Rare PMP22 variants in mild to severe neuropathy uncorrelated to plasma GDF15 or neurofilament light

Charcot-Marie-Tooth disease (CMT) is a heterogeneous set of hereditary neuropathies whose genetic causes are not fully understood. Here, we characterize three previously unknown variants in PMP22 and assess their effect on the recently described potential CMT biomarkers' growth differentiation factor 15 (GDF15) and neurofilament light (NFL): first, a heterozygous PMP22 c.178G > A (p.Glu60Lys) in one mother-son pair with adult-onset mild axonal neuropathy. The variant led to abnormal splicing, confirmed in fibroblasts by reverse transcription PCR. Second, a de novo PMP22 c.35A > C (p.His12Pro), and third, a heterozygous 3.2 kb deletion predicting loss of exon 4. The latter two had severe CMT and ultrasonography showing strong nerve enlargement similar to a previous case of exon 4 loss due to a larger deletion. We further studied patients with PMP22 duplication (CMT1A) finding slightly elevated plasma NFL, as measured by the single molecule array immunoassay (SIMOA). In addition, plasma GDF15, as measured by ELISA, correlated with symptom severity for CMT1A. However, in the severely affected individuals with PMP22 exon 4 deletion or p.His12Pro, these biomarkers were within the range of variability of CMT1A and controls, although they had more pronounced nerve hypertrophy. This study adds p.His12Pro and confirms PMP22 exon 4 deletion as causes of severe CMT, whereas the previously unknown splice variant p.Glu60Lys leads to mild axonal neuropathy. Our results suggest that GDF15 and NFL do not distinguish CMT1A from advanced hypertrophic neuropathy caused by rare PMP22 variants.

Neurofilament Light Regulates Axon Caliber, Synaptic Activity, and Organelle Trafficking in Cultured Human Motor Neurons

Neurofilament light (NFL) is one of the proteins forming multimeric neuron-specific intermediate filaments, neurofilaments, which fill the axonal cytoplasm, establish caliber growth, and provide structural support. Dominant missense mutations and recessive nonsense mutations in the neurofilament light gene (NEFL) are among the causes of Charcot–Marie–Tooth (CMT) neuropathy, which affects the peripheral nerves with the longest axons. We previously demonstrated that a neuropathy-causing homozygous nonsense mutation in NEFL led to the absence of NFL in patient-specific neurons. To understand the disease-causing mechanisms, we investigate here the functional effects of NFL loss in human motor neurons differentiated from induced pluripotent stem cells (iPSC). We used genome editing to generate NEFL knockouts and compared them to patient-specific nonsense mutants and isogenic controls. iPSC lacking NFL differentiated efficiently into motor neurons with normal axon growth and regrowth after mechanical axotomy and contained neurofilaments. Electrophysiological analysis revealed that motor neurons without NFL fired spontaneous and evoked action potentials with similar characteristics as controls. However, we found that, in the absence of NFL, human motor neurons 1) had reduced axonal caliber, 2) the amplitude of miniature excitatory postsynaptic currents (mEPSC) was decreased, 3) neurofilament heavy (NFH) levels were reduced and no compensatory increases in other filament subunits were observed, and 4) the movement of mitochondria and to a lesser extent lysosomes was increased. Our findings elaborate the functional roles of NFL in human motor neurons. NFL is not only a structural protein forming neurofilaments and filling the axonal cytoplasm, but our study supports the role of NFL in the regulation of synaptic transmission and organelle trafficking. To rescue the NFL deficiency in the patient-specific nonsense mutant motor neurons, we used three drugs, amlexanox, ataluren (PTC-124), and gentamicin to induce translational read-through or inhibit nonsense-mediated decay. However, the drugs failed to increase the amount of NFL protein to detectable levels and were toxic to iPSC-derived motor neurons.

Serum Creatine, Not Neurofilament Light, Is Elevated in CHCHD10-Linked Spinal Muscular Atrophy

Objective

To characterize serum biomarkers in mitochondrial CHCHD10-linked spinal muscular atrophy Jokela (SMAJ) type for disease monitoring and for the understanding of pathogenic mechanisms.

Methods

We collected serum samples from a cohort of 49 patients with SMAJ, all carriers of the heterozygous c.197G>T p.G66V variant in CHCHD10. As controls, we used age- and sex-matched serum samples obtained from Helsinki Biobank. Creatine kinase and creatinine were measured by standard methods. Neurofilament light (NfL) and glial fibrillary acidic protein (GFAP) were measured with single molecule array (Simoa), fibroblast growth factor 21 (FGF-21), and growth differentiation factor 15 (GDF-15) with an enzyme-linked immunosorbent assay. For non-targeted plasma metabolite profiling, samples were analyzed with liquid chromatography high-resolution mass spectrometry. Disease severity was evaluated retrospectively by calculating a symptom-based score.

Results

Axon degeneration marker, NfL, was unexpectedly not altered in the serum of patients with SMAJ, whereas astrocytic activation marker, GFAP, was slightly decreased. Creatine kinase was elevated in most patients, particularly men. We identified six metabolites that were significantly altered in serum of patients with SMAJ in comparison to controls: increased creatine and pyruvate, and decreased creatinine, taurine, N-acetyl-carnosine, and succinate. Creatine correlated with disease severity. Altered pyruvate and succinate indicated a metabolic response to mitochondrial dysfunction; however, lactate or mitochondrial myopathy markers FGF-21 or GDF-15 was not changed.

Conclusions

Biomarkers of muscle mass and damage are altered in SMAJ serum, indicating a role for skeletal muscle in disease pathogenesis in addition to neurogenic damage. Despite the minimal mitochondrial pathology in skeletal muscle, signs of a metabolic shift can be detected.