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. 2013 Sep 9;24(3):365-78.
doi: 10.1016/j.ccr.2013.08.004.

The RasGAP Gene, RASAL2, Is a Tumor and Metastasis Suppressor

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Free PMC article

The RasGAP Gene, RASAL2, Is a Tumor and Metastasis Suppressor

Sara Koenig McLaughlin et al. Cancer Cell. .
Free PMC article

Abstract

RAS genes are commonly mutated in cancer; however, RAS mutations are rare in breast cancer, despite frequent hyperactivation of Ras and ERK. Here, we report that the RasGAP gene, RASAL2, functions as a tumor and metastasis suppressor. RASAL2 is mutated or suppressed in human breast cancer, and RASAL2 ablation promotes tumor growth, progression, and metastasis in mouse models. In human breast cancer, RASAL2 loss is associated with metastatic disease; low RASAL2 levels correlate with recurrence of luminal B tumors; and RASAL2 ablation promotes metastasis of luminal mouse tumors. Additional data reveal a broader role for RASAL2 inactivation in other tumor types. These studies highlight the expanding role of RasGAPs and reveal an alternative mechanism of activating Ras in cancer.

Figures

Figure 1
Figure 1. RASAL2 is candidate tumor suppressor
(A) Left: Immortalized MEFs were infected with lentiviral shRNAs targeting Rasal2, Nf1, or control, and plated in soft agar. Data are reported as relative number of colonies ± SEM. Inactivation of Nf1 or Rasal2 induced a statistically significant increase in anchorage-independent growth (p≤0.0001). Right: Western blot confirming knockdown. (B) RASAL2 mutations in human tumor samples (Bamford et al., 2004). Each triangle represents a non-synonymous mutation. Red triangles indicate breast cancer mutations. See also Table S1 and S2. (C) RASAL2 expression in a panel of human breast cancer cell lines in comparison to normal human mammary epithelial cells. Cell lines with very low or no RASAL2 are starred. Luminal (Lu) or basal (Ba) subtype categorization is indicated. (D) Relative RASAL2 expression in subsets of sorted human mammary epithelial cells (Lim et al., 2009). Mammary stem cell-enriched: (CD49hi EpCAM−). LP: luminal progenitor (CD49f+ EpCAM+). ML: mature luminal (CD49f− EpCAM+). Data show relative expression ± SD. Similar results were obtained using two additional RASAL2 probes. There were no statistically significant differences in RASAL2 expression between subsets of cells. (E) Left: Western blot of Ras-GTP and phospho-ERK (pERK) levels in MCF7 cells following expression of LacZ or RASAL2. Right: Western blot of Ras-GTP and phospho-ERK (pERK) levels in MCF10A cells following shRNA-mediated inactivation of RASAL2 or control (non-targeting “Scramble” shRNA).
Figure 2
Figure 2. RASAL2 functions as a tumor suppressor in breast cancer
(A) Growth curves of MDA-MB-361 and MCF7 cells expressing RASAL2 or LacZ. Data points show triplicate averages ± SD. There were no statistically significant differences in proliferation. Western blot on right confirms ectopic RASAL2 expression. (B) Soft agar colony formation of MCF7, BT474, MDA-MB-361, and SUM159PT cells expressing RASAL2 or LacZ. Data show relative number of colonies ± SD. There was a statistically significant decrease in anchorage-independent growth upon ectopic RASAL2 expression in RAS wild type cell lines (MCF7 and BT474 p<0.0001; MDA-MB-361 p=0.002) but not in the HRAS mutant cell line SUM159PT. Western blots confirm ectopic RASAL2 expression. (C) Xenograft tumor formation of MDA-MB-361 and MDA-MB-231 cells expressing RASAL2 or LacZ. MDA-MB-361 cells were injected orthotopically into female NOD/SCID mice; MDA-MB-231 cells were injected subcutaneously into female nude mice. Horizontal bars indicate mean tumor volume. There was a statistically significant decrease in tumor growth upon ectopic RASAL2 expression (p<0.0001) in the RAS wild type cell line MDA-MB-361 but not in the KRAS mutant cell line MDA-MB-231. Western blots below confirm ectopic RASAL2 expression. (D) Xenograft tumor formation of CAMA1 cells infected with shRNAs targeting RASAL2 or non-targeting control shRNA and injected subcutaneously into female NOD/SCID mice. Horizontal bars indicate mean tumor volume. There was a statistically significant increase in tumor growth upon RASAL2 inactivation (p=0.0007). Western blot confirms RASAL2 knockdown.
