The genetic landscape of axonal neuropathies in the middle-aged and elderly: Focus on MME

Abstract

Objective: To test the hypothesis that monogenic neuropathies such as Charcot-Marie-Tooth disease (CMT) contribute to frequent but often unexplained neuropathies in the elderly, we performed genetic analysis of 230 patients with unexplained axonal neuropathies and disease onset ≥35 years.

Methods: We recruited patients, collected clinical data, and conducted whole-exome sequencing (WES; n = 126) and MME single-gene sequencing (n = 104). We further queried WES repositories for MME variants and measured blood levels of the MME-encoded protein neprilysin.

Results: In the WES cohort, the overall detection rate for assumed disease-causing variants in genes for CMT or other conditions associated with neuropathies was 18.3% (familial cases 26.4%, apparently sporadic cases 12.3%). MME was most frequently involved and accounted for 34.8% of genetically solved cases. The relevance of MME for late-onset neuropathies was further supported by detection of a comparable proportion of cases in an independent patient sample, preponderance of MME variants among patients compared to population frequencies, retrieval of additional late-onset neuropathy patients with MME variants from WES repositories, and low neprilysin levels in patients' blood samples. Transmission of MME variants was often consistent with an incompletely penetrant autosomal-dominant trait and less frequently with autosomal-recessive inheritance.

Conclusions: A detectable fraction of unexplained late-onset axonal neuropathies is genetically determined, by variants in either CMT genes or genes involved in other conditions that affect the peripheral nerves and can mimic a CMT phenotype. MME variants can act as completely penetrant recessive alleles but also confer dominantly inherited susceptibility to axonal neuropathies in an aging population.

Figure 1. Genetic findings in late-onset axonal neuropathies

(A) Contribution of genetic causes to the etiology of late-onset axonal neuropathies as identified by whole-exome sequencing of 126 patients. (B) Comparison of variant detection rates in familial and nonfamilial cases. (C) Comparison of variant detection rates specified by age at disease onset.

Figure 2. MME variants associated with late-onset neuropathies

(A) Schematic representation of neprilysin and distribution of variants identified in this and other studies.^, Functionally relevant protein domains are indicated. Variants acting as autosomal-dominantly inherited risk factors are shown above and variants inherited as autosomal-recessive alleles are shown below the protein. Variants found in both groups of patients are shown twice and labeled with individual symbols (+, *, #, &, β, †). (B) Detection of serious MME variants and the p.Met8Val polymorphism in 2 pedigrees. Women are represented by circles; men are represented by squares. Symbols of affected individuals are filled (black, clinically affected; gray, probably affected by history or subclinical disease); those of unaffected individuals are empty. Crossed symbols represent deceased persons. Asterisks indicate index patients whose data are presented in tables S1, S3, and S4. (C) Pedigree of 2 families with both dominantly and recessively inherited late-onset CMT2 due to homozygous and heterozygous MME variant p.Pro156Leufs*14 and p.Tyr347Cys. (D) Comparison of cumulative allele frequencies for MME loss-of-function and rare missense variants between this study and whole-exome sequencing (WES) databases. Severe missense = predicted damaging/disease causing by at least 2 out of 3 in silico algorithms (PolyPhen-2, Scale-Invariant Feature Transform, Combined Annotation Dependent Depletion). (E) Individual frequencies of recurrent MME variants with a minor allele frequency (MAF) (gnomAD all exomes) between 0.0001 and 0.001 in this study and WES databases. (F) Individual frequencies of MME variants with an MAF (gnomAD all exomes) >0.001 in this study and WES databases. TM = transmembrane domain.

Figure 3. MME-related late-onset neuropathies

(A) Distal muscle atrophy and pes cavus deformity in individuals with heterozygous p.Trp24* (PED46) and heterozygous p.Thr100Profs*11 (PED111) MME variants. (B) Comparison of the age at disease onset in cases with autosomal-recessive and assumed autosomal-dominant inheritance of MME variants reported in this study and previous studies.^, Median, quartiles, and whiskers corresponding to 1.5 times the interquartile range (IQR) are shown. (C) Distribution of presenting symptoms in cases with autosomal-recessive and assumed autosomal-dominant inheritance of MME variants. Cases with biallelic variants also include the series of patients reported by other studies.^,, Diameters of the circles correspond to the proportion of cases with the respective symptom. (D) Distribution of disease severity scores from 1 (mild) to 4 (very severe) in cases with autosomal-recessive and assumed autosomal-dominant inheritance of MME variants. Cases with biallelic variants also include the series of patients reported by previous studies.^, Diameters of the circles correspond to the proportion of cases with the respective severity score. (E) Boxplots comparing neprilysin levels in EDTA plasma obtained from healthy controls (n = 22), patients with late-onset neuropathy without MME variants (n = 34), those with serious MME variants (n = 15), and patients with the p.Met8Val low-frequency polymorphism (n = 9). Outliers (3× IQR) are depicted as open circles and were not included in the statistical analyses. **p < 0.01 (Kruskal-Wallis test with Dunn post hoc test and Bonferroni correction).