Clinical and genetic features of CMT2T in Italian patients confirm the importance of MME pathogenic variants in idiopathic, late-onset axonal neuropathies

Abstract

Background and aims: Since 2016, biallelic mutations in the membrane metalloendopeptidase (MME) gene have been associated with late-onset recessive CMT2 (CMT2T). More recently, heterozygous mutations have also been identified in familial and sporadic patients with late-onset axonal neuropathy, ranging from subclinical to severe. This indicates that the heterozygous MME variants may not be fully penetrant, or alternatively, that they may be a potential risk factor for neuropathy. Here, we describe the clinical, neurophysiological, and genetic findings of 32 CM2T Italian patients.

Methods: The patients were recruited from four different Italian referral centers. Following a comprehensive battery of neurological, electrophysiological, and laboratory examinations, the patients' DNA was subjected to sequencing in order to identify any variants in the gene. Bioinformatic and modeling analyses were performed to evaluate the identified variants' effects.

Results: We observe a relatively mild axonal sensory-motor neuropathy with a greater impairment of the lower extremities. Biallelic and monoallelic patients exhibit comparable disease severity, with an earlier onset observed in those with biallelic variants. When considering a subgroup with more than 10 years of disease, it becomes evident that biallelic patients exhibit a more severe form of neuropathy. This suggests that they are more prone to quick progression.

Interpretation: CM2T has been definitively defined as a late-onset neuropathy, with a typical onset in the fifth to sixth decades of life and a more rapidly progressing worsening for biallelic patients. CMT2T can be included in the neuropathies of the elderly, particularly if MME variants heterozygous patients are included.

Keywords: CMT; MME; genotype–phenotype correlation; late‐onset peripheral neuropathies; neprilysin.

FIGURE 1

ACMG‐based variants pathogenicity (A), inheritance (B), and gender (C) distribution among the 32 patients.

FIGURE 2

The box plots show the disease onset (A) of the whole group and compare the homozygous (HMZ) and heterozygous (HTZ) groups (B). The HTZ group experiences a statistically significant later disease onset when deprived of likely benign variants (HTZ‐LB) (p = .0099; p < .05). (C) The disease duration of the entire group. (D) Comparisons of the homozygous (HMZ) and heterozygous (HTZ) groups. No statistically significant differences were found between the three groups.

FIGURE 3

Distribution and type of symptoms at onset.

FIGURE 4

(A) Comparison between disease severity in Heterozygous (HTZ) and homozygous (HMZ) patients, patients with more than 10 (>10) and less than 10 (<10) years of disease duration, heterozygous and homozygous patients with more than 10 and less than 10 years of disease duration (HTZ > 10, HTZ < 10 and HMZ > 10, HMZ <10, ‐ likely benign variants excluded). (B) Overall clinical parameters.

FIGURE 5

Overall values of upper limb nerve conduction studies. CMAP, compound motor amplitude potential; MNCV (m/s), motor nerve conduction velocity.

FIGURE 6

(A) Heterozygous (Htz) versus homozygous (Hmz) variant. (B) The distribution of variants based on their typology.

FIGURE 7

On the left schematic and 3D (front and back) representation of MME protein. In cyan peptidase M‐13, N‐terminal domain, in dark yellow Peptidase M‐13, C‐terminal domain, in green transmembrane domain. Identified variants are marked in red (missense) and cyan (loss of function). On the right localization and interaction with nearby amino acids of the wild‐type (SX) and mutant (DX) codons involved in missense variant. H‐Bonds are depicted with a cyan dotted line; Gly225Ala: No apparent consequences on protein structure; Thr347Cys: Loss of a H‐Bond with Asp209; Arg410Leu: No apparent consequences on protein structure; Tyr414Cys: Loss of a H‐Bond with Asn514.

FIGURE 8

Localization and interaction with nearby amino acids of the wild‐type (SX) and mutant (DX) codons involved in missense variant. H‐Bonds are depicted with a cyan dotted line—clashes between residues are depicted in purple dotted line; Val604Ile: No apparent consequences on protein structure; Ile649Thr: New H‐Bond between Thr649 and Leu644; Ile649Ser: No apparent consequences on protein structure; Asn689Lys: Loss of H‐bonds with Arg68, Leu573, and Pro563—Clashes with Leu573; Gln692His: Loss of H‐Bonds with Ans72, Arg68, Cys88 and Leu69—New H‐Bonds between His692 and Leu 688—Clashes between His692, Arg68, and Asn689; Gly715Asp: Clashes between Asp715 and Trp694.