Elevated p21 mRNA level in skeletal muscle of DMD patients and mdx mice indicates either an exhausted satellite cell pool or a higher p21 expression in dystrophin-deficient cells per se

Abnormalities in proliferation and differentiation of the dystrophin-defici ent muscle are a controversial aspect of the pathogenesis of Duchenne muscu lar dystrophy (DMD). Analyses of molecules involved in cell cycle modulatio n do not exist in this context. Cells withdrawn from the cell cycle permane ntly express p21. The fact that p21, in contrast to other cell cycle protei ns, is not diminished when myotubes are reexposed to growth media, allocate s this cyclin-dependent kinase inhibitor a special function. Here we report for the first time statistically increased p21 mRNA levels in dystrophin-d eficient muscle tissue. Only 42% of conventional RT-PCRs from six muscle sa mples of human controls yielded positive results but almost all skeletal mu scle biopsy samples (87%) from DMD patients (n=5). For p21 mRNA quantificat ion in murine muscle samples we were able to use the exact real-time TaqMan PCR method due to generally higher p21 mRNA levels than in human muscles. In addition, contamination with fibroblasts can be excluded for the murine samples because they do not demonstrate fibrosis at the age of 350 days but start to lose their regenerative capacity. In accord with the results in h umans, we observed p21 mRNA levels in mdx mice that were approx. four times as high as those in control mice. Elevated p21 mRNA level may indicate a s hift in cell composition towards differentiated p21 expressing cells as a r esult of an exhausted pool of undifferentiated, non-p21-expressing satellit e cells due to previous cycles of de- and regeneration. Alternatively, dyst rophin-deficient cells per se may express, higher p21 levels for unknown re asons. Although we cannot distinguish between these possibilities, the even tual transfection of a patient's own satellite cells with p21 antisense oli gonucleotides may enable the dystrophic process to be influenced.