Uncommitted precursor cells might contribute to increased incidence of embryonal rhabdomyosarcoma in heterozygous Patched1-mutant mice.

Embryonal rhabdomyosarcoma (ERMS) is a tumor of the skeletal muscle in children and is frequently initiated by heterozygous germline mutations in the Hedgehog (Hh) receptor Patched1 (Ptch), both in humans and mice. Using a conditional knock-out strategy in Ptch(flox/+) mice, we demonstrate that early embryonic stages are more susceptible to ERMS development than later stages and that cells normally not committed to undergo myogenesis at this stage represent the major source of ERMS. We found that deletion of a single copy of the Ptch allele at E9.5 using the ubiquitously active Rosa26CreERT2 resulted in a tumor incidence of 88% but reached only 44% and 12% when the Ptch allele was inactivated at E11.5 and E13.5, respectively. Induction of the Ptch mutation at E9.5 did also significantly shorten ERMS-free survival and increased tumor multiplicity compared with tumor induction at E11.5 and E13.5. Interestingly, we observed a more that 10-fold reduction of ERMS incidence when the Ptch mutation was specifically introduced in Myf5-expressing cells, which is the myogenic factor expressed in all muscle cells at E9.5. We conclude that Myf5-negative cells are more susceptible to ERMS development than Myf5-positive embryonic precursors. As the propensity to undergo tumorigenic transformation declined with age, concomitant with the increase of stably committed muscle cells, it seems likely that the Ptch mutation favors tumor formation in progenitor cells, which have not yet acquired a muscle cell fate.