Abstract
Introduction
Kyphoscoliosis is defined by a structural lateral curvature of the spine of 10° or more and an excessive thoracic kyphotic curve of 40° or more. Genetic analyses of families in which two or more members had kyphoscoliosis identified a 3·5 Mb area on chromosome 5p containing three genes of the Iroquois (IRX) homeobox family, IRX1, IRX2, and IRX4, which were then sequenced.
Methods
Exons and highly conserved non-coding regions (HNCRs) 500 kb upstream and downstream fromIRX1, IRX2, and IRX4 were sequenced in 46 individuals from six families. Selection of these elements was based on PhastCons Placental Mammal Conserved Elements, Multiz Alignment. Single-nucleotide polymorphism (SNP) genotypes and sequence variants were obtained from all individuals. There were 431 SNPs, 61 in IRX4 regions, 80 in IRX2 regions, and 290 in IRX1 regions. 137 SNPs were novel. Mendelian inconsistencies were detected with PEDCHECK (inconsistency rate: 1·4%; missing data: 2·8%). SNPs and individuals with greater than 10% missing rate were excluded. Association analyses (ASSOC [SAGE version 6.0.1]) of the quantitative trait with patient's largest curve, were undertaken on 391 SNPs.
Results
Association analyses resulted in 12 SNPs with p values less than 0·025, 11 of which were located upstream and downstream from IRX1. The most significant p value (p=0·000382) was obtained for rs35710183 (table). Multiple variants were found surrounding IRX1. The most prominent is a single base-pair deletion in all affected individuals genotyped in one family. All individuals with kyphoscoliosis and those with scoliotic curves greater than 35° had genotypes differing from the reference (unaffected) genotype for 23 SNPs. Several of these SNPs had significant p values for the association analyses done previously.
Conclusions
The phenotype of kyphoscoliosis has been linked to sequence variants that lie within regulatory regions of the IRX homeobox gene family. Further analyses to establish the relevance of these findings will be done through in-vivo and in-vitro assays. The identification of spinal genetic determinants related to axial growth and maturation will help with the understanding of spinal pathology and potentially allow for development of directed therapeutic interventions.
