMN and controls, and evaluated their association with disease course. METHODS: We used Sanger sequencing to genotype five common polymorphisms in the promotor regions of CD46, CD55, and CD59 in 133 patients with MMN and 380 controls. We correlated each polymorphism to clinical parameters. RESULTS: The genotype frequencies of rs28371582, a 21-bp deletion in the CD55 promotor region, were altered in patients with MMN as compared to controls (p .009; Del/Del genotype 16.8% vs. 7.7%, p .005, odds ratio: 2.43 [1.27-4.58]), and patients carrying this deletion had a more favorable disease course (mean difference 0.26 Medical Research Council [MRC] points/year; 95% confidence interval [CI]: 0.040-0.490, p .019). The presence of CD59 rs141385724 was associated with less severe pre-diagnostic disease course (mean difference 0.940 MRC point/year; 95% CI: 0.083-1.80, p .032). INTERPRETATION: MMN susceptibility is associated with a 21-bp deletion in the CD55 promotor region (rs2871582), which is associated with lower CD55 expression. Patients carrying this deletion may have a more favorable long-term disease outcome. Taken together, these results point out the relevance of the pre-C5 level of the complement cascade in the inflammatory processes underlying MMN.

Multifocal motor neuropathy is not associated with altered innate immune responses to endotoxin.

OBJECTIVE: Antibody- and complement-mediated peripheral nerve inflammation are central in the pathogenesis of MMN. Here, we studied innate immune responses to endotoxin in patients with MMN and controls to further our understanding of MMN risk factors and disease modifiers. METHODS: We stimulated whole blood of 52 patients with MMN and 24 controls with endotoxin and collected plasma. With a multiplex assay, we determined levels of the immunoregulating proteins IL-1RA, IL-1ß, IL-6, IL-10, IL-21, TNF-α, IL-8 and CD40L in unstimulated and LPS-stimulated plasma. We compared baseline and stimulated protein levels between patients and controls and correlated concentrations to clinical parameters. RESULTS: Protein level changes after stimulation were comparable between groups (p > 0.05). IL-1RA, IL-1ß, IL-6 and IL-21 baseline concentrations showed a positive correlation with monthly IVIg dosage (all corrected p-values < 0.016). Patients with anti-GM1 IgM antibodies showed a more pronounced IL-21 increase after stimulation (p 0.048). CONCLUSIONS: Altered endotoxin-induced innate immune responses are unlikely to be a susceptibility factor for MMN.

Anti-C2 Antibody ARGX-117 Inhibits Complement in a Disease Model for Multifocal Motor Neuropathy.

BACKGROUND AND OBJECTIVES: To determine the role of complement in the disease pathology of multifocal motor neuropathy (MMN), we investigated complement activation, and inhibition, on binding of MMN patient-derived immunoglobulin M (IgM) antibodies in an induced pluripotent stem cell (iPSC)-derived motor neuron (MN) model for MMN. METHODS: iPSC-derived MNs were characterized for the expression of complement receptors and membrane-bound regulators, for the binding of circulating IgM anti-GM1 from patients with MMN, and for subsequent fixation of C4 and C3 on incubation with fresh serum. The potency of ARGX-117, a novel inhibitory monoclonal antibody targeting C2, to inhibit fixation of complement was assessed. RESULTS: iPSC-derived MNs moderately express the complement regulatory proteins CD46 and CD55 and strongly expressed CD59. Furthermore, MNs express C3aR, C5aR, and complement receptor 1. IgM anti-GM1 antibodies in serum from patients with MMN bind to MNs and induce C3 and C4 fixation on incubation with fresh serum. ARGX-117 inhibits complement activation downstream of C4 induced by patient-derived anti-GM1 antibodies bound to MNs. DISCUSSION: Binding of IgM antibodies from patients with MMN to iPSC-derived MNs induces complement activation. By expressing complement regulatory proteins, particularly CD59, MNs are protected against complement-mediated lysis. Yet, because of expressing C3aR, the function of these cells may be affected by complement activation upstream of membrane attack complex formation. ARGX-117 inhibits complement activation upstream of C3 in this disease model for MMN and therefore represents an intervention strategy to prevent harmful effects of complement in MMN.

SMN1 Duplications Are Associated With Progressive Muscular Atrophy, but Not With Multifocal Motor Neuropathy and Primary Lateral Sclerosis.

OBJECTIVE: To assess the association between copy number (CN) variation in the survival motor neuron (SMN) locus and multifocal motor neuropathy (MMN), progressive muscular atrophy (PMA), and primary lateral sclerosis (PLS) susceptibility and to determine the association of SMN1 and SMN2 CN with MMN, PMA, and PLS disease course. METHODS: In this monocenter study, we used multiplex ligation-dependent probe amplification to determine SMN1 and SMN2 CN in Dutch patients with MMN, PMA, and PLS and controls. We stratified clinical parameters for SMN1 and SMN2 CN. We analyzed SMN1 and SMN2 exons 1-6, intron 6, and exon 8 CN to study the genetic architecture of SMN1 duplications. RESULTS: SMN1 and SMN2 CN were determined in 132 patients with MMN, 150 patients with PMA, 104 patients with PLS, and 956 control subjects. MMN and PLS were not associated with CN variation in SMN1 or SMN2. By contrast, patients with PMA more often than controls carried SMN1 duplications (≥3 SMN1 copies, 12.0% vs 5.0%, odds ratio 2.69 (1.43-4.91), p 0.0020). SMN1 and SMN2 CN status was not associated with MMN, PLS, or PMA disease course. In case of SMN1 exon 7 duplications, exons 1-6, exon 8, and introns 6 and 7 were also duplicated, suggesting full SMN1 duplications. CONCLUSIONS: SMN1 duplications are associated with PMA, but not with PLS and MMN. SMN1 duplications in PMA are balanced duplications. The results of this study highlight the primary effect of altered SMN CN on lower motor neurons.

