Analysis of the IRE1/XBP1 unfolded protein response (UPR) pathway in Schmid metaphyseal chondrodysplasia hypertrophic chondrocyte endoplasmic reticulum (ER) stress

We set out to determine the role of the IRE1/XBP1 pathway, the most ancient and highly conserved endoplasmic reticulum (ER) stress-sensing pathway of the unfolded protein response (UPR), in Schmid metaphyseal chondrodysplasia (MCDS). RNA derived from hypertrophic zones microdissected from growth plates of wildtype mice, mice lacking XBP1 activity in chondrocytes (Xbp1CartΔEx2), mice carrying a COL10A1 pN617K mutation (ColXN617K), and compound mutants (C/X) was analyzed by whole genome microarray analysis. 1633 probes were differentially expressed between ColXN617K and wildtype, 215 probes were differentially expressed between Xbp1CartΔEx2 and wildtype, and 1337 probes were differentially expressed between C/X and wildtype. 885 probes were differentially expressed between ColXN617K and wildtype but not Xbp1CartΔEx2 and wildtype or C/X and wildtype, thus representing the XBP1-dependent response to hypertrophic chondrocyte ER stress. 688 probes were differentially expressed between ColXN617K and wildtype and between C/X and wildtype but not Xbp1CartΔEx2 and wildtype, thus representing the XBP1-independent response to hypertrophic chondrocyte ER stress. Results were validated by qPCR. Entire growth plate hypertrophic zones were microdissected from one tibia from each of three 2-week old wildtype mice, three 2-week old mice carrying a COL10A1 p.N617K mutation (ColXN617K), three 2-week old mice lacking XBP1 activity in chondrocytes (Xbp1CartΔEx2), and three 2-week old mice resulting from a cross between ColXN617K and Xbp1CartΔEx2 (C/X).