Background: Simultaneous or consequent development of multiple solid tumors might be faced in some patients, especially the young. These tumors might be related to certain hereditary cancer syndromes or certain genetic predispositions.
Case Report: We present the case of a 19-year-old woman with metastatic breast cancer to the contralateral axillary lymph node, associated with simultaneous osteosarcoma of the left lower femur. As she did not fit into any of the familial cancer syndromes, genetic predisposition was suspected. We detected MLH1 and MSH2 promotor methylation (PM), microsatellite instability (MSI), and different mutational events in both tumors. BRCA1 gene mutations were detected in the breast tumor, with reduced mRNA expression of BRCA1&2. ERCC1, MLH1 and MSH2, especially in OS, and RRM1 was overexpressed in both tumors.
Conclusions: Aberrations in MMR genes could explain simultaneous or consequent development of multiple solid tumors, especially in a young patient. We recommend detecting these defects, close follow-up for those patients, and genetic counseling for their family members. Further studies in a larger population are essential to support our results.

Keywords: Breast Cancer, double primary, Mismatch Repair Genes