| Monogenic disease is also commonly referred to as Mendelian genetic disease.It is caused by a single gene mutation.Genetic factors play a leading role in the occurrence of the disease.The mode of inheritance follows the Mendelian law of separation.In general,monogenic disease includes five inheritance modes: autosomal dominant(AD),autosomal recessive(AR),X-linked dominant(XLD),Xlinked recessive(XLR)and Y-linked(YL).For a single disease,it is usually rare.However,monogenic diseases are very diverse and about10,000 species have been found.Monogenic diseases have become a severe threat to human health.According to existing studies,the vast majority of monogenic diseases are caused by mutations in proteincoding regions.Since whole exome sequencing(WES)has the advantages of fast detection,high sequencing depth and low cost in discovering the variants in protein-coding region,it has become an important method to study monogenic diseases.Corneal dystrophy(CD)is a kind of monogenic disease that originates in the cornea,mostly autosomal dominant and usually occurs before the age of 20.Clinically,this disease manifests progressive corneal opacity with specific histopathological features.We selected the DNA samples of two distantly related patients in a family with autosomal dominant CD for WES,obtained the non-synonymous variants of target region(exon,splice site and untranslated regions)through sequence alignment with the reference genome hg19,removed the variants which are in 1000 genomes and db SNP,and then screened the variants shared by the two patients.Through searching literature,we found that among candidate genes both TGFBI and KRT3 were associated with CD.Sanger sequencing was used to verify the phenotypic co-segregation of TGFBI gene c.1877A>C and KRT3 gene c.396＿413del variants in 13 family members,respectively.The sequencing results showed that 8 patients carried the c.1877A>C variants of TGFBI and it is absent in 5 normal members.KRT3 gene c.396＿413del variant exists in both family patients and normal members,which excludes the pathogenicity of KRT3 to this family.When SIFT,Poly Phen-2 and Mutation Taster assessed the biological effects of the c.1877A>C variant of TGFBI,the results were “affect protein function”,“probably damaging” and “disease causing”,respectively.In addition,TGFBI gene c.1877A>C variant was not found in 100 normal controls.This study,for the first time in the Chinese Han population found the c.1877A>C variant of TGFBI,which is of great significance to fertility guidance for the Chinese population.Hereditary spastic paraplegia(HSP)is a genetic disease associated with neurodevelopmental disorders and has a considerable clinical and genetic heterogeneity.The vast majority of HSP manifests as autosomal inheritance and its main clinical symptoms are lower limbs spasticity and muscle weakness.The most common pathological feature is axonal lesions of the corticospinal tract.We selected the DNA samples of two patients and one normal member in a family with autosomal dominant HSP for WES,obtained the non-synonymous variants of target region(exon,splice site and untranslated regions)through sequence alignment with the reference genome hg19,removed the variants which are in 1000 genomes and db SNP,and then screened the variants shared by the two patients but not in normal member.Through searching literature,among candidate genes only KIAA0196 was associated with HSP.Sanger sequencing was used to verify the phenotypic co-segregation of the c.1771T>C variant of KIAA0196 in 10 pedigree members.The sequencing results showed that 4 patients and 3 asymptomatic younger members(under 25 years)carried the c.1771T>C variant of KIAA0196,whereas three asymptomatic older members(over 40 years)did not carry the variant.When SIFT,Poly Phen-2 and Mutation Taster assessed the biological effects of the c.1771T>C mutation of KIAA0196 and the results were “affect protein function”,“probably damaging” and“disease causing”,respectively.Three asymptomatic carriers were followed-up and we found that one of them developed into HSP one year later(at age 26)and the other two had no signs of the disease.Considering that the c.1771T>C variants of KIAA0196 is pathogenic in another Chinese pedigree with HSP,we consider this variant is a common pathogenic variant of KIAA0196 in Chinese population and have a considerable clinical heterogeneity.Through whole exome sequencing and Sanger sequencing,we has studied the molecular genetic causes of two monogenic families.The disease-causing gene mutations in the CD and HSP families were identified respectively,providing birth guidance for their families and enriching the theoretical basis of molecular genetics for corresponding diseases. |
