Rhabdomyosarcoma, which arises from abnormal muscle progenitor cells, is the most common soft tissue cancer in children and is sometimes difficult to treat. Now, for the first time, a research team has succeeded in converting degenerated cells of this tumor into normal muscle cells. This was made possible by identifying a protein that plays a key role in activating a cancer gene in immature muscle precursor cells. When the researchers knocked out this NF-Y protein in the cancer cell cultures, the cancer cells turned into normal muscle cells and lost all the characteristics of a degenerate cell, the team found. In her view, this could open up a new approach to combating these and possibly other sarcomas.
Rhabdomyosarcoma (RMS) is an aggressive form of cancer that occurs primarily in childhood. About five percent of all childhood cancers are due to this subtype of soft tissue tumors. The starting point of this type of cancer are myoblasts, embryonic precursor cells of muscle fibers. During the course of their maturation, these spindle-shaped mesenchymal cells form the actin and myosin fibers important for muscle contraction, and several of these cells also fuse with one another to form the mostly multinucleated skeletal muscle fiber cells. In the case of rhabdomyosarcoma, however, this maturation into muscle cells stops, and instead the immature myoblasts degenerate and multiply aggressively. One of the most common triggers for this degeneration is a faulty fusion of two cancer genes that are normally separated on the chromosomes. This fusion creates the oncoprotein PAX3-FOXO1, which stops the normal differentiation of muscle progenitor cells and turns them into rhabdomyosarcoma cells instead.
Gene blockade turns cancer cells into muscle cells
How the oncoprotein PAX3-FOXO1 manages this and which genes are also involved in its mass production and the degeneration of the myoblasts has only been partially clarified so far. That is why Martyna Sroka from the Cold Spring Harbor Laboratory in New York and her colleagues have now searched for genes whose blockade disrupts or even reverses the degeneration of the rhabdomyosarcoma cells. To do this, they used a modified version of the CRISPR/Cas9 gene scissors to silence various genes in these cells. They then used immunofluorescence labeling and cell shape analysis to determine whether this had any effect on the cancer cells. While the rhabdomyosarcoma cells typically have a rounded, rather compact shape, as is typical for the immature progenitor cells, healthy, maturing muscle cells are elongated and spindle-shaped.
This search yielded a hit: In addition to the gene complex for the oncoprotein PAX3-FOXO1, blocking the three gene parts of another protein also had an effect: if the researchers prevented the formation of this protein called NF-Y, spindle-shaped cells developed in the cell culture instead of the rounded cancer cells , which formed actin, myosin and other muscle-typical components. "These cells turned into muscle cells and lost all cancer-like attributes," says senior author Christopher Vakoc of Cold Spring Harbor Laboratory. “The cancer cells change from a cell that just wants to multiply into a cell that wants to contract. Because all of its energy and resources are now dedicated to this new goal, it can no longer return to its previous degenerated state.” This almost complete reconversion of the degenerated cancer cells into relatively normal, differentiated muscle cells came as a surprise, even to the researchers, as they write .
Relevant also for other sarcomas
More detailed analyzes revealed that switching off NF-Y sets a biochemical chain reaction in motion, as a result of which the activity of the gene complex for the oncoprotein PAX3-FOXO1 is also drastically downregulated. "This supports the hypothesis that NF-Y regulates rhabdomyosarcoma cell differentiation by promoting transcription of the PAX3-FOXO1 gene complex," explains the team. The discovery of this control factor could therefore have significance beyond rhabdomyosarcoma: "Impaired differentiation of mesenchymal cells is a key feature of many soft tissue sarcomas," explain Sroka and her colleagues. The screening method they developed could therefore help to identify such regulatory genes in other types of cancer. This also results in new starting points for a therapy that removes this differentiation blockade and thus turns cancer cells back into healthy, mature cells.
"This technology makes it possible to take any type of cancer and look for the levers to get it to mature properly," says Vakoc. "This could be an important step towards the broader application of such differentiation therapies." In an earlier study, Vakoc and his team had already succeeded in converting cells from Ewing's sarcoma, an aggressive bone tumor that is particularly common in children, back into healthy cells . Through their research, the team hopes to find drugs that turn off the newly identified genes and thus fight cancer. However, this is made even more difficult by the fact that, for example, the NF-Y protein is also required by healthy cells - even though the degenerated rhabdomyosarcoma cells react much more sensitively to a blockade. "This could open up a chance for us to induce sarcoma cells to differentiate by temporarily blocking NF-Y, while healthy tissues are only minimally affected," the researchers explain. Further research on this approach is therefore worthwhile and useful.
Source: Martyna Sroka (Cold Spring Harbor Laboratory, New York) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2303859120