Familial Kyphoscoliosis: Continued Linkage on Chromosome 5

Summary: Seven families with kyphoscoliosis underwent genomic screening, linkage analyses, and finemapping utilizing single nucleotide polymorphisms in order to identify the genetic mechanism of this disorder. A candidate locus from the initial screen on chromosome 5p15.3 was narrowed. Three distinct peaks were identified inclusive of 3 specific genes: IRX1, 2, and 4. Genes within the Iroquois family are involved in patterning of the vertebrate embryo, and are excellent candidates for the genetic mechanism of this disorder.

Introduction: Thoracic kyphoscoliosis (KS) is a deformity in which a lateral spinal curvature is seen in conjunction with thoracic hyperkyphosis. As part of a large study of familial idiopathic scoliosis, a subgroup of families with KS was identified and potentially linked to a region on chromosomes 5 and 13 through a genome-wide scan. The purpose of the current work is finemapping to the critical region on chromosome 5 with identification of candidate genes.

Methods: As part of a sample of families with IS (202 families, 1198 individuals) recruited with IRB approval, 7 families (53 individuals) were identified that had >1 individual with kyphoscoliosis. The average degree of scoliosis was 40°; average degree of hyperkyphosis was 53°. Genome-wide scanning and linkage results highlighted chromosomes 5 and 13. Subsequent SNP assays on chromosome 5 were performed on 2 different platforms: 1) sixty-eight SNPs (1SNP/215kb) were analyzed with the Sequenom hME assay, and 2) a custom pool of 317 SNPs (1SNP/82kb) was genotyped on the Illumina platform. Model-independent linkage analyses were performed using SIBPAL.

Results: Initial analyses from the genome-wide scan identified a 10Mb peak on chromosome 5p15.3. Sequenom SNPs narrowed the region to 3.54 Mb (p<0 .05). Illumina SNP linkage analyses resulted in one highly significant peak (1.76Mb), and 2 smaller peaks within the region (0.68Mb 1.22Mb). Within this region, IRX1, 2, 4 genes are positioned.

Discussion: The clinical spectrum of FIS led to the identification of a KS subgroup and the linkage of genetic loci to the phenotype. Subsequent work has narrowed our focus, in particular, to the IRX genes, which code for homeobox proteins associated with embryonic midline development. Currently, mutational analyses and sequencing of the specific candidate genes are in progress.