Integrated multi-omics reveals minor spliceosome inhibition causes molecular stalling and developmental delay of the mouse forelimb.

Developmental insults causing limb progenitor cell cycle defects or death tend to produce micromelic limbs with maintained segmentation. This suggests that the developing limb is plastic yet has a bias towards proximo-distal patterning. Here we use a minor spliceosome-deficient (U11-null) mouse forelimb, which has severe micromelia yet maintains proximo-distal segmentation, to decipher the mechanism(s) underlying this form of developmental robustness. We show that U11 loss triggers transcriptomic stalling upon spatially heterogenous mis-splicing of minor intron-containing genes. Through spatial transcriptomics, we detected a failure of the U11-null forelimb to separate its distal patterning program from its proximal differentiation program, which was supported by single-cell RNAseq-determined developmental delay of U11-null chondroprogenitors. Ultimately, these molecular and cellular deficits culminated in perturbed chondrogenesis, myogenesis, and axonogenesis. Taken together, we suggest that, upon sensing depletion of progenitors, the limb halts its transcriptional networks to pause its cellular trajectory, affording time to restructure its developmental program.