Academic scientist Robert Blelloch’s research has been in constant evolution since he decided to dedicate his life to science. In 2012, The Scientist interviewed Blelloch about the journey of his research career and early work on epigenetics in cell fate determination and cancer.1-3
In celebration of The Scientist’s 40th anniversary, we reached out to Blelloch again to see what paths his scientific work has taken over the last 14 years. From exploring the roles of microRNAs in stem cells and as cancer biomarkers to uncovering key functions of exosome proteins in anti-tumor immunity, Blelloch’s work reveals a mindset that embraces serendipity and is unafraid to take risks to advance scientific discovery.4-8
MicroRNAs: Molecular Regulators of Embryonic Stem Cell Proliferation and Pluripotency
In the mid-2000s, as a freshly-minted group leader at the University of California, San Francisco, Blelloch focused on microRNAs—small, non-coding RNA molecules that regulate gene expression post-transcriptionally—and their effects on embryonic stem cells. Embryonic stem cells can differentiate into any cell type in the human body. Without a differentiation signal, these cells self-renew, meaning they proliferate indefinitely without losing their undifferentiated features.
To understand the role microRNAs play in embryonic stem cell proliferation and pluripotency, Blelloch’s team developed an embryonic stem cell knockout model for the DGCR8 cofactor, which forms a complex with the enzyme Drosha that is essential for microRNA maturation, and they showed that loss of DGCR8 led to defects in embryonic stem cell proliferation.9 Using their newly developed Dgcr8 knockout system, they identified members of the miR-290 family that rescued the embryonic stem cell proliferation capacity.4 Blelloch’s team also found that the same microRNA cluster promotes the dedifferentiation of somatic cells into induced pluripotent stem cells (iPSCs), suggesting that these small RNAs play a role not only in embryonic stem cell self-renewal but could also be used to generate iPSCs.5
From Embryonic Stem Cells to Cancer Biomarkers
While Blelloch recognized that his work on the post-transcriptional regulation of pluripotency established his team’s standing in that field, his desire to explore new scientific questions led him to pursue another path in the microRNA universe: their potential use as cancer biomarkers.
The tumor microenvironment consists of a mix of tumor cells and different types of tumor-infiltrating immune cells. In his lab at the University of California, San Francisco, Robert Blelloch studies tumor immunology.
Li Wang and Hannah Driks, Blelloch’s lab.
Inspired by a student’s idea and emerging evidence of the presence of microRNAs in the plasma of patients with cancer, Blelloch examined whether specific microRNAs in the blood could serve as diagnostic and prognostic markers in people with prostate cancer.10
By developing a method that enabled the detection of even small amounts of microRNAs, his team found that specific microRNAs in the serum were associated with prostate cancer progression. What’s more, adding these small RNAs into the Cancer of the Prostate Risk Assessment (CAPRA) model improved cancer risk prediction, supporting the use of these molecules in clinical settings.6,7
The discovery of microRNAs circulating in the blood led Blelloch and his team to question how exactly these molecules moved inside the human body. One possibility was that extracellular vesicles, particularly in the form of exosomes, carried them around. “Vesicles are basically lipid packages, and they can transfer material from one cell to another,” Blelloch explained. “Does that mean microRNAs are being transferred from one cell to another in the blood?”
Blelloch’s team set out to investigate this question, but it turned out to be a fruitless pursuit. “After three years it was kind of a failure,” he recalled. “[The] poor graduate student working on it was looking for evidence of microRNA transfer via these vesicles, and at least in our hands, we were seeing very little evidence for significant transfer.”
The setback sent the researchers back to the drawing board, where they started rethinking the possible roles these exosomes might have.
Long-Distance Control of the Anti-Tumor Immune Response
Exosomes derive from the plasma membrane of cells, meaning they are covered with insoluble proteins that can act as molecular signals, even over long distances.
This idea prompted Blelloch and his team to shift their focus from microRNAs to proteins. They focused on the programmed cell death protein 1 (PD-L1), a molecule known to suppress the anti-tumor immune response, Blelloch explained. Using genetic knockouts of key enzymes involved in exosome biosynthesis, the researchers showed that tumors secrete PD-L1-packed exosomes that travel away from the tumor microenvironment to the lymph nodes, where PD-L1 inhibits T cell activity.8 Blelloch emphasized that advanced gene-editing technologies, such as CRISPR, were imperative for bringing this work to life. “[In] the paper, everything's CRISPR-ed out. I could've never done that in 2012. No way,” he said.
While their findings suggested that PD-L1 plays a key role in exosome-mediated immunosuppression, Blelloch explained that some results indicated that the protein does not act alone. Using mass spectrometry in the tumor-derived exosomes, his team recently identified the poliovirus receptor-related 2 (PVRL2) molecule, a protein believed to act as an immune checkpoint protein, as another suppressor of the anti-tumor immune response.11 Blelloch and his team are further exploring PVRL2’s role in cancer immunology.
By letting discoveries guide his research, Blelloch’s scientific journey reveals how researchers can chase the most exciting results while making significant contributions to their fields. “The fun of being an academic is just let things flow; let things take you where they take you and hope for the best,” he said. “The downside of this kind of career is [that] it’s very risk-taking [and] very insecure. You get beaten down constantly, but boy, there is a lot of joy.”
- Hochedlinger K, et al. Reprogramming of a melanoma genome by nuclear transplantation. Genes Dev. 2004;18(15):1875-1885.
- Blelloch RH, et al. Nuclear cloning of embryonal carcinoma cells. Proc Natl Acad Sci U S A. 2004;101(39):13985-13990.
- Blelloch R, et al. Reprogramming efficiency following somatic cell nuclear transfer is influenced by the differentiation and methylation state of the donor nucleus. Stem Cells. 2006;24(9):2007-2013.
- Wang Y, et al. Embryonic stem cell-specific microRNAs regulate the G1-S transition and promote rapid proliferation. Nat Genet. 2008;40(12):1478-1483.
- Judson RL, et al. Embryonic stem cell-specific microRNAs promote induced pluripotency. Nat Biotechnol. 2009;27(5):459-461.
- Moltzahn F, et al. Microfluidic-based multiplex qRT-PCR identifies diagnostic and prognostic microRNA signatures in the sera of prostate cancer patients. Cancer Res. 2011;71(2):550-560.
- Wang SY, et al. miR-19, miR-345, miR-519c-5p serum levels predict adverse pathology in prostate cancer patients eligible for active surveillance. PLoS One. 2014;9(6):e98597. Published 2014 Jun 3.
- Poggio M, et al. Suppression of Exosomal PD-L1 Induces Systemic Anti-tumor Immunity and Memory. Cell. 2019;177(2):414-427.e13.
- Wang Y, et al. DGCR8 is essential for microRNA biogenesis and silencing of embryonic stem cell self-renewal. Nat Genet. 2007;39(3):380-385.
- Mitchell PS, et al. Circulating microRNAs as stable blood-based markers for cancer detection. Proc Natl Acad Sci U S A. 2008;105(30):10513-10518.
- Yang J, et al. PVRL2 Suppresses Antitumor Immunity through PVRIG- and TIGIT-independent Pathways. Cancer Immunol Res. 2024;12(5):575-591.
