Posts Tagged ‘chromatin organization’

Y chromosome

Larry H. Bernstein, MD, FCAP, Curator



Y Chromosome Is More Than a Sex Switch


Here to stay. The Y chromosome is small compared with the X, but is required to keep levels of some genes high enough for mammals to survive.


Here to stay. The Y chromosome is small compared with the X, but is required to keep levels of some genes high enough for mammals to survive.

Andrew Syred/Science Source

The small, stumpy Y chromosome—possessed by male mammals but not females, and often shrugged off as doing little more than determining the sex of a developing fetus—may impact human biology in a big way. Two independent studies have concluded that the sex chromosome, which shrank millions of years ago, retains the handful of genes that it does not by chance, but because they are key to our survival. The findings may also explain differences in disease susceptibility between men and women.

“The old textbook description says that once maleness is determined by a few Y chromosome genes and you have gonads, all other sex differences stem from there,” says geneticist Andrew Clark of Cornell University, who was not involved in either study. “These papers open up the door to a much richer and more complex way to think about the Y chromosome.”

The sex chromosomes of mammals have evolved over millions of years, originating from two identical chromosomes. Now, males possess one X and one Y chromosome and females have two Xs. The presence or absence of the Y chromosome is what determines sex—the Y chromosome contains several genes key to testes formation. But while the X chromosome has remained large throughout evolution, with about 2000 genes, the Y chromosome lost most of its genetic material early in its evolution; it now retains less than 100 of those original genes. That’s led some scientists to hypothesize that the chromosome is largely indispensable and could shrink away entirely.

To determine which Y chromosome genes are shared across species, Daniel Winston Bellott, a biologist at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, and colleagues compared the Y chromosomes of eight mammals, including humans, chimpanzees, monkeys, mice, rats, bulls, and opossums. The overlap, they found, wasn’t just in those genes known to determine the sex of an embryo. Eighteen diverse genes stood out as being highly similar between the species. The genes had broad functions including controlling the expression of genes in many other areas of the genome. The fact that all the species have retained these genes, despite massive changes to the overall Y chromosome, hints that they’re vital to mammalian survival.

“The thing that really came home to us was that these ancestral Y chromosome genes—these real survivors of millions of years of evolution—are regulators of lots of different processes,” Bellott says.

Bellott and his colleagues looked closer at the properties of the ancestral Y chromosome genes and found that the majority of them were dosage-dependent—that is, they required two copies of the gene to function. (For many genes on the sex chromosomes, only one copy is needed; in females, the copy on the second X chromosome is turned off and in males, the gene is missing altogether.) But with these genes, the female has one on each X chromosome and the male has a copy on both the X and Y chromosomes. Thus, despite the disappearance of nearby genes, these genes have persisted on the Y chromosome, the team reports online today in Nature.

“The Y chromosome doesn’t just say you’re a male; it doesn’t just say you’re a male and you’re fertile. It says that you’re a male, you’re fertile, and you’re going to survive,” Bellott explains. His group next plans to look in more detail at what the ancestral Y chromosome genes do, where they’re expressed in the body, and which are required for an organism’s survival.

In a second Nature paper, also published online today, another group of researchers used a different genetic sequencing approach, and a different set of mammals, to ask similar questions about the evolution of the Y chromosome. Like Bellott’s paper, the second study concluded that one reason that the Y chromosome has remained stable over recent history is the dosage dependence of the remaining genes.

“Knowing now that the Y chromosome can have effects all over the genome, I think it becomes even more important to look at its implications on diseases,” Clark says. “The chromosome is clearly much more than a single trigger that determines maleness.” Because genes on the Y chromosome often vary slightly in sequence—and even function—from the corresponding genes on the X, males could have slightly different patterns of gene expression throughout the body compared with females, due to not only their hormone levels, but also their entire Y chromosome. These gene expression variances could explain the differences in disease risks, or disease symptoms, between males and females, Clark says.

(1) http://www.sciencemag.org/news/2014/04/y-chromosome-more-sex-switch
(2) https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxuYWxsYXNldGhmc3xneDoxZTJiYjc1OTU5ODFmZmJh
(3) https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxuYWxsYXNldGhmc3xneDo0ZTNmYjQwODA2M2JhY2E
(4) https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxuYWxsYXNldGhmc3xneDo1MzU2YzczNzgxNTI5YmEz

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RNAi Discovery

Larry H. Bernstein, MD, FCAP, Curator




Nobel Lecture, December 8, 2006 by Craig C. Mello



It’s wonderful to be here today, I would like to start with the most important part, by saying thank you. First of all, I want to thank Andy Fire for being such a tremendous colleague, friend and collaborator going back over the years. Without Andy I definitely wouldn’t be here today. I need to thank the University of Massachusetts for providing for my laboratory, for believing in me and for giving me not only a place and money, but great colleagues with whom to pursue my research.

