Microdeletion in the 22q11 region causes a variety of disorders, including the DiGeorge (DGS; OMIM 188400) and Velo-cardio-facial (VCFS; OMIM 192430) syndrome. Summarized as a deletion syndrome (22q11DS), it is the most frequent chromosomal microdeletion syndrome, occurring once in 4,000 live births . Applying FISH with commercially available N25 (D22S75) or TUPLE1 probes located in the proximal commonly deleted region of 22q11.2 is a proven method for detection of the 22q11 deletion. Most of the 22q11.2 deletions can be found using FISH diagnostic probes, although this method has failed to detect deletions that are either proximal or distal to the FISH probes used [7, 19, 20]. Methods employing multiple genetic markers in the 22q11 region are increasingly important for the accurate identification of genomic microdeletions. MLPA analysis with multiple probes has been applied to analyze the 22q11 chromosome in detail , and the commercially available high-definition MLPA 22q11 kit has proven to be a sensitive tool for detecting copy number changes on the long arm of chromosome 22. In our study, in one patient (NO.10D) who tested negative for the 22q11.2 deletion using FISH probes N25 (D22S75) and TUPLE1, MLPA analysis actually identified an atypical deletion from CDC45L to CLTCL1.
In general, CTDs are the disorders most commonly associated with the 22q11.2 deletion syndrome [22, 23]. Of the study subjects, 20% (7/35) of those with PA/VSD, 5.4% (4/74) with TOF, 33% (1/3) with PTA and 2% (1/51) with DORV were detected to have the 22q11.2 deletion. None of the patients with TGA had the 22q11.2 deletion. In the subgroup of patients presenting with the 22q11.2 deletion, 53.8% (7/13) suffered from PA/VSD, 30.8% (4/13) from TOF, and 69.2% from RAA or APCAs (Table 1 and 2). This result is in line with the findings of previous studies, which concluded that a high prevalence of the deletion is noted in patients with TOF, PA/VSD, PTA, as well as RAA and major APCAs [23, 24]. Despite its commonness in patients with the 22q11.2 deletion in previous studies [25–27], none of the patients with the IAA condition examined in this study showed this deletion. This finding may be due to the small number (n = 4) of IAA patients investigated or to possible differences in the 22q11DS pathoanatomy between Western and Asian populations. Further investigation of the deletion in a larger sample size of IAA patients is necessary to clarify this issue.
To date, multiple phenotypic features and associated abnormalities have been observed in patients with the 22q11.2 deletion. However, no consistent correlation between the genotype and phenotype has been shown for this syndrome. Rauch et al. conjectured that conotruncal cardiac defects related to the 22q11.2 deletion are usually associated with the common 3 Mb or 1.5 Mb proximally nested deletions, both of which involve the TBX1 gene . This study shows that all deletion types are found in the commonly deleted region, implicating the involvement of the TBX1 gene. However, a detailed analysis of the clinical manifestations did not reveal any genotype-phenotype correlations in the subjects of our study (Table 2).
TBX1 is thought to be a major candidate gene that influences the cardiac phenotype or its severity in patients carrying the 22q11.2 deletion . Its dose-dependent role in heart development has been demonstrated in mouse models . Therefore, screening of the TBX1 gene on the residual 22q11 homologous chromatid would likely indicate whether the sequence variants or mutations of the remaining TBX1 copy would have an impact on the phenotype, thereby revealing possible correlations between the TBX1 genotype and cardiac phenotype. No mutations were found in this group of patients, but eight sequence variants were found in the haploid TBX1 gene. When comparing the del22q11 CTD patients to the non-del CTDs patients, the frequencies of polymorphism c.933A > G (rs41298840) in the del22q11 patients significantly differed from that in the non-del22q11 CTD patients and healthy controls (P < 0.05). The non-synonymous SNP rs41298838 (c.928G > A, G310S), which is a known mutation  previously predicted to be a tolerant polymorphism , was found in both the non-del22q11 CTD patients and healthy controls at similar frequencies. Prior studies have found that although the TBX1G310S mutation does not prevent transactivation, it does prevent the TBX1-SMAD1 interaction [30, 31], which may have had an impact on the homeostasis of the cardiac progenitor cells . These differences imply that the loci rs41298838 and rs41298840 may have an impact on the pathogenesis of this syndrome. However, the findings presented may be biased due to the small number of del22q11 patients studied.
The allele and genotype frequencies of SNPs rs5748417 (c.-87+242T > C) and rs5748418 (c.-87+256G > A) were significantly different between the non-del22q11 CTDs patients and controls. The alleles c.-87+242T and c.-87+256G were more frequent in the group of non-del22q11 patients with CTDs than in controls. These differences suggest the association of these SNPs in the pathogenesis of CTDs. However, further studies are needed to clarify the mechanism, especially since both of the SNPs are located within the 5'UTR intron of the TBX1 gene.