Figure 3
Figure 3. RASAL2 functions as a tumor suppressor via its effects on Ras
(A) Soft agar colony formation of MCF7 cells expressing HA-tagged LacZ, wild type, or mutant RASAL2 (see also Table S3). Data show relative number of colonies ± SD. * indicates p≤0.05. Western blot confirms expression of constructs. (B) Western blot reflecting the relative activation of the Ras/ERK pathway in the presence of HA-tagged wild type or mutant RASAL2. The pERK/ERK ratio of each sample was calculated and normalized to the vector control. (C) Phospho-ERK (pERK) expression in MDA-MB-361 xenograft tumors. LacZ, RASAL2, or mutant RASAL2 was expressed in MDA-MB-361 cells and cells were injected orthotopically into female NOD/SCID mice. pERK levels were assessed by immunohistochemistry. (D) Western blot showing HRas-GTP and KRas-GTP levels in MCF10A cells following shRNA-mediated inactivation of RASAL2 or control shRNA. As indicated by the asterisk, the blot confirming RASAL2 knockdown is a duplicate from Figure 1E, as these immunoblots were generated from the same samples. (E) Soft agar colony formation of BT474 cells infected with an shRNA targeting HRAS or KRAS or a non-targeting control. Data show relative number of colonies ± SD. Western blot confirms Ras isoform-specific knockdown.
Figure 4
Figure 4. RASAL2 inactivation promotes migration, invasion, and tumor progression
(A) Cell migration of MCF10A cells infected with shRNAs targeting RASAL2 or a non-targeting control. (B) Western blot confirming RASAL2 knockdown in MCF10A cells used in (A) and (C). (C) Transwell invasion of MCF10A cells infected with an shRNA targeting RASAL2 or a non-targeting control. Invasion was measured after 24 hours and reported as average ± SD (p=0.002). (D) Xenograft tumor progression of MCF10ADCIS cells infected with shRNAs targeting RASAL2 or a non-targeting control Top: H&E images of xenograft tumors. Bottom left: Quantification of xenograft tumor progression. Bottom right: Western blot confirming RASAL2 knockdown. (E) Phospho-ERK (pERK) expression in MCF10ADCIS xenograft tumors from (D) as assessed via immunohistochemistry.
Figure 5
Figure 5. Loss of Rasal2 promotes metastasis and Ras activation in a genetically engineered mouse model of breast cancer
(A) Schematic of Rasal2 genomic locus and pNMDi4 genetrap cassette. Un-shaded regions in exons 1 and 18 mark 5′ and 3′ UTRs, respectively. Known domains of Rasal2 are noted (PH, C2, and RasGAP). See Experimental Procedures for detailed description of pNMDi4. The genetrap cassette targets the third intron of Rasal2. (B) Genotyping of Rasal2 mice to distinguish wild type (WT), heterozygous mutant (het), and homozygous mutant (hom). (C) Western blot confirming loss of Rasal2 protein in genetrap animals (mammary gland tissue). WT: wild type, het: heterozygous, hom: homozygous mutant. (D) Top: H&E images of primary mammary adenocarcinomas from MMTVneu; Rasal2 +/+ and MMTVneu; Rasal2 −/− animals. Bottom: H&E images of lung metastases from MMTVneu; Rasal2 +/+ and MMTVneu; Rasal2 −/− animals. M indicates metastases. (E) Lung metastasis burden in MMTVneu; Rasal2 +/+ and MMTVneu; Rasal2 −/− animals. Lung metastasis incidence: percent of tumor-bearing females with lung metastases