Molecular basis of adolescent idiopathic scoliosis

Abstract

Adolescent idiopathic scoliosis (AIS) is the most common form of spinal deformity affecting 1%-4% of adolescents worldwide. "Idiopathic" means "unknown", suggesting that AIS lacks an agreed theory of its etiopathogenesis. AIS patients with a mild spinal curvature generally do not cause any significant clinical symptoms. However, this condition may worsen during the puberty stage until skeletal maturity. In such progressive conditions, painful osteoarthritis of the spine, progressive deformities, radiculopathy, coronary muscle fatigue, sagittal plane imbalance, and psychological effects are noticeably presented. In some severe AIS cases, the spinal curvature intensely compresses the lung and heart, compromising breathing and finally causing death.

Although the pathoetiology of AIS remains elusive, many studies proposed that genetic factors dominate the susceptibility to AIS. In this study, two rare heterozygous missense mutations (c.1984C>T, p.R662W and c.617A>T, p.Y206F) in the SLC6A9 gene in two pedigrees of familial AIS were identified by whole genome sequencing (WGS). Besides, eight more rare variants of SLC6A9 were also identified in sporadic AIS cases. SLC6A9 encodes glycine transporter 1 (GLYT1), which is critical in the clearance of extracellular glycine in excitatory and inhibitory synapses. Concomitantly, we found that the glycine concentrations, in familial AIS patients, are significantly increased compared to familial unaffected members. Moreover, we also showed that the glycine uptake activities of the GLYT1 carrying AIS mutations are significantly reduced, mainly due to the impaired cytomembrane localization. Endoplasmic reticulum or Golgi apparatus retention and quick endocytosis may be the leading causes for the impaired cell membrane localization of GLYT1 mutants.

We further generated a glyt1 mutant zebrafish model to address how glyt1 dysfunction may interfere with the spinal alignment in AIS. Glyt1 mutant zebrafish exhibited a curly body, diminished swimming capacity with a reduced survival rate, which is believed to result from disruption of left and right body axis coordination in the absence of malformation in motor circuits, skeletal muscle, and central nervous system (CNS). Interestingly, some glyt1 mutant zebrafish exhibited apparent spinal curvature that first appeared at the juvenile stage, recapitulating human AIS. Pharmacological inhibition of glyt1 in wild type larvae phenocopied the spinal curvature observed in glyt1 mutant zebrafish. Furthermore, selective blockage of glycine receptors or neutralization of glycine partially rescued the spinal phenotypes in glyt1 mutant zebrafish. Our studies reveal that glyt1 regulates glycine concentration in CNS and thereby maintains the proper left and right coordination of the body axis, providing new insights into the etiology and pathogenesis of AIS.