Monoallelic variants resulting in substitutions of MAB21L1 Arg51 Cause Aniridia and microphthalmia

Monoallelic missense variants in MAB21L1 cause a novel autosomal dominant microphthalmia

PURPOSE

The biallelic variant of MAB21L1 has previously been documented in conjunction with the autosomal recessive cerebellar, ocular, craniofacial, and genital syndrome (COFG). The purpose of this study was to investigate the gene-disease association of MAB21L1 and the newly discovered autosomal dominant (AD) microphthalmia.

METHODS

We report the presence of an exceptionally rare missense variant in a single allele of the Arg51 codon of MAB21L1 among four individuals from a single family diagnosed with microphthalmia, which suggesting an autosomal dominant inheritance pattern. Subsequently, based on comprehensive literature review, we identified another 13 families that have reported cases of autosomal dominant microphthalmos.

RESULTS

Genotype-phenotype analysis revealed that patients with a single allele missense variant in MAB21L1 exhibited solely eye abnormalities. This starkly diverged from the clinical presentation of COFG, typified by the concurrent occurrence of ocular and extraocular symptoms stemming from the biallelic variant in MAB21L1. Our findings revealed that the heterozygous pathogenic variant in MAB21L1 resulted in the emergence of autosomal dominant microphthalmia. By combining these genetic and experimental evidence, the clinical validity of MAB21L1 and the emerging autosomal dominant microphthalmia can be regarded as moderate.

CONCLUSION

In summary, there is sufficient convincing evidence to prove that MAB21L1 is a novel pathogenic gene responsible for autosomal dominant microphthalmia, thus offering valuable insights for precise diagnosis and targeted therapeutic interventions in cases of microphthalmia.

Congenital anterior segment ocular disorders: Genotype-phenotype correlations and emerging novel mechanisms

Development of the anterior segment of the eye requires reciprocal sequential interactions between the arising tissues, facilitated by numerous genetic factors. Disruption of any of these processes results in congenital anomalies in the affected tissue(s) leading to anterior segment disorders (ASD) including aniridia, Axenfeld-Rieger anomaly, congenital corneal opacities (Peters anomaly, cornea plana, congenital primary aphakia), and primary congenital glaucoma. Current understanding of the genetic factors involved in ASD remains incomplete, with approximately 50% overall receiving a genetic diagnosis. While some genes are strongly associated with a specific clinical diagnosis, the majority of known factors are linked with highly variable phenotypic presentations, with pathogenic variants in FOXC1, CYP1B1, and PITX2 associated with the broadest spectrum of ASD conditions. This review discusses typical clinical presentations including associated systemic features of various forms of ASD; the latest functional data and genotype-phenotype correlations related to 25 ASD factors including newly identified genes; promising novel candidates; and current and emerging treatments for these complex conditions. Recent developments of interest in the genetics of ASD include identification of phenotypic expansions for several factors, discovery of multiple modes of inheritance for some genes, and novel mechanisms including a growing number of non-coding variants and alleles affecting specific domains/residues and requiring further studies.

A bird's eye view on the use of whole exome sequencing in rare congenital ophthalmic diseases

Phenotypic and genotypic heterogeneity in congenital ocular diseases, especially in anterior segment dysgenesis (ASD), have created challenges for proper diagnosis and classification of diseases. Over the last decade, genomic research has indeed boosted our understanding in the molecular basis of ASD and genes associated with both autosomal dominant and recessive patterns of inheritance have been described with a wide range of expressivity. Here we describe the molecular characterization of a cohort of 162 patients displaying isolated or syndromic congenital ocular dysgenesis. Samples were analyzed with diverse techniques, such as direct sequencing, multiplex ligation-dependent probe amplification, and whole exome sequencing (WES), over 20 years. Our data reiterate the notion that PAX6 alterations are primarily associated with ASD, mostly aniridia, since the majority of the cohort (66.7%) has a pathogenic or likely pathogenic variant in the PAX6 locus. Unexpectedly, a high fraction of positive samples (20.3%) displayed deletions involving the 11p13 locus, either partially/totally involving PAX6 coding region or abolishing its critical regulatory region, underlying its significance. Most importantly, the use of WES has allowed us to both assess variants in known ASD genes (i.e., CYP1B1, ITPR1, MAB21L1, PXDN, and PITX2) and to identify rarer phenotypes (i.e., MIDAS, oculogastrointestinal-neurodevelopmental syndrome and Jacobsen syndrome). Our data clearly suggest that WES allows expanding the analytical portfolio of ocular dysgenesis, both isolated and syndromic, and that is pivotal for the differential diagnosis of those conditions in which there may be phenotypic overlaps and in general in ASD.

Missense Mutations in MAB21L1: Causation of Novel Autosomal Dominant Ocular BAMD Syndrome

Purpose

Biallelic MAB21L1 variants have been reported to cause autosomal recessive cerebellar, ocular, craniofacial, and genital syndrome (COFG), whereas only five heterozygous pathogenic variants have been suspected to cause autosomal dominant (AD) microphthalmia and aniridia in eight families. This study aimed to report an AD ocular syndrome (blepharophimosis plus anterior segment and macular dysgenesis [BAMD]) syndrome based on clinical and genetic findings from patients with monoallelic MAB21L1 pathogenic variants in our cohort and reported cases.

Methods

Potential pathogenic variants in MAB21L1 were detected from a large in-house exome sequencing dataset. Ocular phenotypes of the patients with potential pathogenic variants in MAB21L1 were summarized, and the genotype-phenotype correlation was analyzed through a comprehensive literature review.

Results

Three heterozygous missense variants in MAB21L1, predicted to be damaging, were detected in 5 unrelated families, including c.152G>T in 2, c.152G>A in 2, and c.155T>G in one. All were absent from gnomAD. The variants were de novo in two families, transmitted from affected parents to offspring in two families, and with an unknown origin in the other family, demonstrating strong evidence of AD inheritance. All patients revealed similar BAMD phenotypes, including blepharophimosis, anterior segment dysgenesis, and macular dysgenesis. Genotype-phenotype analysis suggested that patients with monoallelic MAB21L1 missense variants had only ocular anomalies (BAMD), whereas patients with biallelic variants presented both ocular and extraocular symptoms.

Conclusions

Heterozygous pathogenic variants in MAB21L1 account for a new AD BAMD syndrome, which is completely different from COFG caused by homozygous variants in MAB21L1. Nucleotide c.152 is likely a mutation hot spot, and the encoded residue of p.Arg51 might be critical for MAB21L1.