Mice carrying an ovine 18-lactoglobulin (BLG) transgene secrete BLG protein into their milk. To explore transgene expression stability, we studied expression levels in three BLG transgenic mouse lines. Unexpectedly, two lines exhibited variable levels of transgene expression. Copy number within lines appeared to be stable and there was no evidence of transgene rearrangement. In the most variable line, BLG production levels were stable within individual mice in two successive lactations. Backcrossing demonstrated that genetic background did not contribute significantly to variable expression. Tissue in situ hybridization revealed mosaicism of transgene expression within individual mammary glands from the two variable lines; in low expressors, discrete patches of cells expressing the transgene were observed. Transgene protein concentrations in milk reflected the proportion of epithelial cells expressing BLG mRNA. Furthermore, chromosomal in situ hybridization revealed that transgene arrays in both lines are situated close to the centromere. We propose that mosaicism of transgene expression is a consequence of the chromosomal location and/or the nature of the primary transgene integration event.f3-Lactoglobulin (BLG) is a major ovine milk protein. The function of BLG is unknown, though the crystal structure of bovine BLG is consistent with a role in vitamin A transport (1). We previously reported that mice carrying a sheep BLG transgene secrete BLG into their milk (2); BLG regulatory regions can direct expression of biomedical proteins into the milk of transgenic mice and sheep (3)(4)(5). In this context, it is important that transgene expression is stable. Unstable transgene expression has been described previously; Palmiter et al. (6) reported that the level of herpes simplex virus thymidine kinase expression could vary by more than an order of magnitude among progeny of the same founder. Although other transgene insertions express to variable degrees within individual cell lines or transgenic mouse lines (refs. 7-19; M. Mehtali and R.L., unpublished data), there has been no common explanation for the instability of expression. Unstable expression may be due to strong selection against the transgene, for instance by the failure of sperm fertility engendered by testicular thymidine kinase expression (7,8) or by the toxicity of high-level hepatic expression of plasminogen activator (9). A transgene inserted into the X chromosome (10) or an X-autosome translocation (20) generates mosaic expression due to stochastic X chromosome inactivation. Silencing has also been observed when the transgene integrates into repeat sequence or satellite DNA (11,12), whereas different levels of transgene expression between animals of the same lineage have been attributed to strain-specific modifier genes (13-15).Mosaic patterns of expression were also observed in transgenic animals bearing intestinal fatty-acid binding protein fusion transgenes (16). Here, mosaicism was attributed to a deficit of cis-acting elements in the tr...

Background: During mammary gland development, massive coordinated changes in protein expression govern the progression through pregnancy to lactation and involution. These dramatic changes are likely regulated in part at the translational level by changes in microRNAs (miRNAs). We profiled miRNA expression in mammary epithelial cells (MECs) isolated from mice at pregnancy day 14 (P14) or lactation day 2 (L2) and found that miR-150 is the most significantly downregulated miRNA between pregnancy and lactation. Interestingly, miR-150 was recently discovered to be decreased in mouse mammary tumors compared to normal mammary tissue in numerous transgenic models. However, a causal role for miR-150 has yet to be studied in human breast cancer and little is known about its functional role and relevant targets in the normal breast or breast cancer. Hypothesis: We hypothesized that miR-150 may be a tumor suppressor whose loss in breast cancer cells is an important event that allows for expression of multiple pro-tumorigenic genes. Methods: miR-150 levels in human breast cancers were evaluated in specimens of both ER+ and triple-negative breast cancer (TNBC) as compared to adjacent, non-involved normal breast epithelium by in situ hybridization. miR-150 levels were also measured by qRT-PCR in cell lines representative of multiple breast cancer subtypes. We stably restored miR-150 in TNBC cell lines by lentiviral infection and evaluated its effects on clonogenicity, growth in 3D culture, migration/invasion, and tumorigenicity. Expression of predicted miR-150 targets was assessed by immunoblot. Crossing of miR-150fl/fl mice with BLG-Cre or MMTV/NIC transgenic mice will be utilized to determine the effects of MEC-specific loss of this miRNA on mammary tumorigenesis. Results: In clinical samples, in situ hybridization reveals that miR-150 levels are lower in both ER+ and TNBC tumor specimens compared to adjacent normal epithelium, with triple-negative tumors having the lowest expression. All breast cancer cell lines tested also have low miR-150 expression as compared to normal mammary epithelial tissue, with TNBC cell lines expressing the lowest levels. Exogenous expression of miR-150 in breast cancer cell lines caused a dramatic decrease in migration and invasion in vitro, and we are testing predicted miR-150 targets for their role in this phenotype. Experiments with transgenic models will determine if loss of miR-150 in mammary epithelial cells in the MMTV/NIC mouse model results in decreased latency or increased tumor formation and metastasis, or if miR-150 loss during pregnancy results in alterations in lactation, hyperplasia, or tumor formation. Conclusions: TNBC specimens and cell lines have the lowest expression of miR-150, though decreased miR-150 expression compared to normal mammary epithelium is a common feature of breast cancers regardless of subtype. miR-150 expression dramatically inhibits breast cancer cell migration and invasion in vitro, and ongoing experiments will determine the relevant target genes. Supported by Susan G. Komen Grant KG090415 and NIH NICHD P01 PAR-10-245 (Project 3) to JKR Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-10-06.