Introduction

Molecular regulation of the development of the cardiovascular system is a complex process whose defect leads to abnormalities in the heart. Therefore, congenital heart disease is common because defects in various parts of the heart, such as the atrium, ventricle, or cardiovascular system lead to the disease (Lin, Lin, Chen, Zhou, & Chang, 2012). The primary septum of the heart is formed between days 27 and 37, and defects in its formation lead to either atrial septal defects (ASD) or ventricle septal defects (VSD) (Gittenberger-de Groot, Calkoen, Poelmann, Bartelings, & Jongbloed, 2014). The prevalence of ASD is 7 per 10,000 births and women have double the prevalence of men (Fahed, Gelb, Seidman, & Seidman, 2013; Hoffman & Kaplan, 2002). The defects of the ventricular septal affecting the membrane or musculature of the septum are the most common congenital anomaly of the heart, occurring at a rate of 12 per 10,000 births (Bhatt et al., 2015).
Gene expression in the pathway of heart development is regulated by multiple transcription factors. Members of the T-box gene family (Tbx) are essential for normal heart development (Zhu et al., 2017). The Tbx5 gene consists of 9 exons with exons 2 to 9 coding the protein. The gene is located at 12q24.1 in the human genome and encodes a transcription factor that contains a highly conserved DNA binding motif composed of 181 amino acid residues called the T-box or T-domain (Liu et al., 2016; Stirnimann, Ptchelkine, Grimm, & Müller, 2010). In precursor cells, TBX5 acts with GATA4 and NKX2-5 to activate a large number of genes involved in cardiac specialization (Granados‐Riveron et al., 2012; van Weerd & Christoffels, 2016). Subsequently, the expression of TBX5 is limited to the atrium and left ventricle, and the ventricle septal forms on the border between the cells which express TBX5 and those that do not (van Weerd & Christoffels, 2016). TBX5 competes with TBX20, which is in the right ventricle (Brown et al., 2005). If the TBX5 expressing region is misplaced, the ventricular septal is formed in this new region. Also, TBX5 is important in the formation of upper human organs and its mutation causes Holt-Oram syndrome (HOS) (Basson et al., 1999; Nimura & Kaneda, 2016). Thus, the transcription factor of the homeobox TBX5, expressed during heart formation, acts as an important regulator and plays a vital role in activating the genes which cooperate in the formation of the right and left heart (Fujita et al., 2017).
Many studies have shown that mutations in the TBX5 gene mainly result in HOS. For example, the study by Postma et al. showed that a Gly125Arg mutation leads to a novel function in the TBX5 protein and affects the development of other organs, leading to the above-mentioned syndrome (Postma et al., 2008). Also, studies by Dreßen et al. showed that the Pro85Thr mutation in this protein resulted in this syndrome due to a severe decrease in the activation of other promoters (Dreßen et al., 2016). Other studies, such as Basson and Granados-Riveron, showed that the G80R and D111Y mutations led to a non-syndromic malformation of the cardiac septum (Basson et al., 1999; Granados‐Riveron et al., 2012). Also, mutations in this gene lead to non-syndromic diseases such as atrial fibrillation (Baban et al., 2014; Guo et al., 2016; Ma et al., 2016). The numerous mutations leading to HOS and other varied clinical symptoms point to the importance of the TBX5 gene and the complexity and variation of the gene regulation it participates in.
The purpose of this study was to find mutations in the TBX5 gene that are involved in the development of non-syndromic cardiac defect phenotypes. Therefore, in this study, screening of these gene mutations in patients with congenital non-syndromic defects of heart septal using high resolution melt (HRM) technique was done in Kurdish people.