Detection And Functional Study Of DOCK6 Gene Mutations In Adams-Oliver Syndrome

Adams-Oliver syndrome(AOS)(OMIM#100300)is an autosomal recessive multiple congenital anomaly syndrome characterized by aplasia cutis congenita(ACC),terminal transverse limb defects(TTLD)and variable involvement of brain,eyes,and cardiovascular system.According to the inheritance mode of AOS,it can be divided into autosomal dominant inheritance and autosomal recessive inheritance.Among them,Adams-Oliver syndrome,which is autosomal recessive inheritance caused by DOCK6 gene mutation,is called AOS2.The main clinical manifestations of AOS2 patients are terminal limb defects such as finger(toe)absence,short finger(toe),syndactyly and nail dysplasia and scalp dysplasia,such as skin defect on the top of the head and scalp atrophy.Some patients may have abnormal brain,eye and cardiovascular system development.DOCK6(Dedicator of cytokinesis 6)gene is located in 19 p13.2,and consists of 48 exons.DOCK6 gene encoding a member of the dedicator of cytokinesis(DOCK)family of atypical guanine nucleotide exchange factors(GEFs).GEFs activate and inhibit small GTPases and are components of intracellular signaling networks.The DOCK6 protein can activate two GTPases known as Cdc42 and Rac1 by exchanging GTP for the attached GDP,which transmit signals that are critical for various aspects of embryonic development,such as increment,polarization,migration,adhesion,secretion,maintenance of cell morphology,cell division,apoptosis and phagocytosis The deletion of DHR2 domain can result in the inactivation of DOCK6 protein,which leads to the formation of AOS phenotype.Here,we reported a child with AOS phenotype who has compound heterozygous mutations in the DOCK6 gene(c.4106+2T>C and c.3063 C>G(p.Y1021*)).Both mutations were reported for the first time.Sanger DNA sequencing further validated her lineal consanguinity,indicating that the mutations were inherited maternally and paternally,respectively.The mutation of c.4106+2T>C is located at the splice site and is predicted to damage the normal splicing.To explore the pathological feature of this mutation,minigene assay was used to investigate the effects of the c.4106+2T>C mutation on splicing.The results showed that the c.4106+2T>C mutation in minigene resulted aberrant splicing which caused a four-nucleotide insertion in the transcript and a premature stop codon.Our results showed that c.4106+2T>C and c.3063 C>G(p.Y1021*)of DOCK6 gene were the pathogenic factors in this AOS family.The nonsense mutation c.3063 C>G(p.Y1021*)produced the termination codon ahead of time,resulting in the absence of an abnormal protein of N-terminal 1026 amino acid the c.4106+2T>C mutation results in the activation of covert splicing sites in intron 32,resulting in the retention of 4 bp bases between exon 32 and exon 33.The redundant 4 bp base causes the transcript to shift its reading frame and terminate its codon in 1374 amino acids in advance.Our findings expanded the number of reported cases of this rare disease and the mutation spectrum of DOCK6 mutations,which can serve as the basis for prenatal diagnosis and genetic counseling.