Linardic Corinne M, Naini Sarasija, Herndon James E, Kesserwan Chimen, Qualman Stephen J, Counter Christopher M
Department of Pediatrics, Pharmacology and Cancer Biology, Duke University Medical Center, Durham, NC 27710, USA.
Cancer Res. 2007 Jul 15;67(14):6691-9. doi: 10.1158/0008-5472.CAN-06-3210.
Rhabdomyosarcoma is the most common soft tissue sarcoma of childhood and adolescence. Despite advances in therapy, patients with a histologic variant of rhabdomyosarcoma known as alveolar rhabdomyosarcoma (ARMS) have a 5-year survival of <30%. ARMS is characterized by a chromosomal translocation generating the PAX3-FKHR fusion gene. However, ectopic expression of PAX3-FKHR often induces inhibition of cell proliferation, or cell death, when expressed in nonmuscle cells. This prompted us to explore the effect of expressing PAX3-FKHR in more relevant cells, specifically primary human skeletal muscle cells because these cells can be converted to a tumorigenic state that mimics rhabdomyosarcoma. PAX3-FKHR expression promoted both fetal and postnatal primary human skeletal muscle cell precursors to bypass the senescence growth arrest checkpoint. This bypass was accompanied by epigenetic DNA methylation of the p16(INK4A) promoter and correspondingly a loss of expression of this tumor suppressor. Knockdown of p16(INK4A) cooperated with PAX3-FKHR to drive proliferation past senescence, whereas reintroduction of wild-type p16(INK4A) in post-senescent cells caused growth arrest. Thus, PAX3-FKHR acts in concert with loss of p16(INK4A) to promote inappropriate proliferation of skeletal muscle cells. This association between PAX3-FKHR expression and p16(INK4A) loss was seen in human ARMS tumor tissue, as both human rhabdomyosarcoma cell lines and tissue microarrays showed a trend toward down-regulation of p16(INK4A) protein in alveolar subsets. We surmise that the generation of the PAX3-FKHR fusion protein may require loss of p16(INK4A) to promote malignant proliferation of skeletal muscle cells as an early step in ARMS tumorigenesis.
横纹肌肉瘤是儿童和青少年中最常见的软组织肉瘤。尽管治疗取得了进展，但患有横纹肌肉瘤组织学变体即肺泡型横纹肌肉瘤（ARMS）的患者5年生存率仍低于30%。ARMS的特征是发生染色体易位，产生PAX3 - FKHR融合基因。然而，PAX3 - FKHR的异位表达在非肌肉细胞中表达时通常会诱导细胞增殖抑制或细胞死亡。这促使我们探索在更相关的细胞中表达PAX3 - FKHR的效果，特别是原代人骨骼肌细胞，因为这些细胞可以转化为模拟横纹肌肉瘤的致瘤状态。PAX3 - FKHR的表达促进了胎儿和出生后的原代人骨骼肌细胞前体绕过衰老生长停滞检查点。这种绕过伴随着p16（INK4A）启动子的表观遗传DNA甲基化，相应地这种肿瘤抑制因子的表达丧失。p16（INK4A）的敲低与PAX3 - FKHR协同作用，驱动增殖超过衰老阶段，而在衰老后细胞中重新引入野生型p16（INK4A）则导致生长停滞。因此，PAX3 - FKHR与p16（INK4A）的丧失协同作用，促进骨骼肌细胞的不适当增殖。在人ARMS肿瘤组织中也观察到了PAX3 - FKHR表达与p16（INK4A）丧失之间的这种关联，因为人横纹肌肉瘤细胞系和组织微阵列均显示肺泡亚群中p16（INK4A）蛋白有下调趋势。我们推测，PAX3 - FKHR融合蛋白的产生可能需要p16（INK4A）的丧失，以促进骨骼肌细胞的恶性增殖，这是ARMS肿瘤发生的早期步骤。
