Expanding the Molecular Spectrum of MMP21 Missense Variants: Clinical Insights and Literature Review

Abstract

Background/objectives: The failure of physiological left-right (LR) patterning, a critical embryological process responsible for establishing the asymmetric positioning of internal organs, leads to a spectrum of congenital abnormalities characterized by laterality defects, collectively known as "heterotaxy". MMP21 biallelic variants have recently been associated with heterotaxy syndrome and congenital heart defects (CHD). However, the genotype-phenotype correlations and the underlying pathogenic mechanisms remain poorly understood.

Methods: Patients harboring biallelic MMP21 missense variants who underwent diagnostic genetic testing for CHD or heterotaxy were recruited at the Institute for Maternal and Child Health-I.R.C.C.S. "Burlo Garofolo". Additionally, a literature review on MMP21 missense variants was conducted, and clinical data from reported patients, along with molecular data from in silico and modeling tools, were collected.

Results: A total of 18 MMP21 missense variants were reported in 26 patients, with the majority exhibiting CHD (94%) and variable extra-cardiac manifestations (64%). In our cohort, through Whole-Exome Sequencing (WES) analysis, the missense p.(Met301Ile) variant was identified in two unrelated patients, who both presented with heterotaxy syndrome.

Conclusions: Our comprehensive analysis of MMP21 missense variants supports the pathogenic role of the p.(Met301Ile) variant and provides significant insights into the disease pathogenesis. Specifically, missense variants are distributed throughout the gene without clustering in specific regions, and phenotype comparisons between patients carrying missense variants in compound heterozygosity or homozygosity do not reveal significant differences. These findings may suggest a potential loss-of-function mechanism for MMP21 missense variants, especially those located in the catalytic domain, and highlight their critical role in the pathogenesis of heterotaxy syndrome.

Keywords: Congenital Heart Defects; Heterotaxy; MMP21.

Figure 1

(A) Family pedigree of Patient 1, showing grade of parents’ consanguinity, (B) ECG of Patient 1 displaying multifocal atrial beats with junctional rhythm, right axis deviation and positive T wave in right precordial leads; (C) Family pedigree of Patient 2, (D) ECG of Patient 2 showing suggestive findings of ventricular inversion, including absence of q waves in V5–V6 and presence of q waves in V1. Filled symbols with black arrows represent index patients; half-filled symbols represent healthy carrier individuals; numbers in pedigree represent the number of individuals with the same degree of biological relationship (e.g., Patient 2 has two paternal uncles).

Figure 2

(A) Schematic representation of the MMP21 gene and the MMP21 protein. Exons are depicted in blue and introns in light blue. Protein domains are represented in different colors and aminoacidic positions at the beginning and end of each domain are reported. Missense variants identified in the literature are reported. (B) Structural model of MMP21 as predicted by AlphaFold (XZ plane). Protein backbone is represented in gray, while domains are reported in the same color as Figure 2A (i.e., ZnMc domain in green and HX domains in pink). Aminoacidic positions in which missense variants occur are colored in red in the 3D structure. (C) Backbone of the ZnMc domain of MMP21 is reported in green. Positions in which missense variants occur are colored in red and amino acid structures are highlighted (carbon atoms are depicted in grey, oxygen in red, nitrogen in blue, and sulfur in yellow). (D) Protein alignment showing conservation of methionine 301 across species, highlighted by the red frame; amino acids with similar physicochemical properties are represented in the same colors, as defined by the Clustal Omega software. (ZnMc: Zinc-dependent metalloprotease; HX: Hemopexin-like repeats). In figures, the variant identified in our patients, c.903G>A, p.(Met301Ile), is reported in red.