A comparison of MyoD1 and fetal acetylcholine receptor expression in childhood tumors and normal tissues - Implications for the molecular diagnosis of minimal disease in rhabdomyosarcomas

Detection of minimal residual disease or micrometastases in rhabdomyosarcom a (RMS) has been an unresolved problem In 70 to 80% of RMS patients. In pat ients with alveolar type RMS, which harbors chromosomal translocations and produces tumor-specific fusion products, polymerase chain reaction (PCR)-ba sed diagnosis is clear-cut. In the more frequent embryonal RIMS, however, n o such PCR-based marker has been described. Recently it has been suggested that the PCR-based detection of MyoD1 may be a valuable adjunct in the diag nosis of minimal disease in embryonal RMS. We report here that MyoD1 mRNA i s not specific for RMS, but can be amplified from ex vivo samples of many o ther childhood tumors and some normal tissues. By contrast, simultaneous am plification of alpha and gamma subunit message of the fetal type acetylchol ine receptor (AChR), by a novel duplex PCR, and the quantification of both transcripts resulting in a alpha/gamma AChR ratio <1 was 100% sensitive in alveolar (n = 8) and embryonal (n = 10) RMS, Moreover, gamma AChR was not d etected in other childhood (n = 27) or adult tumors (n = 12), or normal tis sues, except thymus, The high sensitivity and specificity of the method wer e confirmed by the successful detection of five cases of cytologically or m olecularly verified RMS bone marrow micrometastases among 47 bone marrow sa mples from childhood tumor patients. By contrast, MyoD1 showed no amplifica tion because of its low level of transcription. We conclude that mRNA of th e fetal type AChR is a more specific and (about 100 times) more sensitive m arker for the molecular detection of RMS than MyoD1, and thus appears to be a promising candidate for the detection of minimal disease in RMS lacking tumor-specific translocations.