The aberrant methylation of the CpG island promoter regions acquired by
tumor cells is one mechanism for loss of gene function. The high methylation rate for RB1 and
death-associated protein-kinase gene (
DAP-kinase) (60 and 90%, respectively) previously found in
brain metastases suggests this mechanism could be non-randomly associated to
tumor progression and
metastasis. Thus, in addition to these two genes, we determined the methylation status of the genes p16INK4a,
glutathione S-transferase P1 (GSTP1),
O6-methylguanine DNA methyltransferase (MGMT), thrombospondin-1 (THBS1),
p14ARF, TP53, p73, and
tissue inhibitor of metalloproteinase 3 (TIMP-3), in 18
brain metastases of solid
tumors, with methylation specific PCR. The
metastases were derived from
malignant melanoma (three cases), lung
carcinoma (six cases),
breast carcinoma (three cases), ovarian
carcinoma (two cases) and one each from colon, kidney, bladder and
undifferentiated carcinoma. We detected methylation levels in the
tumor samples of 83% in p16INK4a, 72% in
DAP-kinase, 56% in THBS1, 50% in RB1, 39% in MGMT, 33% in GSTP1 and
p14ARF each, 22% in p73 and
TIMP-3 each, and 11% in TP53. The methylation index (number of genes methylated/number of genes tested) varied between 0.1 and 0.6, with an average of 0.42, indicating that a high grade of gene methylation accumulates parallel to the
tumor metastasis process. Our data suggest an important role for gene methylation in the development of
brain metastases, primarily involving epigenetic silencing of
DAP-kinase, THBS1 and the cell-cycle regulators RB1/p16INK4a.