Trigger warning: Long science post!
Yesterday I provided a bunch of scientists’ reactions—and these were big names in the field of gene regulation—to Siddhartha Mukherjee’s ill-informed piece in The New Yorker, “Same but different” (subtitle: “How epigenetics can blur the line between nature and nurture”). Today, in part 2, I provide a sentence-by-sentence analysis and reaction by two renowned researchers in that area. We’ll start with a set of definitions (provided by the authors) that we need to understand the debate, and then proceed to the critique.
Let me add one thing to avoid confusion: everything below the line, including the definition (except for my one comment at the end) was written by Ptashne and Greally.
by Mark Ptashne and John Greally
Ptashne is The Ludwig Professor of Molecular Biology at the Memorial Sloan Kettering Cancer Center in New York. He wrote A Genetic Switch, now in its third edition, which describes the principles of gene regulation and the workings of a ‘switch’; and, with Alex Gann, Genes and Signals, which extends these principles and ideas to higher organisms and to other cellular processes as well. John Greally is the Director of the Center for Epigenomics at the Albert Einstein College of Medicine in New York.
The New Yorker (May 2, 2016) published an article entitled “Same But Different” written by Siddhartha Mukherjee. As readers will have gathered from the letters posted yesterday, there is a concern that the article is misleading, especially for a non-scientific audience. The issue concerns our current understanding of “gene regulation” and how that understanding has been arrived at.
First some definitions/concepts:
Gene regulation refers to the “turning on and off of genes”. The primary event in turning a gene “on” is to transcribe (copy) it into messenger RNA (mRNA). That mRNA is then decoded, usually, into a specific protein. Genes are transcribed by the enzyme called RNA polymerase.
Development: the process in which a fertilized egg (e.g., a human egg) divides many times and eventually forms an organism. During this process, many of the roughly 23,000 genes of a human are turned “on” or “off” in different combinations, at different times and places in the developing organism. The process produces many different cell types in different organs (e.g. liver and brain), but all retain the original set of genes.
Transcription factors: proteins that bind to specific DNA sequences near specific genes and turn transcription of those genes on and off. A transcriptional ‘activator’, for example, bears two surfaces: one binds a specific sequence in DNA, and the other binds to, and thereby recruits to the gene, protein complexes that include RNA polymerase. It is widely acknowledged that the identity of a cell in the body depends on the array of transcription factors present in the cell, and the cell’s history. RNA molecules can also recognize specific genomic sequences, and they too sometimes work as regulators. Neither transcription factors nor these kinds of RNA molecules – the fundamental regulators of gene expression and development – are mentioned in the New Yorker article.
Signals: these come in many forms (small molecules like estrogen, larger molecules (often proteins such as cytokines) that determine the ability of transcription factors to work. For example, estrogen binds directly to a transcription factor (the estrogen receptor) and, by changing its shape, permits it to bind DNA and activate transcription.
“Memory”: a dividing cell can (often does) produce daughters that are identical, and that express identical genes as does the mother cell. This occurs because the transcription factors present in the mother cell are passively transmitted to the daughters as the cell divides, and they go to work in their new contexts as before. To make two different daughters, the cell must distribute its transcription factors asymmetrically.
Positive Feedback: An activator can maintain its own expression by positive feedback. This requires, simply, that a copy of the DNA sequence to which the activator binds is present near its own gene. Expression of the activator then becomes self-perpetuating. The activator (of which there now are many copies in the cell) activates other target genes as it maintains its own expression. This kind of ‘memory circuit’, first described in bacteria, is found in higher organisms as well. Positive feedback can explain how a fully differentiated cell (that is, a cell that has reached its developmental endpoint) maintains its identity.
Nucleosomes: DNA in higher organisms (eukaryotes) is wrapped, like beads on a string, around certain proteins (called histones), to form nucleosomes. The histones are subject to enzymatic modifications: e.g., acetyl, methyl, phosphate, etc. groups can be added to these structures. In bacteria there are no nucleosomes, and the DNA is more or less ‘naked’.
“Epigenetic modifications”: please don’t worry about the word ”epigenetic”; it is misused in any case. What Mukherjee refers to by this term are the histone modifications mentioned above, and a modification to DNA itself: the addition of methyl groups. Keep in mind that the organisms that have taught us the most about development – flies (Drosophila) and worms (C. elegans)—do not have the enzymes required for DNA methylation. That does not mean that DNA methylation cannot do interesting things in humans, for example, but it is obviously not at the heart of gene regulation.
