Fluorescence in situ hybridization of 12p in germ cell tumors using a bacterial artificial chromosome clone 12p probe on paraffin-embedded tissue: clinical test validation

Most germ cell tumors have an isochromosome 12p (detected by metaphase cytogenetics), 12p overrepresentation (detected by fluorescence in situ hybridization [FISH]), or both. Although interphase FISH on paraffin-embedded tissue is a sensitive method of detection of 12p anomalies, use of FISH for clinical diagnostic purposes is not well defined. We describe an interphase FISH assay for detection of increased 12p copy number in germ cell tumors using a bacterial artificial chromosome–derived probe localized to 12p12.1 and a commercially available probe for the centromere of chromosome 12. Twenty-four paraffin-embedded blocks from 14 tumor cases (7 malignant mixed germ cell tumors, 2 dysgerminomas, 4 non–germ cell malignancies arising in germ cell tumors, and 1 mediastinal adenocarcinoma) and 18 normal controls were studied. Negative controls included normal lymph node, lung, and mediastinal tissue. The signals for 12p and 12cen were counted, and the ratio of the averaged signals was calculated; a ratio of 1.3 was considered positive. All germ cell tumors and non–germ cell malignancies arising in germ cell tumors were positive for 12p overrepresentation. All control cases were negative. Because germ cell tumors may metastasize with non–germ cell tumor morphology, interphase FISH may be helpful in distinguishing de novo malignancy from germ cell tumor recurrence in its various forms.