SIX1+PAX3+ identify a progenitor for myogenic lineage commitment from hPSCs.

The earliest skeletal muscle progenitor cells (SMPCs) derived from human pluripotent stem cells (hPSCs) are often identified by factors expressed by a diverse number of progenitors. An early transcriptional checkpoint that defines myogenic commitment could improve hPSC differentiation to skeletal muscle. Analysis of several myogenic factors in human embryos and early hPSC differentiations found SIX1+PAX3+ co-expression was most indictive of myogenesis. Using dCas9-KRAB hPSCs, we demonstrate that early inhibition of SIX1 alone significantly decreased PAX3 expression, reduced PAX7+ SMPCs, and myotubes later in differentiation. Emergence of SIX1+PAX3+ precursors can be improved by manipulating seeding density, monitoring metabolic secretion and altering the concentration of CHIR99021. These modifications resulted in the co-emergence of hPSC-derived sclerotome, cardiac and neural crest that we hypothesized enhanced hPSC myogenic differentiation. Inhibition of non-myogenic lineages modulated PAX3 independent of SIX1. To better understand SIX1 expression, we compared directed differentiations to fetal progenitors and adult satellite cells by RNA-seq. Although SIX1 continued to be expressed across human development, SIX1 co-factor expression was dependent on developmental timing. We provide a resource to enable efficient derivation of skeletal muscle from hPSCs.