Specificity Development requires the highly specific sequential turning on and off of sets of genes. Transcription factors and RNA supply this specificity, but enzymes that impart modifications to histones cannot: every nucleosome (and hence every gene) appears the same to the enzyme. Thus such enzymes cannot pick out particular nucleosomes associated with particular genes to modify. Histone modifications might be imagined to convey ‘memory’ as cells divide – but there are no convincing indications that this happens, nor are there molecular models that might explain why they would have the imputed effects.
Analysis and critique of Mukherjee’s article
The picture we have just sketched has taken the combined efforts of many scientists over 50 years to develop. So what, then, is the problem with the New Yorker article?
There are two: first, the picture we have just sketched, emphasizing the primary role of transcription factors and RNA, is absent. Second, that picture is replaced by highly dubious speculations, some of which don’t make sense, and none of which has been shown to work as imagined in the article.
(Quotes from the Mukherjee article are indented and in plain text; they are followed by comments, flush left and in bold, by Ptashne and Greally.)
In 1978, having obtained a Ph.D. in biology at Indiana University, Allis began to tackle a problem that had long troubled geneticists and cell biologists: if all the cells in the body have the same genome, how does one become a nerve cell, say, and another a blood cell, which looks and functions very differently?
The problems referred to were recognized long before 1978. In fact, these were exactly the problems that the great French scientists François Jacob and Jacques Monod took on in the 1950s-60s. In a series of brilliant experiments, Jacob and Monod showed that in bacteria, certain genes encode products that regulate (turn on and off) specific other genes. Those regulatory molecules turned out to be proteins, some of which respond to signals from the environment. Much of the story of modern biology has been figuring out how these proteins – in bacteria and in higher organisms – bind to and regulate specific genes. Of note is that in higher organisms, the regulatory proteins look and act like those in bacteria, despite the fact that eukaryotic DNA is wrapped in nucleosomes whereas bacterial DNA is not. We have also learned that certain RNA molecules can play a regulatory role, a phenomenon made possible by the fact that RNA molecules, like regulatory proteins, can recognize specific genomic sequences.
In the nineteen-forties, Conrad Waddington, an English embryologist, had proposed an ingenious answer: cells acquired their identities just as humans do—by letting nurture (environmental signals) modify nature (genes). For that to happen, Waddington concluded, an additional layer of information must exist within a cell—a layer that hovered, ghostlike, above the genome. This layer would carry the “memory” of the cell, recording its past and establishing its future, marking its identity and its destiny but permitting that identity to be changed, if needed. He termed the phenomenon “epigenetics”—“above genetics.”
This description greatly misrepresents the original concept. Waddington argued that development proceeds not by the loss (or gain) of genes, which would be a “genetic” process, but rather that some genes would be selectively expressed in specific and complex cellular patterns as development proceeds. He referred to this intersection of embryology (then called “epigenesis”) and genetics as “epigenetic”. We now understand that regulatory proteins work in combinations to turn on and off genes, including their own genes, and that sometimes the regulatory proteins respond to signals sent by other cells. It should be emphasized that Waddington never proposed any “ghost-like” layer of additional information hovering above the gene. This is a later misinterpretation of a literal translation of the term epigenetics, with “epi-“ meaning “above/upon” the genetic information encoded in DNA sequence. Unfortunately, this new and pervasive definition encompasses all of transcriptional regulation and is of no practical value.
Waddington’s hypothesis was perhaps a little too inspired. No one had visualized a gene in the nineteen-forties, and the notion of a layer of information levitating above the genome was an abstraction built atop an abstraction, impossible to test experimentally. “By the time I began graduate school, it had largely been forgotten,” Allis said. . . Had Allis started his experiments in the nineteen-eighties trying to pin down words like “identity” and “memory,” he might have found himself lost in a maze of metaphysics.
