Blood cancers, making up almost 10% of all cancer diagnoses, are some of the most common cancer subtypes. Yet, they are among the deadliest cancers because of the lack of accurate biomarkers to detect early signs. According to the Leukemia and Lymphoma Society (LLS) reports, someone in the US dies from blood cancer every 9 min. That’s why it is increasingly important to recognize the early symptoms and onset of leukemia and lymphoma not only to raise awareness in society but also to encourage more investment in biomarkers and therapeutic discovery.
We have discussed the devastating impact of aberrant glycan structures and glycosylation networks on cancer progression several times on our blog. Blood cancers are no exception to the effects of these structures and networks. To highlight the fight against these malignant blood cancer types, we will walk you through recent research publications that shed light on the complex mechanisms behind lymphoma and leukemia.
Before we dive in, let us clarify the parallels and differences between leukemia and lymphoma. They are both considered blood cancers, as they both affect blood cells (usually white blood cells) and the immune system.
Leukemia mainly targets the blood cells in the bone marrow, while lymphoma involves lymphocytes, white blood cells formed in our lymph nodes.
Symptoms of lymphoma and leukemia are similar, although they might vary depending on the subtype. These symptoms include heavy bleeding; bruising; muscle and bone pain; fever; fatigue; and swelling of the liver, spleen, and lymph nodes.
In the US in 2021, leukemia and lymphoma accounted for 9.5% of all cancer deaths. 5-year survival rates significantly vary depending on subtypes, age, ethnicity, and gender. While most subtypes have 5-year survival rates between 60–80%, others, such as Acute Myeloid Leukemia, have a low 5-year survival rate of 29%.
This divergence derives from the malignant and metastatic behavior of each subtype. Therefore, it is essential to correlate molecular mechanisms (e.g., glycosylation) to tumor aggressiveness.
The first publication focused on B-cell precursor acute lymphoblastic leukemia (BCP-ALL), an aggressive subtype where the bone marrow produces an excess of B-cell lymphoblasts (premature white blood cells). The primary focus was the expression of sialyltransferases, which add sialic acids to glycan structures as terminal sugars. Differential expressions of these enzymes, in particular the ST6Gal enzymes, were previously discovered in several cancers, including breast (1), pancreatic (2), prostate (3), and ovarian cancer (4).
Researchers from the Beckman Research Institute City of Hope wanted to explore the impact of ST6Gal1 on cancer cell behavior in BPC-ALL (5). First, they aimed to detect α2-6 sialylation, catalyzed by ST6Gal1 in BCP-ALL cells. For that purpose, they used Sambucus Nigra Agglutinin (SNA) from Vector Laboratories in Western blotting. Fluorescence-activated cell sorting (FACS) revealed an increased relative abundance of α2-6-linked Sia on the majority of the N-linked cell surface glycans. This observation was in agreement with relative ST6Gal1 expression levels in healthy and patient blood samples.
The next step was to investigate the impact of ST6Gal1 on malignancy. Researchers transplanted BCP-ALL cells with eitherendogenous level or overly-expressed ST6Gal1 into mice. Compared to the average expression levels, overexpression of ST6Gal1 accelerated tumor spread, increased weight loss, and decreased survival rate. When treated with the chemo-drug vincristine, both groups exhibited modulation in tumor growth and spread. However, both groups experienced relapse shortly after the treatment ended. As expected, the symptoms in mice with ST6Gal1 overexpression were much more rapid and severe.
Does that mean ST6Gal1 overexpression made the BCP-ALL cells treatment-resistant? To confirm this hypothesis, researchers applied an ST6Gal1 knockdown to vincristine-treated cells. FACS and Western Blotting with SNA lectin revealed that the relation between ST6Gal1 expression and drug resistance was more perplexing than a simple linear correlation, as gene knockdown also increased drug resistance. It was evident that ST6Gal1 expression level was not the sole indicator of aberrant sialylation and drug resistance. Researchers must evaluate other factors affecting α2-6 sialylation.
A multitude of studies show strong correlations between cell surface glycans and lymphoma cell behavior, such as adhesion and cell death (6). With the help of cell surface proteins, lymphoma cells can adhere to extracellular matrix (ECM) proteins, which increases their metastatic capacity. Researchers from the University of Osaka had previously demonstrated that decreased expression of oligosaccharides, which are recognized by plant lectins, such as Peanut agglutinin (PNA), L-PHA, and ConA, resulted in poor prognosis in Burkitt’s lymphoma, a type of non-Hodgkin’s lymphoma, where the immature B-cells differentiate into cancer cells (7).
Researchers treated lymphoma cell lines with glycosylation inhibitors to fully comprehend the relationship between glycosylation and adhesion-induced cancer cell death (8). Cell lines were initially incubated with avidin-FiTC from Vector Laboratories as the control, followed by a series of plant lectins to monitor oligosaccharide presence and lectin reactivity on the cell surface via flow cytometry. Researchers showed that inhibiting different glycosylation types (N- or O-) resulted in distinct effects on cell surface lectin reactivity.
The researchers also shed light on the glycosylation mechanisms driving adhesion to ECM. They found that inhibiting cell surface glycosylation prompted lymphoma cells to adhere to fibronectin, an ECM glycoprotein that regulates cell migration and proliferation. This adhesion pattern was mediated by an integrin called Very Late Antigen-4 (VLA-4), which normally recruits healthy leukocytes to inflammatory sites.
Next, the researchers looked at the effects of sialylation on adhesion to galectin-3 and how this adhesion influences the metastatic potential of lymphocytes. Tumor cells can evade the immune system and migrate throughout the body by adhering to galectin-3. They observed the presence of sialic acid on O-glycans modulated cell adhesion to adhesion, and kept invasion under control. Interestingly, this was the opposite of what was observed in other cancers. For example, sialic acid was found to encourage cell adhesion in pancreatic cancer (9).
In addition to the above analyses, researchers ran inhibition experiments and identified the Arginylglycylaspartic acid (RGD) peptide and through immunohistochemical analysis, identified the Guanosine-5′-triphosphate (GTP)-binding proteins, such as Ras homologous (Rho), Rac1, and Cdc42, as essential drivers of adhesion to galectin-3.
Overall, the study provided preliminary insights into the complicated mechanisms behind tumor cell metastasis through ECM.
The two studies mentioned above are two pieces of a very large puzzle. Detecting and acknowledging the presence of glycans on leukocytes and lymphocytes is simply not enough. Further research is required to fragment cell glycome to elaborate on the relative abundances of specific glycan structures since these unique glycans interact with various extracellular proteins to help the tumor cell thrive. Plant lectins are key to detecting specific glycan epitopes through various imaging and quantification techniques.
Read more articles on the blog and check out our Glycobiology Resource Page to learn how to leverage lectins in your research workflow.