Mapping of homozygous deletions on human chromosome 10q23 has led to the isolation of a candidate tumor suppressor gene, PTEN, that appears to be mutated at considerable frequency in human cancers. In preliminary screens, mutations of PTEN were detected in 31% (13/42) of glioblastoma cell lines and xenografts, 100% (4/4) of prostate cancer cell lines, 6% (4/65) of breast cancer cell lines and xenografts, and 17% (3/18) of primary glioblastomas. The predicted PTEN product has a protein tyrosine phosphatase domain and extensive homology to tensin, a protein that interacts with actin filaments at focal adhesions. These homologies suggest that PTEN may suppress tumor cell growth by antagonizing protein tyrosine kinases and may regulate tumor cell invasion and metastasis through interactions at focal adhesions.

Representational difference analysis (RDA) of human breast cancer was used to discover a novel amplicon located at chromosomal region 8q24.3. We examined a series of breast cancer samples harboring amplification of this region and determined that KCNK9 is the sole overexpressed gene within the amplification epicenter. KCNK9 encodes a potassium channel that is amplified from 3-fold to 10-fold in 10% of breast tumors and overexpressed from 5-fold to over 100-fold in 44% of breast tumors. Overexpression of KCNK9 in cell lines promotes tumor formation and confers resistance to both hypoxia and serum deprivation, suggesting that its amplification and overexpression plays a physiologically important role in human breast cancer.

Poster abstracts 60 ulated by at least fivefold, and 60 similarly down-regulated genes. Among those that increased are neuron-associated genes such as APP and neuronal protein 3.1, and genes associated with tissue remodeling (proteinases and inhibitors). Many of the affected genes are unannotated expressed sequence tags (ESTs), and these experiments will contribute to an understanding of their functions. Since differentiation of ES cells can be directed in several ways in culture, our studies will define a set of genes for each cell type. These gene sets may be involved in the determination and differentiation of embryonic cells in vivo.

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