Researchers led by Christopher Garcia of the Ludwig Center at Stanford University have solved the long-sought structure of a large signaling protein involved in responses to infection, inflammation, the generation of immune cells, and when dysregulated by mutation-; the emergence of blood cancers known as myeloproliferative neoplasms. Published in the journal Science, the structure reveals the mechanism by which this protein, Janus kinase (JAK), transmits signals sent by immune cell growth factors called cytokines.
That structural information has direct implications for the development of new drugs for myeloproliferative neoplasms, which are currently treated with drugs often referred to as “jakinibs” that target all JAK proteins, not just the mutants that drive cancer. This broad targeting of JAK proteins causes side effects that include anemia and thrombocytopenia, a blood clotting disorder.
“Our model of JAK structure gives us an atomic blueprint for how one could make mutant-selective medicines to treat these cancers,” said Garcia. “This is the essence of basic discovery biology leading to translational insights.”
Cytokines signal through receptors found on the cell surface that thread through the cell’s outer membrane into its cytoplasm. Each receptor has attached to its cytoplasmic tail, a single JAK protein in an inactive state. Each cytokine protein binds two of these receptors, drawing their attached JAKs together.
“When the JAKs are brought close together, they activate one another,” Garcia explained. “That’s the activated complex that gets the signaling engine running.”
Each JAK phosphorylates-; or adds a phosphate molecule-; to a specific spot on its partner. So activated, the JAKs then phosphorylate the cytokine receptor to which they’re attached, drawing a protein known as STAT to the complex. It is this complex that transmits the cytokine’s growth-promoting signal.
The structure solved by Garcia’s lab, which has pursued this prize for more than two decades, is of the JAKs in their juxtaposed and activated state.
“Small pieces of the JAK structure had been published over the years-; a toe here, a finger there, an ear there-; but nobody had seen what the whole body looked like, so to speak,” said Garcia.
The full structure shows that when drawn together by the cytokine, the JAKs meet at a roughly flattened region in their middle. The change induced by the oncogenic JAK mutation-; swapping the smaller amino acid valine for the much larger phenylalanine-; falls in the middle of this region. The mutation alters the flat interface between the JAKs, creating a sort of ball and socket connection between the two that makes them adhere far more firmly to one another.
“When people have this mutation-; the most classical mutation in blood cancers-; the JAKs come together and start working all the time because there’s like a little dab of glue in there that’s bringing them together even when there’s no cytokine around,” Garcia said. They thus continuously transmit growth signals, driving cell proliferation.
The finding opens the door to developing small molecules that disrupt the ball-and-socket connection of the mutant JAKs. Such drugs would not affect normal JAK proteins and would, therefore, be less likely to have toxic side effects.
Garcia and his colleagues solved the structure of the mouse JAK-1 protein. The “V617F” mutation that drives myeloproliferative neoplasms is, however, found in its cousin, JAK-2. But, Garcia says, the two are sufficiently alike to share the same signaling mechanism.
Garcia’s group is now working on developing drugs to target V617F mutant JAKs and capturing the structure of the larger JAK-STAT complex and that of the complexes formed between different types of JAKs, which are also involved in cytokine signaling.
Claire Harrison, MD, DM, FRCP, FRCPath is a Professor of Hematology at the Guy’s and St. Thomas’ NHS Foundation Trust in London, UK. Prof. Harrison became a Consultant at the Guy’s and St. Thomas’ NHS Foundation Trust in 2001, where she is now a Clinical Director. 
Dr. Hobbs is the clinical director of the adult leukemia service at Massachusetts General Hospital. Her clinical and research interests are myeloproliferative neoplasms and chronic myeloid leukemia.
Professor Wendy Erber is Professor of Pathology and Laboratory Medicine and a diagnostic haematologist at the University of Western Australia.