Friday, February 22, 2008


Fluorescence in situ Hybridization (FISH): FISH is a molecular cytogenetic technique that can detect chromosomal abnormalities that cannot be appreciated by standard chromosomal analysis (e.g. microdeletion syndromes) or when mitotic cells are not available for chromosomal analysis (e.g. X/Y FISH for cross-sex transplants). Briefly, metaphase chromosomes or interphase nuclei are denatured on the slide, as is the fluorescently labeled DNA probe. The probe and the chromosomes/nuclei are then hybridized, slides are washed, counterstained and analyzed by fluorescent microscopy. There are a number of different types of FISH probes including unique sequence probes (e.g. microdeletion syndromes), whole chromosome painting probes, repetitive probes (e.g. centromeric alpha satellite probes, subtelomeric probes), gene fusion probes (e.g. BCR/ABL in t(9;22) in CML and ALL) and break apart probes (e.g. MLL in 11q23 rearrangements in ALL and AML).

HER2 Amplification by FISH in Breast Cancer: The HER2 oncogene is overexpressed in 25-30% of human breast cancer. In 90-95% of these cases, the overexpression is a direct result of gene amplification. This amplification correlates with a poor clinical prognosis, and therefore, amplification status has become increasingly important in therapeutic decisions for patients with breast cancer. Specifically, women with breast cancer are eligible for treatment with the monoclonal antibody trastuzumab (Herceptin, Genentech), which targets the HER2 gene product, if they show amplification and/or overexpression of HER2.

Screening for Recurrent Bladder Cancer Using FISH: Transitional cell carcinoma (TCC) constitutes 90% of human bladder cancer, 75% of which are considered “superficial”, i.e., carcinoma in situ, non-invasive papillary TCC or minimally invasion TCC. Recurrence of superficial TCC or progression to muscle invasive TCC occurs in 50-80% and 15-25% of cases, respectively. Patients with TCC are monitored with cystoscopy and/or urine cytology for recurrent disease, both of which are relatively insensitive methods of testing. Numerous studies have correlated various chromosomal aneuploidies with TCC. Fluorescence in situ hybridization (FISH) using a variety of probes has been utilized for detection of aneuploidies on cells from voided urine. The UroVysion Bladder Cancer Screening Kit (Abbott, Vysis) provides a FISH probe mixture that includes centromeric probes for chromosomes #3, #7 and #17 and a unique sequence probe for the p16 gene (located at 9p21). Specifically, concurrent extra copies of two or more chromosomes and/or loss of 9p21 is consistent with a diagnosis of recurrent TCC.

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