Current phenotypic and genetic spectrum of syndromic deafness in Tunisia: paving the way for precision auditory health.

Hearing impairment (HI) is a prevalent neurosensory condition globally, impacting 5% of the population, with over 50% of congenital cases attributed to genetic etiologies. In Tunisia, HI underdiagnosis prevails, primarily due to limited access to comprehensive clinical tools, particularly for syndromic deafness (SD), characterized by clinical and genetic heterogeneity. This study aimed to uncover the SD spectrum through a 14-year investigation of a Tunisian cohort encompassing over 700 patients from four referral centers (2007-2021). Employing Sanger sequencing, Targeted Panel Gene Sequencing, and Whole Exome Sequencing, genetic analysis in 30 SD patients identified diagnoses such as Usher syndrome, Waardenburg syndrome, cranio-facial-hand-deafness syndrome, and H syndrome. This latter is a rare genodermatosis characterized by HI, hyperpigmentation, hypertrichosis, and systemic manifestations. A meta-analysis integrating our findings with existing data revealed that nearly 50% of Tunisian SD cases corresponded to rare inherited metabolic disorders. Distinguishing between non-syndromic and syndromic HI poses a challenge, where the age of onset and progression of features significantly impact accurate diagnoses. Despite advancements in local genetic characterization capabilities, certain ultra-rare forms of SD remain underdiagnosed. This research contributes critical insights to inform molecular diagnosis approaches for SD in Tunisia and the broader North-African region, thereby facilitating informed decision-making in clinical practice.

Granular type I corneal dystrophy in a large consanguineous Tunisian family with homozygous p.R124S mutation in the TGFBI gene.

Purpose: We report the clinical features and the mutational analysis in a large Tunisian family with granular corneal dystrophy type I (GCD1). Patients and Methods: Thirty-three members of the Tunisian family underwent a complete ophthalmologic examination. DNA extraction and direct Sanger sequencing of the exons 4 and 12 of transforming growth factor ß Induced (TGFBI) gene was performed for 42 members. For the molecular modeling of TGFBI protein, we used pGenTHREADER method to identify templates, 3D-EXPRESSO program to align sequences, MODELLER to get a homology model for the FAS1 (fasciclin-like) domains and finally NOMAD-ref web server for the energy minimization. Results: The diagnosis of GCD1 was clinically and genetically confirmed. Sequencing of exon 4 of TGFBI gene revealed the p.[R124S] mutation at heterozygous and homozygous states in patients with different clinical severities. Visual acuity was severely affected in the homozygous patients leading to a first penetrating keratoplasty. Recurrence occurred rapidly, began in the seat of the corneal stitches and remained superficial up to 40 years after the graft. For heterozygous cases, visual acuity ranged from 6/10 to 10/10. Corneal opacities were deeper and predominating in the stromal center. According to bioinformatic analysis, this mutation likely perturbs the protein physicochemical properties and reduces its solubility without structural modification. Conclusions: Our study describes for the first time phenotype-genotype correlation in a large Tunisian family with GCDI and illustrates for the first time clinical and histopathological presentation of homozygous p.[R124S] mutation. These results help to understand pathophysiology of the disease.

Cytogenetic and molecular diagnosis of Fanconi anemia revealed two hidden phenotypes: Disorder of sex development and cerebro-oculo-facio-skeletal syndrome.

BACKGROUND: Several studies have shown a high rate of consanguinity and endogamy in North African populations. As a result, the frequency of autosomal recessive diseases is relatively high in the region with the co-occurrence of two or more diseases. METHODS: We report here on a consanguineous Libyan family whose child was initially diagnosed as presenting Fanconi anemia (FA) with uncommon skeletal deformities. The chromosome breakage test has been performed using mitomycin C (MMC) while molecular analysis was performed by a combined approach of linkage analysis and whole exome sequencing. RESULTS: Cytogenetic analyses showed that the karyotype of the female patient is 46,XY suggesting the diagnosis of a disorder of sex development (DSD). By looking at the genetic etiology of FA and DSD, we have identified p.[Arg798*];[Arg798*] mutation in FANCJ (OMIM #605882) gene responsible for FA and p.[Arg108*];[Arg1497Trp] in EFCAB6 (Gene #64800) gene responsible for DSD. In addition, we have incidentally discovered a novel mutation p.[Gly1372Arg];[Gly1372Arg] in the ERCC6 (CSB) (OMIM #609413) gene responsible for COFS that might explain the atypical severe skeletal deformities. CONCLUSION: The co-occurrence of clinical and overlapping genetic heterogeneous entities should be taken into consideration for better molecular and genetic counseling.

A Tunisian family with a novel mutation in the gene CYP4F22 for lamellar ichthyosis and co-occurrence of hearing loss in a child due to mutation in the SLC26A4 gene.

BACKGROUND: Co-occurrence of two genetic diseases is challenging for accurate diagnosis and genetic counseling. The recent availability of whole exome sequencing (WES) has dramatically improved the molecular diagnosis of rare genetic diseases in particular in consanguineous populations. METHODS: We report here on a consanguineous family from Southern Tunisia including three members affected with congenital ichthyosis. The index case had a hearing loss (HL) and ichthyosis and was primarily suspected as suffering from keratitis-ichthyosis-deafness (KID) syndrome. WES was performed for the index case, and all members of the nuclear family were sequenced (Sanger method). RESULTS: The WES approach allowed the identification of two strong candidate variants in two different genes; a missense mutation c.1334T>G (p.Leu445Trp) in exon 11 of SLC26A4 gene, associated with isolated HL and a novel missense mutation c.728G>T (p.Arg243Leu) in exon 8 of CYP4F22 gene likely responsible for ichthyosis. These two mutations were predicted to be pathogenic by three pathogenicity prediction softwares (Scale-Invariant Feature Transform [SIFT], Polymorphism Phenotyping [PolyPhen], Mutation Taster) to underlie the HL and ichthyosis, respectively. CONCLUSIONS: The present study raises awareness about the importance of familial history for accurate diagnosis of syndromic genetic diseases and differential diagnosis with co-occurrence of two distinct clinical entities. In addition, in countries with limited resources, WES sequencing for a single individual provides a cost effective tool for molecular diagnosis confirmation and genetic counseling.

TOP3B: A Novel Candidate Gene in Juvenile Myoclonic Epilepsy?

Juvenile myoclonic epilepsy (JME) is characterized by seizures, severe cognitive abnormalities, and behavior impairments. These features could evolve over time and get worse, especially when the encephalopathy is pharmacoresistant. Thus, genetic studies should provide a better understanding of infantile epilepsy syndromes. Herein, we investigate the genetics of JME in a consanguineous family analyzing the copy number variations detected using over 700 K SNP arrays. We identified a 254-kb deletion in the 22q11.2 region, including only the TOP3B gene, detected in the patient and her father. TOP3B encodes a topoisomerase DNA (III) ß protein and has been implicated in several neurological diseases such as schizophrenia and autism. In this study, we discuss the implication of the 22q11.2 region in neurodevelopmental disorders and the association of TOP3B with epilepsy.