The Relationship Between TBX5 And Ventricular Septal Defect Was Explored By Exon Sequencing

Background. A ventricular septal defect (VSD) is defined as an openingor hole in the interventricular septum. VSD is one of the commonestcongenital malformations of the heart, accounting for up to40%of allcardiac anomalies. It is not only a common isolated cardiac malformationbut also an intrinsic component of several complex malformations,including tetralogy of Fallot or functional univentricle. It might alsobe associated with lesions including transposition of the great arteries,congenitally corrected transposition, and aortic coarctation orinterruption.Many tiny VSDs present at birth and close shortlyafterwards or disappear during the first year of life. Largeinterventricular communication or doubly committed juxtarterialdefectswon’t close spontaneously. Consequently,patients with these VSDswill develop pulmonary hypertension or even pulmonary venousobstructive disease, which is a threat to life.Our understanding of the origins of ventricular septal defect islimited by our knowledge of mechanisms that lead to normal cardiacseptation. At present, information suggests that the septum has bothmesenchymal and muscular components. The mesenchymal element originatesmainly from fusion of the conotruncal and atrioventricular endocardialcushions. Mechanisms that initiate development of the muscular septumremains debate. Some researchers postulate that the muscular septumforms from coalescence of the part of the ventricular wall that isinterposed between the enlarging free walls of the developing right andleft ventricles, therefore, as the ventricular cavities become deeperthe septum grows passively inwards. An alternative hypothesis suggests that the muscular septum originates from a cluster of cells, the so-called primitive interventricular septum, which expands actively towardsthe cushions of the atrioventricular canal. Different factors probablylead to different locations of the defects. Failure of completeformation of the primitive interventricular septum could contribute totrabecular defects. Failure of fusion of the atrioventricular cushions—with each other or with the primary septum—could result in an inletdefect, whereas malalignment or poor development of outlet cushionsmight add to outlet defects. Finally, failure of complete closure of thearea that forms the membranous septum is in association with aperimembranous defect.TBX5is a member of the T-box transcription factor family and it islocated in12q24.1. It includes a total length of2441bp and9exonswhich encode518amino acid residues. Exon3-7encode a highly conservedDNA binding domain of about180amino acid residues which is offunctional importance. The C-terminal region had a helix-span-helix area,a8-bp-long DNA half-site (A/G)GGTGT (C/G/T)(A/G)， and a nuclearlocalization signal. Basson et al first reported that TBX5mutationcontributes to Holt-Oram syndrome, which is characterized by upper limbmalformations and cardiac septation defect, in1997. And they suggestedthat haploinsufficience of TBX5causes Holt-Oram syndrome. Claudia founda new mutation in the TBX5gene in two unrelated ASD-type HOS familiesin2003. In the same year, Stella analyzed68explanted hearts fromunrelated patients with various cardiac abnormalities including atrial(ASD), ventricular (VSD) and atrioventricular septal defects (AVSD).Direct sequencing detected nine mutations in diseased cardiac tissues ofpatients, eight of which are novel. Mutations were found in patients with ASD and AVSD, but not with VSD; and mutations were absent in normalheart tissue of same patients. A TBX5polymorphism was also associatedwith ventricular septal defect (without limb abnormalities) in theChinese Han population according the research conducted by Liu in2009.Recently, Smemo et al uncovered a low-frequency single-nucleotidepolymorphism (SNP) segregating in the population that abrogates acardiac-specific TBX5enhancer, thereby decoupling the heart (VSD) andhandphenotypes seen in the HOS patients. This study provides us a newinsight to the potential biological function of TBX5.Methods. There were216patients in the VSD group and250patients inthe control group. There were125males and91females in the VSD groupcompared with135and115patients, respectively in the control group(p=0.52). Mean age was128±321days in VSD group, compared with4.93±1.14years in the control group (p<0.001). Echocardiography wasperformed in all patients, mean size of theVSD was7.65±3.73mm. Thisreview was approved by Ethics Committee of Shanghai Children’s MedicalCenter. We collected the blood sample (2ml) from the patients and thehealthy children. Polymerase chain reaction (PCR) was used to amplify8exons and flanking introns of the TBX5gene. Direct forward and reversesequencing of the PCR products was performed. We aligned the acquiredsequences with those in GenBank by using basic local alignment searchtool (BLAST).Results. No mutation was found in the coding area of TBX5genge. One novel single-nucleotide polymorphisms (SNPs), c.663+36T>Cwas detected in the introns. The variant was identified63bp downstreamof the end of the exon6in the TBX5gene. Significant differences in the prevalence of the alleles T and C were observed in the controls andVSD patients.Conclusions. The mutation in the coding area of TBX5does notnecessarily contribute to a perimembranous or malalignment VSD.Transcriptional factor TBX5may play an important role in the emergence,fusion and proliferation of the ventricular trabeculations. VSD mayhavemultifactorial origins. An underlying inherited genetic predisposition,environmental causes, disrupt TBX5interaction with co-factors, andregulatory variation in a TBX5enhancer can produce isolated andsporadic VSD. Besides, impairness of the nuclear localisation of TBX5protein may be another causativefactor for VSD except thehaploinsufficiency of TBX5.