Alveolar Rhabdomyosarcoma : Molecular Genetics
Cytogenetics and Molecular Genetics of Alveolar Rhabdomyosarcoma (ARMS): ARMS shows distinctive cytogenetic and molecular genetic abnormalities. About 80% of cases are defined by tumor-specific reciprocal t(2;13) or t(1;13) translocations, that result in fusion of the N-terminus of PAX3 or PAX7 to the C-terminus of FOXO1 gene. FUSION-POSITIVE ARMS: About 60% of ARMS show t(2;13)(q36;q14). The breakpoint involves PAX3 gene on chromosome 2 and the FOXO1A gene on chromosome 13. The resultant chimeric proteins PAX3-FOXO1A and FOXO1A-PAX3 cause overexpression of genes like MYCN that interact with PAX3. About 20% of ARMS are associated with t(1;13)(p36;q14) which juxtaposes PAX7 on chromosome 1 to FOXO1A on chromosome 13. These molecular abnormalities can be detected by breakapart FISH probes (images 56-58) or by RT-PCR methods. Fusion-positive ARMS show amplification of several genes, including MYCN, CDK4, and MIR17HG and are much more aggressive clinically than fusion-negative cases. The image shows solid pattern of ARMS. Irregularly shaped nests of tumor cells are separated by fibrous septa. The cells in the center of the nests show loss of cohesion; however, alveolar spaces are not well-developed.