Current progress in genetic research of adolescent idiopathic scoliosis

Adolescent idiopathic scoliosis (AIS) is a structural deformity of the spine estimated to affect millions of children with a prevalence of 2-4% (1-3). Commonly understood as a complex disease, AIS is believed to be resulted from the interaction among multiple genetic loci as well as certain environmental factors (4-6). It is possible that genetic factors may be involved in specific aspects of scoliosis including the shape of a scoliosis curve and the risk for curve progression. A number of population studies have documented that scoliosis runs within families and that there is a higher prevalence of scoliosis among relatives of patients with scoliosis than within the general population (7).

In the past decades, genetic linkage analysis and candidate gene association analysis were utilized broadly to reveal the genetic basis underling AIS (7-14). Since year 2000, several genes have been reported to be involved in the etiopathogenesis of AIS, including estrogen receptor estrogen receptor 1 (ESR1) (14), estrogen receptor 2 (ESR2) (9), matrilin 1 (MATN1) (10), melatonin receptor 1B (MTNR1B) (13), tryptophan hydroxylase 1 (TPH1) (11), DOT1L (15), TGFB1 (16) and so on. Seemingly great progress being made to clarify the aetiology of AIS, it should be noted that validation of these genes in different populations is always challenging. Replication of the associations in other studies with larger sample sizes or different populations is of great importance to validate the results of genetic association studies. Tang et al. (17) reported that ESR1 was not associated with the risk of AIS in HongKong. Similarly, Takahashi et al. (18) also found no association between polymorphisms of ESR1 and occurrence of AIS in Japan. As for ESR2, TPH1, MATN1, and MTNR1B gene, none of them can be successfully validated (19). Recently, we analysed the genotype of TGFB1 in a large cohort of AIS patients and healthy controls from Chinese population, whereas no significant association was found (under submission).

As a powerful tool for the investigation of complex disease, genome-wide association studies (GWAS) was recently applied to the genetic research of AIS (20,21). In 2011, Ikegawa and his fellows performed the first GWAS in Japanese AIS population, and reported that ladybird homeobox 1 (LBX1) gene were significantly associated with the occurrence of AIS (21). Two years later, on the basis of previous GWAS data, the group of Ikegawa et al. reported another gene, G protein–coupled receptor 126 (GPR126), could be implicated in the development of AIS (22). Compared with previously reported genetic locus of AIS, variants of LBX1 and GPR126 seemed far more promising with regard to their contributions to the development of AIS. The sample size of the GWAS performed by Ikegawa was several times larger than earlier association studies. In the stage of chips experiment, they included a cohort of more than 2,000 patients and controls. And in the validation stage, over 10,000 normal controls were recruited (21). In addition to the large sample size, it is noteworthy that both genes were successfully replicated in other ethnics including southern and northern Chinese Han population and European ancestry (22-24). More importantly, high expression level of LBX1 was detectable and specific to skeletal muscle and spinal cord of both adult and fetal (21). And GPR126 was highly expressed in the cartilage of human and the proliferating chondrocytes of the vertebral body of the embryonic mouse (22). Moreover, the functional consequence of GPR126 was also confirmed by the studies of Ikegawa et al. (22). They found that GPR126 knockdown zebrafish had shorter body lengths and delayed ossification of the vertebrae, as well as slower escape responses, indicating possible neurological defects (22). Taken together, these findings greatly extended the genetic research of AIS, and expanded our understanding on the possible etiology.

It is widely hypothesized that AIS could be induced by abnormal skeletal growth, abnormal somatosensory function and abnormal development of neural system (4). The function of the two AIS-related genes reported by Ikegawa et al. appears well coincided with the classical hypothesis of the aetiology of AIS. In mouse, LBX1 is an important determinant of dorsal spinal neurons and hindbrain somatosensory neurons (25). A recent study reported that GPR126-null mice have limb posture abnormalities and growth failure (26). The most significantly associated SNP rs6570507 of GPR126 gene was also associated with trunk length in a GWAS meta-analysis of height in European populations (27). GPR126 also functions in nervous system control, and this process may contribute to AIS susceptibility. Several studies have indicated that GPR126 is essential for myelination, a process necessary for proper nerve conduction velocity (28). To conclude, these observations extraordinarily suggest the possibility that LBX1 and GPR126 gene may be involved in the susceptibility of AIS through abnormal spinal development or neural system growth.

