Class II transactivator: is it a master switch for MHC class II gene expression?

The quantitative assessment of MHC II on thymic epithelium: implications in cortical thymocyte development

The dynamics of MHC II expression in various thymic stromal compartments was investigated. By including MHC II in flow cytometry in addition to the cortical CDR1, medullary UEA-1 and pan-epithelial G8.8 markers, thymic stromal compartments were subdivided into at least six different populations. The total level of surface and cytoplasmic MHC II from fresh cortical thymic epithelial cells (cTECs) of normal mouse was as high as MHC II levels in medullary thymic epithelial cells (mTECs). MHC II levels as well as the percentages and cycling status of thymic epithelial cell populations expressing MHC II were not static during post-natal development, suggesting quantitative flexibility in presenting signals to the developing thymocytes. Although there was no evidence found for regulation of surface MHC II levels by TCR or by IFN-gamma, the absence of class II transactivator reduced both the level of MHC II expression and the number of MHC II+ cells. Surprisingly, MHC II molecules were found to form distinct focal aggregates on the surface of cTEC but not mTEC using high-resolution analysis by confocal microscopy. Moreover, these aggregates were formed independent of TCR or TCR-bearing cells in the thymus. These aggregates could potentially generate a functional unit containing a much higher local MHC II concentration to yield a higher avidity interaction. We discuss possible mechanisms for positive selection by weak interactions in the presence of such preformed MHC II aggregate units in cTEC.

Gene expression profiles of BRCA1-linked, BRCA2-linked, and sporadic ovarian cancers

BACKGROUND

Germline mutations in BRCA1 and BRCA2 are responsible for 5%-10% of epithelial ovarian cancers, but the molecular pathways affected by these mutations are unknown. We used complementary DNA (cDNA) microarrays to compare gene expression patterns in ovarian cancers associated with BRCA1 or BRCA2 mutations with gene expression patterns in sporadic epithelial ovarian cancers and to identify patterns common to both hereditary and sporadic tumors.

METHODS

Tumor samples from 61 patients with pathologically confirmed epithelial ovarian adenocarcinoma with matched clinicopathologic features were studied, including 18 with BRCA1 founder mutations, 16 with BRCA2 founder mutations, and 27 without either founder mutation (termed sporadic cancers). The cDNA microarrays contained 7651 sequence-verified features. Gene expression data were analyzed with a modified two-sided F test, with P<.0001 considered statistically significant. The expression level of six genes was also studied with reverse transcription-polymerase chain reaction.

RESULTS

The greatest contrast in gene expression was observed between tumors with BRCA1 mutations and those with BRCA2 mutations; 110 genes showed statistically significantly different expression levels (P<.0001). This group of genes could segregate sporadic tumors into two subgroups, "BRCA1-like" and "BRCA2-like," suggesting that BRCA1-related and BRCA2-related pathways are also involved in sporadic ovarian cancers. Fifty-three genes were differentially expressed between tumors with BRCA1 mutations and sporadic tumors; six of the 53 mapped to Xp11.23 and were expressed at higher levels in tumors with BRCA1 mutations than in sporadic tumors. Compared with the immortalized ovarian surface epithelial cells used as reference, several interferon-inducible genes were overexpressed in the majority of tumors with a BRCA mutation and in sporadic tumors.

CONCLUSIONS

Mutations in BRCA1 and BRCA2 may lead to carcinogenesis through distinct molecular pathways that also appear to be involved in sporadic cancers. Sporadic carcinogenic pathways may result from epigenetic aberrations of BRCA1 and BRCA2 or their downstream effectors.