By the 1980’s there had been significant advances in our understanding of the biological problems of “identity and “memory”. We had learned not only how regulatory proteins bind specific sequences in DNA, but also how such proteins can work together, in response to extracellular signals, to make a “switch” in turning one set of genes on and another off. It was apparent by that time that these ideas and findings were applicable to the study of development in higher organisms, and explained different cell identities. The problem of cellular “memory”—then and now—can be explained by positive feedback mechanisms involving regulatory proteins, as discussed in the Introduction.
But part of his scientific genius lies in radical simplification: he has a knack for boiling problems down to their tar. What allows a cell to maintain its specialized identity? A neuron in the brain is a neuron (and not a lymphocyte) because a specific set of genes is turned “on” and another set of genes is turned “off.” The genome is not a passive blueprint: the selective activation or repression of genes allows an individual cell to acquire its identity and to perform its function. When one twin breaks an ankle and acquires a gash in the skin, wound-healing and bone-repairing genes are turned on, thereby recording a scar in one body but not the other.
As noted above, this would hardly have been news at the time. The specificity of cellular identity and the response to stress has been known for decades to be due to the actions of specific DNA binding proteins (and, more rarely, RNA molecules) that regulate gene transcription.
But what turns those genes on and off, and keeps them turned on or off? Why doesn’t a liver cell wake up one morning and find itself transformed into a neuron? Allis unpacked the problem further: suppose he could find an organism with two distinct sets of genes—an active set and an inactive set—between which it regularly toggled. If he could identify the molecular switches that maintain one state, or toggle between the two states, he might be able to identify the mechanism responsible for cellular memory. “What I really needed, then, was a cell with these properties,” he recalled when we spoke at his office a few weeks ago. “Two sets of genes, turned ‘on’ or ‘off’ by some signal.”
The question raised in the first sentence here had, as we have noted, already been answered. The lambda phage switch mechanism is one well-known example of how regulatory proteins can be used to switch a gene “on”, with the gene then persisting in this ‘on’ state in the absence of the protein/signal that first switched it on. The mechanism is an instantiation of positive feedback (see Introduction). The more detailed explanation is readily apparent, and does not involve extra layers of information. The mechanism has been well-established in many cases in higher organisms as well.
In 1996, Allis and his research group deepened this theory with a seminal discovery. “We became interested in the process of histone modification,” he said. “What is the signal that changes the structure of the histone so that DNA can be packed into such radically different states? We finally found a protein that makes a specific chemical change in the histone, possibly forcing the DNA coil to open. And when we studied the properties of this protein it became quite clear that it was also changing the activity of genes.” The coils of DNA seemed to open and close in response to histone modifications—inhaling, exhaling, inhaling, like life.
This attributes an autonomy to and an effect of histone modifications that is grossly misleading. And there is no evidence that coiling and uncoiling of DNA has a causal effect on gene activity.
“Two features of histone modifications are notable,” Allis said. “First, changing histones can change the activity of a gene without affecting the sequence of the DNA.” It is, in short, formally epi-genetic, just as Waddington had imagined. “And, second, the histone modifications are passed from a parent cell to its daughter cells when cells divide. A cell can thus record ‘memory,’ and not just for itself but for all its daughter cells.”
There is no evidence, despite years of research, that nucleosome states can be “copied” for transmission to daughter cells. The one experiment performed in yeast that appeared to show persistence of histone modifications was performed using mutant strains lacking the enzyme that erases the modification tested. In the Introduction, we describe how states of expression are transmitted from as cells divide.
By 2000, Allis and his colleagues around the world had identified a gamut of proteins that could modify histones, and so modulate the activity of genes. Other systems, too, that could scratch different kinds of code on the genome were identified (some of these discoveries predating the identification of histone modifications). One involved the addition of a chemical side chain, called a methyl group, to DNA. The methyl groups hang off the DNA string like Christmas ornaments, and specific proteins add and remove the ornaments, in effect “decorating” the genome. The most heavily methylated parts of the genome tend to be dampened in their activity.
It is true that enzymes that modify histones have been found—lots of them. A striking problem is that, after all this time, it is not at all clear what the vast majority of these modifications do. When these enzymatic activities are eliminated by mutation of their active sites (a task substantially easier to accomplish in yeast than in higher organisms) they mostly have little or no effect on transcription. It is not even clear that histones are the biologically relevant substrates of most of these enzymes.
