Clinical application of next generation sequencing for Mendelian disease diagnosis in the Iranian population.

Next-generation sequencing (NGS) has been proven to be one of the most powerful diagnostic tools for rare Mendelian disorders. Several studies on the clinical application of NGS in unselected cohorts of Middle Eastern patients have reported a high diagnostic yield of up to 48%, correlated with a high level of consanguinity in these populations. We evaluated the diagnostic utility of NGS-based testing across different clinical indications in 1436 patients from Iran, representing the first study of its kind in this highly consanguineous population. A total of 1075 exome sequencing and 361 targeted gene panel sequencing were performed over 8 years at a single clinical genetics laboratory, with the majority of cases tested as proband-only (91.6%). The overall diagnostic rate was 46.7%, ranging from 24% in patients with an abnormality of prenatal development to over 67% in patients with an abnormality of the skin. We identified 660 pathogenic or likely pathogenic variants, including 241 novel variants, associated with over 342 known genetic conditions. The highly consanguineous nature of this cohort led to the diagnosis of autosomal recessive disorders in the majority of patients (79.1%) and allowed us to determine the shared carrier status of couples for suspected recessive phenotypes in their deceased child(ren) when direct testing was not possible. We also highlight the observations of recessive inheritance of genes previously associated only with dominant disorders and provide an expanded genotype-phenotype spectrum for multiple less-characterized genes. We present the largest mutational spectrum of known Mendelian disease, including possible founder variants, throughout the Iranian population, which can serve as a unique resource for clinical genomic studies locally and beyond.

Guidelines for clinical interpretation of variant pathogenicity using RNA phenotypes.

Over the last 5 years, RNA sequencing (RNA-seq) has been established and is increasingly applied as an effective approach complementary to DNA sequencing in molecular diagnostics. Currently, three RNA phenotypes, aberrant expression, aberrant splicing, and allelic imbalance, are considered to provide information about pathogenic variants. By providing a high-throughput, transcriptome-wide functional readout on variants causing aberrant RNA phenotypes, RNA-seq has increased diagnostic rates by about 15% over whole-exome sequencing. This breakthrough encouraged the development of computational tools and pipelines aiming to streamline RNA-seq analysis for implementation in clinical diagnostics. Although a number of studies showed the added value of RNA-seq for the molecular diagnosis of individuals with Mendelian disorders, there is no formal consensus on assessing variant pathogenicity strength based on RNA phenotypes. Taking RNA-seq as a functional assay for genetic variants, we evaluated the value of statistical significance and effect size of RNA phenotypes as evidence for the strength of variant pathogenicity. This was determined by the analysis of 394 pathogenic variants, of which 198 were associated with aberrant RNA phenotypes and 723 benign variants. Overall, this study seeks to establish recommendations for integrating functional RNA-seq data into the the American College of Medical Genetics and Genomics and the Association for Molecular Pathology guidelines classification system.

RNA sequencing role and application in clinical diagnostic.

Although whole-exome sequencing and whole-genome sequencing has tremendously improved our understanding of the genetic etiology of human disorders, about half of the patients still do not receive a molecular diagnosis. The high fraction of variants with uncertain significance and the challenges of interpretation of noncoding variants have urged scientists to implement RNA sequencing (RNA-seq) in the diagnostic approach as a high throughput assay to complement genomic data with functional evidence. RNA-seq data can be used to identify aberrantly spliced genes, detect allele-specific expression, and identify gene expression outliers. Amongst eight studies utilizing RNA-seq, a mean diagnostic uplift of 15% has been reported. Here, we provide an overview of how RNA-seq has been implemented to aid in identifying the causal variants of Mendelian disorders.

Novel Mutation in LARP7 in Two Iranian Consanguineous Families with Syndromic Intellectual Disability and Facial Dysmorphism.

BACKGROUND: Recently, we have reported mutations in LARP7 gene, leading to neurodevelopmental disorders (NDDs), the most frequent cause of disability in children with a broad phenotype spectrum and diverse genetic landscape. METHODS: Here, we present two Iranian patients from consanguineous families with syndromic intellectual disability, facial dysmorphism, and short stature. RESULTS: Whole-exome sequencing (WES) revealed a novel homozygous stop-gain (c.C925T, p.R309X) variant and a previously known homozygous acceptor splice-site (c.1669-1_1671del) variant in LARP7 gene, indicating the diagnosis of Alazami syndrome. CONCLUSION: These identified variants in patients with Alazami syndrome were consistent with previously reported loss of function variants in LARP7 and provide further evidence that loss of function of LARP7 is the disease mechanism.

Whole-Transcriptome Analysis Reveals Dysregulation of Actin-Cytoskeleton Pathway in Intellectual Disability Patients.

A significant level of genetic heterogeneity has been demonstrated in intellectual disability (ID). More than 700 genes have been identified in ID patients. To identify molecular pathways underlying this heterogeneity, we applied whole-transcriptome analysis using RNA-Seq in consanguineous families with ID. Significant changes in expression of genes related to neuronal and actin cytoskeletal functions were observed in all the ID families. Remarkably, we found a significant down-regulation of SHTN1 gene and up-regulation of FGFR2 gene in all ID patients. FGFR2, but not SHTN1, was previously reported as an ID causing gene. Detailed gene ontology analyses identified pathways linked to tyrosine protein kinase, actin cytoskeleton, and axonogenesis to be affected in ID patients. The findings reported here provide new insights into the candidate genes and molecular pathways underling ID and highlight the key role of actin cytoskeleton in etiology of ID.