Identification And Analysis Of New Mutation Sites Of NSUN2 Pathogenic Genes In Mental Retardation Families Based On High-throughput Sequencing Technology

Intellectual Disability(or Mental Retardation)is a common childhood neurological disease.This study was designed to determine the genetic causes of three sibling-female patients with intellectual retardation in a Chinese non-consanguinity pedigree using Whole‐exome sequencing(WES).This study provides a theoretical basis for prenatal diagnosis of mental retardation caused by NSUN2 gene variation,and lays a foundation for subsequent treatment.In this study,8 members in the 2 generations of this family were collected as the research objects to draw a family map.After obtaining informed consent,peripheral blood samples from 8 people were collected,anticoagulated,and whole blood genomic DNA(g DNA)was extracted.At the same time,detailed medical history collection and physical examination were mainly performed on 3 patients,and other clinical auxiliary examinations including Magnetic Resonance Imaging(MRI),Electrocardiogram(ECG),ultrasound examination of abdomen,thyroid and heart,and chromosome karyotype analysis were performed on the three patients.Using WES to screen pathogenic variants in the genome,We excluded low‐confidence variants,common variants with allele frequencies(AF)>1% in the gnom AD database(http://gnomad.broadinstitute.org/),benign variants,including synonymous,harmless missense variants predicted by Poly Phen‐2 and Mutation Taster,and those with no impact on splicing predicted by Max Ent Scan software.Thereafter,clinical symptoms of global DD and ID served as filtering indexes to analyze candidate variants.And the candidate variants were subsequently verified using Sanger sequencing.WES revealed a previously unreported homozygous nonsense variant in exon 9 of the NSUN2 gene(NM＿017755.5: c.1004T>A,p.Leu335*),which was consistent with the clinical phenotype of the patients and co‐segregated with the disease in their family.This mutation site was discovered for the first time in China and reported for the first time in the world.Comparing the phenotypes of these patients with the phenotypes of reported patients,in addition to stunting,short stature,microcephaly and facial deformities,several novel clinical features related to NSUN2 variants were found,including the absence of secondary sexual characteristics,difficulty in feeding,slender hands and fingers,severe limitation of finger movement,hallux valgus,varus foot.Data from clinical examination and genetic testing indicated that the disease in this family is mental retardation,and the homozygous nonsense variant NM＿017755.5:c.1004T>A,p.Leu335* in exon 9 of NSUN2 gene were the causative gene mutations of this family.This is the first time that a homozygous mutation at a new mutation site in the NSUN2 gene has been discovered in a non-consanguineous Chinese line.We have expanded the phenotypic spectrum associated with NSUN2 mutations.