A novel variant of C12orf4 in a consanguineous Armenian family confirms the etiology of autosomal recessive intellectual disability type 66 with delineation of the phenotype

Neurogenetic disorders associated with mutations in the FERRY complex: a novel disease class?

The five-subunit endosomal Rab5 and RNA/ribose intermediary (FERRY) complex is a newly described protein complex consisting of TBCK, PPP1R21, FERRY3 (previously C12orf4), CRYZL1, and GATD1. The FERRY complex is proposed to function as a Rab5 effector to shuttle mRNA to the cell periphery for local translation, a process especially important in cells with far reaching processes. Interestingly, three members of the FERRY complex are associated with ultra-rare neurogenetic disorders. Mutation of TBCK causes TBCK syndrome, mutation of PPP1R21 is associated with PPP1R21-related intellectual disability, and mutation of FERRY3 results in an autosomal recessive intellectual disability. Neurologic disorders have yet to be associated with mutation of GATD1 or CRYZL1. Here, we provide a review of each FERRY complex-related neurologic disorder and draw clinical comparisons between the disease states. We also discuss data from the current cellular and animal models available to study these disorders, which is notably disparate and scattered across different cell types and systems. Taken together, we explore the possibility that these three diseases may represent one shared disease class, which could be further understood by combining and comparing known information about each individual disease. If true, this could have substantial implications on our understanding of the cellular role of the FERRY complex and on treatment strategies for affected individuals, allowing researchers, clinicians, and patient organizations to maximize the utility of research efforts and resources to support patients with these disorders.

The Rab5 effector FERRY links early endosomes with mRNA localization

Localized translation is vital to polarized cells and requires precise and robust distribution of different mRNAs and ribosomes across the cell. However, the underlying molecular mechanisms are poorly understood and important players are lacking. Here, we discovered a Rab5 effector, the five-subunit endosomal Rab5 and RNA/ribosome intermediary (FERRY) complex, that recruits mRNAs and ribosomes to early endosomes through direct mRNA-interaction. FERRY displays preferential binding to certain groups of transcripts, including mRNAs encoding mitochondrial proteins. Deletion of FERRY subunits reduces the endosomal localization of transcripts in cells and has a significant impact on mRNA levels. Clinical studies show that genetic disruption of FERRY causes severe brain damage. We found that, in neurons, FERRY co-localizes with mRNA on early endosomes, and mRNA loaded FERRY-positive endosomes are in close proximity of mitochondria. FERRY thus transforms endosomes into mRNA carriers and plays a key role in regulating mRNA distribution and transport.

A novel variant of C12orf4 linked to autosomal recessive intellectual disability type 66 with phenotype expansion

BACKGROUND

Intellectual disability (ID) is a hallmark of many rare disorders that are highly heterogeneous and complex. A large number of specific genes are involved in development of this heterogeneity, and each of these genes is only found in a small number of patients. This weakens the definition of the predominant genotype and the phenotypic characteristics associated with that gene. Autosomal recessive ID type 66 (OMIM # 618221) is one of these very rare diseases created by defects in C12orf4 gene.

METHODS

The researchers in the current study included two patients from an Iranian family with initial diagnosis of non-syndromic ID to identify the possible genetic cause(s), and whole-exome sequencing (WES) was performed for the proband. The obtained variant was confirmed by Sanger sequencing and co-segregated in the family.

RESULT

The patients carried a novel pathogenic splicing variant called c.1441-1G>A in exon 12 of C12orf4 gene (NM_001304811). They predominantly manifested ID, behavioral problems, speech impairment, and dysmorphic facial features, some of which had not been reported in previous studies.

CONCLUSION

A novel pathogenic splicing variant was identified named c.1441-1G>A in C12orf4 gene. So far, only seven families have been reported with defects in this gene. Previous studies have not highlighted the exact clinical manifestations of these patients, thus, this study could contribute to better delineation of the genotype-phenotype correlation and interpretation of very rare variants of the gene.

A novel variant of C12orf4 in a consanguineous Armenian family confirms the etiology of autosomal recessive intellectual disability type 66 with delineation of the phenotype

Background

Intellectual disability (ID) is a feature of many rare diseases caused by thousands of genes. This genetic heterogeneity implies that pathogenic variants in a specific gene are found only in a small number of patients, and difficulties arise in the definition of prevailing genotype and characteristic phenotype associated with that gene. One of such very rare disorders is autosomal recessive ID type 66 (OMIM #618221) caused by defects in C12orf4. Up to now, six families have been reported with mostly truncating variants. The spectrum of the clinical phenotype was not emphasized in previous reports, and detailed phenotype was not always available from previous patients, especially from large cohort studies.

Methods

Exome sequencing was performed in a consanguineous Armenian family with two affected adult brothers.

Results

The patients carry a novel homozygous nonsense C12orf4 variant. The integration of previous data and phenotyping of the brothers indicate that the clinical picture of C12orf4 defects involves hypotonia in infancy, rather severe ID, speech impairment, and behavioral problems such as aggressiveness, unstable mood, and autistic features. Several other symptoms are more variable and less consistent.

Conclusion

This rather nonsyndromic and nonspecific clinical picture implies that additional patients with C12orf4 defects will likely continue to be identified using the “genotype‐first” approach, rather than based on clinical assessment. The phenotype needs further delineation in future reports.