The Phenotype And Gene Mutation Analysis Of HOXD13 In A Congenital Synpolydactyly Pedigree

Objective: (Syndactyly,SD)is the most common type of finger(toe) malformation and belongs to chromosome dominant inheritance. Clinically, SD can be classified into five subtypes among which the causative genes of Syndactyly Type I, II and III have been definitely mapped to 2q34～36,2q31～32,2q21～23.2 respectively. Syndactyly type II is named as congenital synpolydactyly (SPD). Clinical presentation, in general, is syndactyly accompanied with hyperdactylia while the typical presentation is complete and partial webbing between 3rd and 4th fingers and 4th and 5th toes. Patients with malformation in hands may present with abnormal palm lines and those with severe deformation in feet may have hyperdactylia or failure of development in the base of metatarsus. Urethra disruption occurs in some cases simultaneously.Hox genes (homEobox), i.e., homeotic genes, as the major genes in controling development, were found to be gene clusters associated with human body's development especially embryo development and are crucial to the regulation of human organ initiation and cell differentiation. Current research suggests that SPD is the first human limb malformation syndrome caused by heterozygous mutaution of HOXD13 resulting in polyalanine expansion. HOXD13 is located in 2q31 region containing two exons with the length of gene 1365bp and code region 1008bp. The code protein production contains a homologous functional region in the 3'end and two polyserinechain and a polyalaninechain 5'end. It is proved that typical SPD pedigree is caused by polyalanine expansion,PAE encoded by the first exon of HOXD13 gene.SPD is a malformation of autosomal dominant inheritance. The prevalence of SPD in men and women is equal with a tendency of pedigree and incomplete penetrance and variable expressivity. Severely diseased individuals suffer the great loss of hand function and fail to perform normal activities, thus contributing to a heavy burden to the familes and society. The study of genetics not only helps to make diagnosis for patients and locate the mutation position, but also provides genetic counseling for pregnant women with high risks to promote the quality of population through reducing the birh trate of fetus with congenital defects. This study is to analyze the clinical characterists and genetic modes of patients through a survey of the phenotype of a SPD pedigree. In the study, venous blood of the family members is collected followed by a chromosome karyosome analysis of the blood. Then the genome DNA is extracted to augment the aimed fragment and analyze the sequence. Method:1 Family members from a four-generation-SPD pedigree in Pingyang, Hebei Province were defined as subjects with a total of 10 members including the propositus in the pedigree enrolled. Based on the fact that all the patients in this pedigree present with congenital SPD, the living four of them accepted detailed clinical physical examination, and hands and feet of the propositus IV and II1,III1 were X rayed.2 With the consent of SPD patients and their family members, peripheral blood (EDTA anticoagulant) was extracted from the patients and their first-degree relatives.Then 1ml venous blood of the propositus and his father was extracted and anticoagulated with Heparinum to be cultured as perioheral lymphocyte and the chromosome karyotype was analyzed.3 DNA -20℃was extracted from the periperepheral anticoagulant blood of the patients and their first-degree relatives for standby preservation. With the purpose of detecting the gene mutation, specific solicitation was designed and the mutation hot pot in the first exon that contains encoding polyalaninechain incomplete trinucleotide acid repeating arrangement of the HOXD13 genes was augmented while the production was detected through 4% agar sugar gel electrophoresis to measure whether mutation exists. Following a sequence measure of the purified augmented production, a comparison was made between the sequence and normal ones in Genebank. Results:1 Both men and women in this pedigree present with SPD in sequence of generation without intersection and atavism. It belongs to inherited enchromosome dominant condition.2 Patients in the pedigree present with SPD to varying degrees whereas there exists a significant discrepancy of expressivity among them.3 Based on the analysis of chromosome karytype of the propositus and his father, abnormalities in the structure and number of chromosome were not found.4 The agar sugar gel electrophoresis detection showed that both normal persons and patients all have a belt with the length of 505bp. After purifying and measuring the sequence of the augmented production, another 9 trinucleotide acid with all together 27bp repeating sequence was observed in the HOXD13 polyalanine encoding sequence of all the patients. The duplication expands the stretch of alanines from 15 to 24.Conclusions:1 Four generations in the pedigree present with SPD in sequence without atavism and gender association and one of the parents of the patients possesses SPD, all of which accord with the enchromosome dominant inheritance traits.2 Detection of chromosome karyotype of the propositus's father was 46, XY, belonging to a normal one.3 Alanines in encoding polyalaninechain of HOXD13 gene in the patients were expanded from 15 to 24. Accodingly, HOXD13 genes are the causative genes for congenital synpolydactyly pedigree.