Lack of association between AKAP2 and the susceptibility of adolescent idiopathic scoliosis in the Chinese population

Adolescent idiopathic scoliosis (AIS) is a well characterized spinal deformity that affects millions of children world-widely [1]. As a condition exclusive to human beings, the pathogenesis of AIS has been widely investigated in the past decades [2‐6]. Among the multiple proposed factors, the role of genetic factor in the development of AIS has been of great interest, since obvious hereditary trend has been observed in AIS families [7‐9]. Large population studies showed that 11% of the first-degree relatives of AIS patients could present scoliosis [9]. Moreover, AIS Twins showed a significantly higher concordance rate for the disease as compared with those reported in the first-degree relatives [10]. Despite consensus on the familial nature of AIS, the mode of inheritance has been on debate. Different inheritance patterns has been proposed, including an autosomal dominant, X-linked, and multifactorial pattern [8, 11].

To explore the genetic background of AIS, Miller et al. firstly applied genetic linkage analysis was to familial patients, which unveiled several candidate regions [12]. Later, using genetic association study, a number of susceptible genes were reported for AIS, such as ESR1, NTF3, and CDH13 [13‐15]. Recently, genome-wide association studies (GWASs) conducted revealed a few more susceptible loci of AIS in the Caucasian, the Japanese and the Chinese populations [16‐20]. Though a meta-analysis of the Caucasian and the Japanese GWAS dataset, Wise et al. [21] reported a functional variant in PAX1 was significantly associated with AIS. Nevertheless, the common variants reported by these association studies can only explain limited variation of AIS, since they all confer a small odds ratio to the risk of the disease.

Compared with common variants, rare variants are generally thought to exert a greater influence on the risk of inherited disease [22]. For AIS, variants of smaller effect have yet to be found, which could contribute substantially to the missing heritability [22]. With the development of sequencing technologies and reduction in sequencing cost, large-scale sequencing studies has been performed to detect rare variants associated with AIS [23‐27]. Using whole exome sequencing, Buchan et al. [25] firstly analyzed rare variants in severe AIS cases and identified FBN1 and FBN2 as candidate genes for AIS, which were successfully replicated in two independent cohorts of patients from European ancestry and Han Chinese population, respectively. Through genetic linkage analyses combined with exome sequencing, Patten et al. [24] identified 3 rare missense variants in the POC5 that cosegregated with the disease in a large family with IS. Rare variants of POC5 were further replicated in an independent cohort of AIS patients, which added to the reliability of their findings [24]. Baschal et al. [27] performed exome sequencing in a four-generation family with IS and identified the variant p.Asn786Ser in the HSPG2 gene as a potential contributor to the phenotype. Enrichment of this variant was also confirmed in two independent cohorts of unrelated IS patients. In a recent study of Chinese population [23], a novel mutation of AKAP2 was observed in a family with AIS, which was believed to play a role in the etiology of AIS. However, the sample size of affected family members is small and there was a lack of functional analysis supporting the association of AKAP2 with AIS. In this study, we aimed to validate the relationship between AKAP2 and AIS in a large-scale general population.

Aiming to validate the association of the rare variant in AKAP2 with the development AIS, we performed the genotyping of c.2645A > C in a cohort of 1254 AIS patients and 1232 normal controls, but found none case of mutation. Among a total of 116 common variants covering AKAP2, none was found to have significantly different allele frequency between the patients and the normal controls. Moreover, the mRNA expression of AKAP2 in the blood was comparable between the AIS patients and the controls. Obviously, the previously reported association of AKAP2 with the susceptibility of AIS in Chinese population was not replicated in the present study. It is noteworthy that there were different study designs between our study using population-based samples and the previous study of Li et al. [23] using familial patients. In their study, a mutation of c.2645A > C was detected through whole exome sequencing in the 5 affected members with AIS, 2 of whom were found to have remarkably decreased expression level of AKAP2. In this study, we recruited a large number of sporadic patients and normal controls which we believed were powerful enough to detect the effect sizes as reported by the study of Li et al. [23]. Herein, it is likely that the mutation of AKAP2 may play a role in the development of familial AIS, while its role in the general AIS population should be further validated.

Panza et al. [28] reported a balanced de novo translocation t(7;9)(p14.1;q31.3) that disrupts AKAP2 gene on chromosome 9. In such case of rare translocation linked to the disease, genes on chromosome 7 may demonstrate stronger association with AIS. Therefore, we cannot rule out that there could also be such event in AIS patients, since previous linkage study has reported evidence suggestive of linkage in Chromosome 7 [29]. Pinpointing the susceptible genes located at 7p14.1 may be an interesting topic worthy of further investigation.

Studying the genetics of AIS is usually difficult since there is a high degree of genetic heterogenicity among patients [7]. AIS can be a potential presence of many different genetic defects, all of which may eventually lead to the same clinical phenotype recognized as spinal curves. It is well documented that there is a lack of replication on most of the previously reported predisposition genes of AIS except for LBX1, MMP3 and IL6 [30‐33]. Recently, mechanism underlying the role of LBX1 in the development of AIS was illustrated by the functional characterization of the most significantly associated SNP which presents a novel pathological feature of LBX1 in body axis deformation [34]. Herein, without strong evidence produced through functional experiment, research merely based on simple association analysis cannot be sound enough to establish a reproducible result. Of note, the primary limitation of the study reported by Li et al. [23] lies in that the relationship between mutation of c.2645A > C and the expression of AKAP2 was not validated through in-vivo cellular experiment such as luciferase assay. Taken together, the function of AKAP2 as a predisposition gene of AIS should be cautiously interpreted.

Our large-scale replication study of the variants in AKAP2 gene did not support its association with the susceptibility of AIS in the Chinese population. In future study, functional studies of the previously reported rare variant are warranted to clarify whether the variant can regulate the expression of AKAP2. The whole AKAP2 gene can be sequenced in larger AIS cohorts to identify potentially missing mutations.