Prior to the GWAS performed by Ikegawa et al., Sharma et al. performed GWAS of approximately 327,000 SNPs in 419 white AIS families. They found the rs1400180 of the CHL1 gene was strongly associated with AIS. It was an interesting finding since CHL1 encodes an axon guidance protein related to Robo3, the mutations of which can lead horizontal gaze palsy with progressive scoliosis (20). As a well-established AIS susceptibility gene in Caucasian population, however, CHL1 gene failed to be replicated in Chinese Han population (29). In another GWAS consisted of 196 cases and 401 controls from southern Chinese population, Fan et al. (from personal communication) reported two SNPs on one particular chromosome showed marginal significant association (snp1: P=1.32×10^-6, odds ratio =0.52; snp2: P=1.23×10^-5, odds ratio =0.55), both lack of further validation in an enlarged sample of cases and controls. Therefore, for the genetic research of AIS, the following points should be kept in mind to have a sound and reliable result. First, there exists a significant divergence between different populations regarding the association of susceptible genes with pathogenesis of AIS. Second, independent replication study with enlarged sample size is always warranted for the determination of the susceptible gene. Third, population stratification should be well controlled when recruiting the subjects.

Conclusions

As estimated by Ikegawa et al., variants of LBX1 and GPR126 can explain approximately 1% of the trait variance in AIS (22). It is obvious that additional AIS risk factors wait to be discovered. A global meta-analysis of published GWAS data should be helpful for the elucidation of the veiled genetic locus related to AIS. Further functional studies are also necessary to elucidate how the variants in LBX1 and GPR126 alter the risk of AIS in humans.

Acknowledgements

Disclosure: The authors declare no conflict of interest.