In the ensuing decade, Allis wrote enormous, magisterial papers in which a rich cast of histone-modifying proteins appear and reappear through various roles, mapping out a hatchwork of complexity. . . These protein systems, overlaying information on the genome, interacted with one another, reinforcing or attenuating their signals. Together, they generated the bewildering intricacy necessary for a cell to build a constellation of other cells out of the same genes, and for the cells to add “memories” to their genomes and transmit these memories to their progeny. “There’s an epigenetic code, just like there’s a genetic code,” Allis said. “There are codes to make parts of the genome more active, and codes to make them inactive.”
By ‘epigenetic code’ the author seems to mean specific arrays of nucleosome modifications, imparted over time and cell divisions, marking genes for expression. This idea has been tested in many experiments and has been found not to hold.
But Reinberg sought a more advanced instance of epigenetic instruction—a whole animal, not just a cell, whose form and identity could be shifted by shifting the epigenetic code. “So imagine that you tighten some parts of the DNA and loosen other parts by changing the structures of the histones,” Reinberg said. “Can you change the form or nature of an animal simply by coiling and uncoiling various parts of its genome?”
This is once again subscribing to the view that chromatin structure is the primary determinant of cellular and organismal states. If that is the view, then the question must be asked – if you could magically change chromatin structure at specific genomic locations, why would cell physiology alter? If the answer is that “this will allow regulatory proteins to bind at these specific sequences,” then the question becomes why invoke a mysterious mechanism for targeted chromatin structure changes with secondary binding of regulatory proteins, when a primary event of binding of these proteins accomplishes both steps?
Perhaps the most startling demonstration of the power of epigenetics to set cellular memory and identity arises from an experiment performed by the Japanese stem-cell biologist Shinya Yamanaka in 2006. Yamanaka was taken by the idea that chemical marks attached to genes in a cell might function as a record of cellular identity. What if he could erase these marks? Would the adult cell revert to an original state and turn into an embryonic cell? He began his experiments with a normal skin cell from an adult mouse. After a decades-long hunt for identity-switching factors, he and his colleagues figured out a way to erase a cell’s memory. The process, they found, involved a cascade of events. Circuits of genes were activated or repressed. The metabolism of the cell was reset. Most important, epigenetic marks were erased and rewritten, resetting the landscape of active and inactive genes. The cell changed shape and size. Its wrinkles unmarked, its stiffening joints made supple, its youth restored, the cell could now become any cell type in the body. Yamanaka had reversed not just cellular memory but the direction of biological time.
This is an extremely inappropriate example to use in a story about the supposed primacy of histone modifications. The Yamanaka experiment, in fact, showed the opposite: that you can change cell identity by expressing certain DNA-binding proteins that bind to and activate specific genes. Any changes in chromatin organization—presumably the “epigenetic marks” referred to, given the context of the entire piece—found during this process are the result of the activities of the DNA-binding regulatory proteins Yamanaka used. There are now many examples of cell “reprogramming” elicited by expression of specific DNA-binding regulatory proteins. This reprogramming example is used by Mukherjee in attempting to establish a primary role for ‘epigenetic’ regulation, but instead provides an excellent example of the higher level control by regulatory proteins.
Both Allis and Reinberg understand the implications of transgenerational epigenetic transmission: it would overturn fundamental principles of biology, including our understanding of evolution.
. . . . Conceptually, a key element of classical Darwinian evolution is that genes do not retain an organism’s experiences in a permanently heritable manner. Jean-Baptiste Lamarck, in the early nineteenth century, had supposed that when an antelope strained its neck to reach a tree its efforts were somehow passed down and its progeny evolved into giraffes. Darwin discredited that model. Giraffes, he proposed, arose through heritable variation and natural selection—a tall-necked specimen appears in an ancestral tree-grazing animal, and, perhaps during a period of famine, this mutant survives and is naturally selected. But, if epigenetic information can be transmitted through sperm and eggs, an organism would seem to have a direct conduit to the heritable features of its progeny. Such a system would act as a wormhole for evolution—a shortcut through the glum cycles of mutation and natural selection.
We agree with the author that this is highly speculative and not currently supported by any mechanistic studies involving so-called epigenetic regulatory processes.
JERRY’S ADDENDUM: Yes, and until there is evidence for this kind of evolutionary transformation—ANY evidence—people should stop yammering about this kind of “Lamarckian” evolution.