Role of Pax1 in the homeostasis of the spine and pathogenesis of Adolescent Idiopathic Scoliosis

Abstract

Background: Adolescent idiopathic scoliosis (AIS) is of high prevalence affecting children between the ages of 10 to skeletal maturity. Affected individuals are born without noticeable spinal defects and subsequent changes are contributed by interplay between genetic and environmental factors. Wedging of the intervertebral discs (IVD) leading to curvature is considered as a factor contributing to the etiology of AIS. IVD consists of a central jelly-like nucleus pulposus surrounding by lamellar structure annulus fibrosus (AF). They function as a unit to dissipate mechanical load from spinal movements. PAX1 is a transcription factor with known functions in early AF formation, however its role in IVD homeostasis after birth is unclear. SNPs in an enhancer region of the PAX1 gene has been shown to be associated with AIS from a reduced PAX1 expression. This project aims to exploit how the level of Pax1 affect the formation and homeostasis of IVD. Since wedging of IVDs cause mechanical loading to AF tissue, we will also address PAX1 expression in response to mechanical loading. Methods: IVD changes in heterozygous (Pax1+/-) or homozygous (Pax1-/-) mice for a Pax1-null allele were characterized. Pax1 expression in postnatal mouse IVDs was determined with bulk and single-cell transcriptome analyses. Mechanical loading on the IVD was induced using a mouse tail-looping model, simulating the IVD wedging condition in AIS, and the impact on the expression of Pax1 and targeted genes studied. Results: Pax1-/- mice developed malformed and asymmetric IVDs, hence contributing to a spinal curvature phenotype. Pax1+/- mice developed normal spine and IVDs, however, static compression lead to a dramatic cell lost in AF. Transcriptome analyses showed that Pax1 is expressed in 80% of AF cells of 8-weeks old wildtype mouse and it was upregulated in the AF of looped tails, indicating Pax1 is responsive to mechanical stimulus. Conclusion: PAX1 level is important for normal IVD development and homeostasis and responsive to mechanical loading. AF cells in Pax1+/- mice are sensitive to apoptosis in response to abnormal loading affecting AF function. This finding provides a novel correlation of PAX1 to mechanosensing and IVD homeostasis, providing new insights into the etiology for AIS.