High-resolution mapping identifies HLA class II associations with multifocal motor neuropathy.

OBJECTIVE: To gain further insight in the immunopathology underlying multifocal motor neuropathy (MMN) by exploring the association between MMN and the human leukocyte antigen (HLA) class II DRB1, DQB1, and DQA loci in depth and by correlating associated haplotypes to detailed clinical and anti-ganglioside antibody data. METHODS: We performed high-resolution HLA-class II typing for the DRB1, DQB1, and DQA1 loci in 126 well-characterized MMN patients and assessed disease associations with haplotypes. We used a cohort of 1305 random individuals as a reference for haplotype distribution in the Dutch population. RESULTS: The DRB1*15:01-DQB1*06:02 haplotype (OR 1.6 [95% CI 1.1-2.2], p < 0.05) and the DRB1*12:01-DQB1*03:01 haplotype (OR 2.7 [95% CI 1.2-5.5], p < 0.05) were more frequent in patients with MMN than in controls. These haplotypes were not associated with disease course, response to treatment or anti-ganglioside antibodies. CONCLUSIONS: MMN is associated with the DRB1*15:01-DQB1*06:02 and DRB1*12:01-DQB1*03:01 haplotypes. These HLA molecules or gene variants in their immediate vicinity may promote the specific inflammatory processes underlying MMN.

Biallelic variants in POLR3GL cause endosteal hyperostosis and oligodontia.

RNA polymerase III (Pol III) is an essential 17-subunit complex responsible for the transcription of small housekeeping RNAs such as transfer RNAs and 5S ribosomal RNA. Biallelic variants in four genes (POLR3A, POLR3B, and POLR1C and POLR3K) encoding Pol III subunits have previously been found in individuals with (neuro-) developmental disorders. In this report, we describe three individuals with biallelic variants in POLR3GL, a gene encoding a Pol III subunit that has not been associated with disease before. Using whole exome sequencing in a monozygotic twin and an unrelated individual, we detected homozygous and compound heterozygous POLR3GL splice acceptor site variants. RNA sequencing confirmed the loss of full-length POLR3GL RNA transcripts in blood samples of the individuals. The phenotypes of the described individuals are mainly characterized by axial endosteal hyperostosis, oligodontia, short stature, and mild facial dysmorphisms. These features largely fit within the spectrum of phenotypes caused by previously described biallelic variants in POLR3A, POLR3B, POLR1C, and POLR3K. These findings further expand the spectrum of POLR3-related disorders and implicate that POLR3GL should be included in genetic testing if such disorders are suspected.

Brachial plexus magnetic resonance imaging differentiates between inflammatory neuropathies and does not predict disease course.

OBJECTIVE: The main objective of this study was to evaluate the correlation between the distribution of brachial plexus magnetic resonance imaging (MRI) abnormalities and clinical weakness, and to evaluate the value of brachial plexus MRI in predicting disease course and response to treatment in multifocal motor neuropathy (MMN), Lewis-Sumner syndrome (LSS) and chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). METHODS: Sixty-seven patients with an inflammatory neuropathy diagnosed at our tertiary referral center for neuromuscular diseases had undergone bilateral T2-weighted short tau inversion recovery (STIR) MRI of the brachial plexus. We obtained clinical follow-up data and scored all MRIs for abnormalities and the symmetry of their distribution. RESULTS: Brachial plexus MRI abnormalities were detected in 45% of the patients. An abnormal MRI did not predict disease course in terms of patterns of weakness, sensory disturbances or response to treatment. Within the spectrum of radiological abnormalities, asymmetrical clinical syndromes, MMN and LSS were significantly associated with asymmetrical radiological abnormalities, whereas symmetrical abnormalities predominated in CIDP (p < .001, phi 0.791). CONCLUSION: T2 STIR brachial plexus MRI abnormalities correspond with the distribution of neurological deficits in inflammatory neuropathies, but do not correlate with specific clinical characteristics, response to treatment or disease course.

Comparative study of peripheral nerve Mri and ultrasound in multifocal motor neuropathy and amyotrophic lateral sclerosis.

INTRODUCTION: Differentiating multifocal motor neuropathy (MMN) from amyotrophic lateral sclerosis (ALS) is important, as MMN is a difficult, but treatable disorder. METHODS: We studied peripheral nerve imaging techniques in differentiating MMN from ALS by measuring the cross-sectional area (CSA) of the median and ulnar nerves in the forearms using high resolution ultrasound (HRUS) and MRI. RESULTS: HRUS CSA values of the median nerve in the forearm (P = 0.002) and the ulnar nerve distal to the sulcus (P = 0.009) were significantly enlarged in patients with MMN. There was a positive correlation between CSA as measured with HRUS and MRI (Spearman rho 0.60; P < 0.001). CONCLUSIONS: Peripheral nerve imaging is a potentially powerful technique to distinguish MMN from ALS. Muscle Nerve, 2016 Muscle Nerve 54: 1133-1135, 2016.