You’ve got something in your eye: cellular reprogramming in development and cancer

From the Henikoff Lab, Basic Sciences Division

Sometimes in biology the medical relevance of an experiment is obvious, its potential to improve human health crystal clear even to the casual observer. This is not uncommon in the field of cancer biology, where much research is focused on understanding and learning how to treat genetic mutations known to promote tumor growth and progression. Sometimes, on the other hand, an experiment is so odd, its results so unexpected, that only years later can its true importance to biomedical science be fully appreciated. Such is the realm of the basic sciences, a premise of which, it could be said, is precisely to make discoveries whose impacts cannot be appreciated in the context of current (and incomplete) biomedical paradigms. This is a story of the latter type of work, and of its ultimate convergence with the former to provide new insights into both developmental biology and cancer.

This story begins in the fruit fly, Drosophila melanogaster, with one of the weirdest, wackiest, and ultimately most transformative (pun intended) findings in the field of developmental biology: the homeotic transformation. In the 1960s and 1970s, researchers studying fruit flies began identifying genes that, when mutated, had the power to transform entire body parts into other body parts. The most famous, and stunning, of these discoveries were Antennapedia, in which fully formed legs grew out of the fly’s forehead where the antennae should have been, and Ultrabithorax, in which a complete second set of wings grew out just behind the first. Not long after, genes with similar qualities were found in mammals. Perhaps the most well-known early example, discovered by Dr. Hal Weintraub, a former Fred Hutch Member and the namesake of the building in which our Basic Sciences Division resides, was MyoD, a transcription factor with the power to transform fibroblasts into muscle cells.  These and subsequent findings led to two Nobel Prizes (1995 & 2012) and a new appreciation of the flexibility of cells to adopt different identities, the power of transcription factors to regulate cell fate, and the concept of the so-called “master regulator” to describe genes with the ability to reprogram cell fates. More recently, work in this area led to the invention of induced Pluripotent Stem Cells (iPSCs), cells with exceptional biomedical potential that are created by the application of a cocktail of master regulators that essentially reverse the developmental process. And, notably, as cancer has come to be seen more and more as a perversion of developmental processes, cell fate transformations, in some cases driven by the very genes identified in those early Drosophila studies, have been recognized as a critical element of tumor formation and growth.

homeotic transformation mutants
Left: An Antennapedia mutant fly with legs growing out of its face. Right: An Ultrabithorax mutant fly with a second set of wings.

It is in this context that we consider the work, recently published in PLoS Genetics, of Drs. Kami Ahmad and Steve Henikoff in the Fred Hutch Basic Sciences Division. This story, too, begins with a question of basic science and ends with an unexpected insight into cancer formation. The authors were initially intrigued by the question of how master regulators manage the multitude of transcriptional changes required to reprogram cell fates. As they explain, cell fate determination normally requires two opposing processes: “Development proceeds by the activation of genes by transcription factors and the inactivation of others by chromatin-mediated gene silencing.” In the case of reprogramming, they muse, “This must involve the activation of previously silenced genes [by the master regulator].” But what about the silencing of previously activated genes? To answer this question, the authors used two genetic tools. First, the master regulator: Vestigial (Vg), a gene that is normally expressed in the wing of the fly, will, if mis-expressed in the eye, transform eye cells into wing cells, causing rudimentary wings to protrude bizarrely from the eye. Second, the regulator of gene silencing: expression of a mutant version of a histone protein, called H3.3K27M, blocks H3K27 trimethylation, a modification important for turning off genes. This mutation, therefore, acts to block chromatin-mediated gene silencing. By combining these two conditions, the authors pondered how reprogramming proceeds in the absence of gene silencing. And, to their surprise, it didn’t. Cells expressing Vg and H3.3K27M failed to turn off eye-specific genes and struggled to turn on wing-specific genes. This finding, speculates Dr. Ahmad, “suggests that silencing old developmental programs is a crucial part of initiating new developmental steps.”

Even more surprising, though, was what did happen when these two genes were expressed together. Expression of either gene on its own resulted in a decrease in cell proliferation in the eye. But putting the two together caused extensive tumor-like overgrowth. “Our results demonstrate that growth dysregulation can result from the simple combination of crippled silencing and transcription factor mis-expression,” they explain. To Dr. Ahmad, this finding also made sense of an interesting characteristic of some cancers. H3.3K27M is a so-called oncohistone – a mutant histone protein that promotes tumor formation. “Studies of patients have shown that oncohistone-associated gliomas are surprisingly genetically simple, but these mutations are only neoplastic [meaning they cause tumors] in specific developmental contexts”, he explains. “Our discovery that conflicting developmental programs drive neoplastic growth explains how context matters.” Dr. Ahmad is excited about the prospect of using this system to further understand the mechanism of oncohistone-driven tumor growth. “We're really interested in using these genetically-defined systems to tease apart the early events in tumor growth, particularly…to distinguish aberrant cells when they first arise in a tissue.” Sometimes the best results are the ones you least expect.

fly eye mutants
Left: normal fly eyes. Center: Fly eyes expressing Vestigial with wings growing out of the eyes. Right: Fly eyes expressing Vestigial and H3.3K27M with tumor-like overgrowth of eye tissue. Images provided by Kami Ahmad.

This work was supported by the National Institutes of Health.

Fred Hutch/UW Cancer Consortium member Steve Henikoff contributed to this work

Ahmad K, Henikoff S. (2021) The H3.3K27M oncohistone antagonizes reprogramming in Drosophila. PLoS Genet. 17(7):e1009225.