References

1. Horne JP, Flannery R, Usman S. Adolescent idiopathic scoliosis: diagnosis and management. Am Fam Physician 2014;89:193-8. [PubMed]
2. Weinstein SL. Advances in the diagnosis and management of adolescent idiopathic scoliosis. J Pediatr Orthop 1994;14:561-3. [PubMed]
3. Lonstein JE. Adolescent idiopathic scoliosis: screening and diagnosis. Instr Course Lect 1989;38:105-13. [PubMed]
4. Ahn UM, Ahn NU, Nallamshetty L, et al. The etiology of adolescent idiopathic scoliosis. Am J Orthop (Belle Mead NJ) 2002;31:387-95. [PubMed]
5. Miller NH. Cause and natural history of adolescent idiopathic scoliosis. Orthop Clin North Am 1999;30:343-52. [PubMed]
6. Miller NH, Mims B, Child A, et al. Genetic analysis of structural elastic fiber and collagen genes in familial adolescent idiopathic scoliosis. J Orthop Res 1996;14:994-9. [PubMed]
7. Miller NH, Justice CM, Marosy B, et al. Identification of candidate regions for familial idiopathic scoliosis. Spine (Phila Pa 1976) 2005;30:1181-7. [PubMed]
8. Girardo M, Bettini N, Dema E, et al. The role of melatonin in the pathogenesis of adolescent idiopathic scoliosis (AIS). Eur Spine J 2011;20 Suppl 1:S68-74. [PubMed]
9. Zhang HQ, Lu SJ, Tang MX, et al. Association of estrogen receptor beta gene polymorphisms with susceptibility to adolescent idiopathic scoliosis. Spine (Phila Pa 1976) 2009;34:760-4. [PubMed]
10. Chen Z, Tang NL, Cao X, et al. Promoter polymorphism of matrilin-1 gene predisposes to adolescent idiopathic scoliosis in a Chinese population. Eur J Hum Genet 2009;17:525-32. [PubMed]
11. Wang H, Wu Z, Zhuang Q, et al. Association study of tryptophan hydroxylase 1 and arylalkylamine N-acetyltransferase polymorphisms with adolescent idiopathic scoliosis in Han Chinese. Spine (Phila Pa 1976) 2008;33:2199-203. [PubMed]
12. Gao X, Gordon D, Zhang D, et al. CHD7 gene polymorphisms are associated with susceptibility to idiopathic scoliosis. Am J Hum Genet 2007;80:957-65. [PubMed]
13. Qiu XS, Tang NL, Yeung HY, et al. Melatonin receptor 1B (MTNR1B) gene polymorphism is associated with the occurrence of adolescent idiopathic scoliosis. Spine (Phila Pa 1976) 2007;32:1748-53. [PubMed]
14. Wu J, Qiu Y, Zhang L, et al. Association of estrogen receptor gene polymorphisms with susceptibility to adolescent idiopathic scoliosis. Spine (Phila Pa 1976) 2006;31:1131-6. [PubMed]
15. Mao S, Xu L, Zhu Z, et al. Association between genetic determinants of peak height velocity during puberty and predisposition to adolescent idiopathic scoliosis. Spine (Phila Pa 1976) 2013. [Epub ahead of print]. [PubMed]
16. Ryzhkov II, Borzilov EE, Churnosov MI, et al. Transforming Growth Factor Beta 1 is a Novel Susceptibility Gene for Adolescent Idiopathic Scoliosis. Spine (Phila Pa 1976) 2013. [Epub ahead of print]. [PubMed]
17. Tang NL, Yeung HY, Lee KM, et al. A relook into the association of the estrogen receptor [alpha] gene (PvuII, XbaI) and adolescent idiopathic scoliosis: a study of 540 Chinese cases. Spine (Phila Pa 1976) 2006;31:2463-8. [PubMed]
18. Takahashi Y, Matsumoto M, Karasugi T, et al. Replication study of the association between adolescent idiopathic scoliosis and two estrogen receptor genes. J Orthop Res 2011;29:834-7. [PubMed]
19. Takahashi Y, Matsumoto M, Karasugi T, et al. Lack of association between adolescent idiopathic scoliosis and previously reported single nucleotide polymorphisms in MATN1, MTNR1B, TPH1, and IGF1 in a Japanese population. J Orthop Res 2011;29:1055-8. [PubMed]
20. Sharma S, Gao X, Londono D, et al. Genome-wide association studies of adolescent idiopathic scoliosis suggest candidate susceptibility genes. Hum Mol Genet 2011;20:1456-66. [PubMed]
21. Takahashi Y, Kou I, Takahashi A, et al. A genome-wide association study identifies common variants near LBX1 associated with adolescent idiopathic scoliosis. Nat Genet 2011;43:1237-40. [PubMed]
22. Kou I, Takahashi Y, Johnson TA, et al. Genetic variants in GPR126 are associated with adolescent idiopathic scoliosis. Nat Genet 2013;45:676-9. [PubMed]
23. Jiang H, Qiu X, Dai J, et al. Association of rs11190870 near LBX1 with adolescent idiopathic scoliosis susceptibility in a Han Chinese population. Eur Spine J 2013;22:282-6. [PubMed]
24. Fan YH, Song YQ, Chan D, et al. SNP rs11190870 near LBX1 is associated with adolescent idiopathic scoliosis in southern Chinese. J Hum Genet 2012;57:244-6. [PubMed]
25. Uchiyama K, Ishikawa A, Hanaoka K. Expression of lbx1 involved in the hypaxial musculature formation of the mouse embryo. J Exp Zool 2000;286:270-9. [PubMed]
26. Waller-Evans H, Prömel S, Langenhan T, et al. The orphan adhesion-GPCR GPR126 is required for embryonic development in the mouse. PLoS One 2010;5:e14047. [PubMed]
27. Soranzo N, Rivadeneira F, Chinappen-Horsley U, et al. Meta-analysis of genome-wide scans for human adult stature identifies novel Loci and associations with measures of skeletal frame size. PLoS Genet 2009;5:e1000445. [PubMed]
28. Monk KR, Oshima K, Jörs S, et al. Gpr126 is essential for peripheral nerve development and myelination in mammals. Development 2011;138:2673-80. [PubMed]
29. Qiu XS, Lv F, Zhu ZZ, et al. Lack of association between the CHL1 gene and adolescent idiopathic scoliosis susceptibility in Han Chinese: a case-control study. BMC Musculoskelet Disord 2014;15:38. [PubMed]