at sacrifice (p=0.05; n=24 MMTVneu; Rasal2 +/+, n=23 MMTVneu; Rasal2 −/−). Average number of lung metastases per animal: counted per representative section of lungs for each tumor-bearing female (p=0.04). Average metastasis burden per animal: average total area of metastasis in a representative section of lung for each tumor-bearing female (arbitrary units; p=0.04). See also Figure S2. (F) H&E images of metastases to brain (a), gut (b), ovary (c), and kidney (d) in compound tumor-bearing females. M indicates regions of metastasis. (G) Western blot analysis of phospho-ERK (pERK) and phospho-AKT (pAKT) levels in primary mammary tumors from MMTVneu; Rasal2 +/+ animals (numbers 1–9) and MMTVneu; Rasal2 −/− animals (numbers 10–18).
Figure 6
Figure 6. RASAL2 expression is lost/low in primary human breast cancers and low levels are associated with metastasis and recurrence
(A) RASAL2 dot blot of whole cell RIPA extracts from human breast cancer cell lines with high or low RASAL2 expression (MDA-MB-231 “231” and T47D, respectively) (top) or MCF10ADCIS cells infected with control or RASAL2-targeting shRNAs (bottom). (B) Dot blot images from human breast tumor lysate array. Six sets of RASAL2 and total protein stains are shown. Each set contains triplicate spots of tumor lysate (right) and triplicate spots of paired normal tissue lysate (left). (C) Quantification of RASAL2 expression in tumor lysate arrays. Each bar depicts the change in RASAL2 expression in one sample as compared to its matched normal control as described in Experimental Procedures. Shaded bars indicate metastatic samples. (D) RASAL2 protein expression in tumor versus normal in non-metastatic (Stages I, II, III) versus metastatic (Stage IV) tumors. Graph shows the Log2 fold change in RASAL2 protein expression in tumor versus normal. Data are reported as average ± 95% CI. p=0.006. (E) Heatmap of RASAL2 gene expression as a function of robust molecular subtype predictor classification, which is based on patients classified in the same tumor subtype. Percentages of tumors with high, intermediate, and low RASAL2 expression per molecular subtype are given in the gene expression table. (F) RASAL2 expression table. For each breast cancer subtype, the number of samples and percentage of samples with low, intermediate, or high RASAL2 mRNA expression are indicated. (G) Kaplan-Meier curve showing recurrence-free survival of luminal B tumors with high or low RASAL2 expression (logrank p = 0.013). (H) Kaplan-Meier curve showing overall survival of luminal B tumors with high or low RASAL2 expression (logrank p = 0.013).
Figure 7
Figure 7. Rasal2, Trp53 compound mutant mice develop highly metastatic tumors
(A) Phenotypes in Rasal2/Trp53 compound mutant mice. Pie charts display the array of phenotypes in each genotype. New phenotypes in Rasal2 mutant compound mice are shown in color. n=21 Rasal2 +/+; Trp53 −/−, 18 Rasal2 +/−; Trp53 −/−, 31 Rasal2 −/−; Trp53 −/−, 16 Rasal2 +/+; Trp53 +/−, 21 Rasal2 +/−; Trp53 +/−, 21 Rasal2 −/−; Trp53 +/−. (B) Western blot analysis of phospho-ERK (pERK) levels in primary tumors from Rasal2 +/+; Trp53 −/− and Rasal2 −/−; Trp53 −/− compound mice. (C) Percentage of metastatic solid tumors in Rasal2 +/+, +/−, and −/−; Trp53 +/− compound mice. Increased metastasis in compound animals is statistically significant (p=0.003). The paucity of metastatic solid tumors in Trp53 +/− mice is supported by historical data. (D) Images of metastatic lesions (bottom) and their primary tumors (top). From left to right: mammary adenocarcinoma and lung metastasis, osteosarcoma and liver metastasis, hepatocellular carcinoma and lung metastasis, and lung adenocarcinoma and liver metastasis. M: metastasis, Lu: lung, Liv: liver.

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