TISSUE-SPECIFIC EXPRESSION OF NEURAL CELL-ADHESION MOLECULE (NCAM) MAY ALLOW DIFFERENTIAL-DIAGNOSIS OF NEUROBLASTOMA FROM EMBRYONAL RHABDOMYOSARCOMA

The MSD1 region of neural cell adhesion molecule (NCAM) was originally described as being spliced into the 120-kDa isoform of NCAM isolated from muscle. The 105 bp region is inserted between exons 12 and 13 and actually consists of three separate exons, MSD1a, MSD1b and MSD1c of 15, 48, 42 bp, respectively. In addition, a further exon consisting of a single triplet has been designated MSD1d, making the full insert si ze 108 bp. As the MSD1 region was originally described as being select ively expressed in muscle tissue, we have investigated whether it is a lso present on tumours of rhabdoid origins and whether its presence ca n be used as the diagnostic marker to distinguish other small round ce ll tumours of childhood, such as neuroblastoma. Using a variety of hum an tumour cell lines, we demonstrated the presence of the MSD1 region on all rhabdomyosarcomas investigated. However, neuroblastoma cell lin es only expressed subcompartments of the MSD1 region. The MSD1c exon w as not spliced into the NCAM molecules isolated from any of the neurob lastoma cell lines investigated. On the basis of this finding, it appe ars that neuroblastoma and rhabdomyosarcoma can be distinguished by th e expression of the MSD1c mini-exon. Further studies are underway to a ttempt to define a monoclonal antibody that recognises the region, usi ng mice immunised with synthetic peptides, and to confirm the finding using fresh biopsy material.