I want to get down to the theme of my talk today, which really is, in part, about how we continually underestimate the complexity of life. It’s the correction of these underestimations that is quite often what this prize is really recognizing. As science progresses, our knowledge expands, we think we understand, and too often we become overconfident. The fact is, I think we almost always underestimate the complexity of life and of nature. Today has been a true celebration of that beauty and complexity.

If one looks carefully, the complexity of living things becomes strikingly clear. Consider for example the natural environment of C. elegans. Figure 3 is an electron micrograph taken by George Barron, who works on nematophagous fungi. The unfortunate worm shown here has become ensnared in a trap set by a fungus that preys on nematodes….These fungi can sense the motion or contact of a worm and, after the worm has entered its lariat, the fungus inflates it to constrict the snare around the animal, trapping it.

In Roger Kornberg’s talk, we heard about an RNA polymerase that can transcribe the DNA to produce RNA copies of the genetic information. These copies provide templates for the polymerization of the proteins through another elaborate and really beautiful process, called translation.

Does the DNA sequence information control all of the events in the cell? Cells are constantly responding to their environment and to surrounding cells, and these external influences can alter the cell in heritable ways that do not require changes in the primary sequence information in the DNA. Consider the early C. elegans embryo. During these early divisions, maternal mRNA and protein products that are stored in the egg direct numerous cellcell signaling and differentiation events that give rise to the multicellular organism. These are exemplified by the distribution of the PIE-1 protein (Figure 4). PIE-1 tracks with, and is essential for, germline specification…..two daughter cells differ with respect to their content of maternally provided products, like PIE-1. These products, in turn, can direct the subsequent development of these cells such that, once differentiated in this way, these cells remain committed to their specific tasks in the animal through numerous rounds of cell division. These remarkably stable differentation events can be maintained for the entire life of an organism without any underlying changes in the DNA sequence.

How do developing cells, all with the same DNA content, lock in different programs of gene expression that are stable through so many rounds of cell division? One possibility, as I will discuss below, is that mechanisms related to those that mediate RNA interference have a role in this process. It has been suggested that the origin of life on Earth may have begun with selfreplica­ting nucleic acid polymers that were more similar chemically to RNA than to DNA, a classic hypothesis referred to as the “RNA World” hypothesis.

….primordial germ cells in the common metazoan (probably worm-like) ancestor of worms and humans, and going even farther back are direct descendants of the hypothetical self-replicating RNA molecules that gave rise to all life on Earth some 3.5 billion years ago….. RNAi itself is at least one billion years old. Biological mechanisms are far more constant than the positions of continents on our planet. That fact and the implicit concept of deep time are among the most profound discoveries of science.

1) there is a particle, containing siRNAs, for some traits; 2) these siRNAs can grow and multiply independent of cell division; 3) both the nucleus and the cytoplasm can contain the siRNAs; 4) a given siRNA may be represented by many replicas; and 5) that during cell division the daughter cells may receive different kinds and numbers of siRNAs through unequal cell division.

…here’s what CBS Evening News came up with (Figure 5). In the movie, the double stranded RNA flies onto the scene then opens at one end and begins to open and close as though it’s chewing. Defective genes, shaped like colored cheese puffs, then begin to fly into the mouth of the RNA from the left. The RNA is literally eating the DNA for lunch. Now, Andy and I knew that RNA interference was something incredible when we started working on it, but we really didn’t have any idea that it was this incredible.

Public broadcasting has the luxury of an audience that tends to have a bit more patience, and they came up with a 15 minute segment and another strategy, “the cop”, to explain RNAi (Figure 6). They describe a little policeman who looks out for viruses and other misbehavior in the cell. When he sees double-stranded RNA he realizes something is not right. He then goes on to use “enzymatic kung fu” to destroy not only the dsRNA with that sequence, but all RNAs with that sequence that he encounters in the cell.

I like both of these movies because they illustrate a really important concept; that is, that RNAi is an active process, that there is an organismal response to the dsRNA [4]. We realized this at an early stage, because, first of all, as Andy mentioned, the silencing was heritable. RNA injected into an animal resulted in silencing that was transmitted to progeny and even transmitted through crosses for multiple generations via the egg or the sperm. So, the interference mechanism can be initiated in one generation and then transmitted in the germline.

The inheritance properties and systemic nature of RNAi, along with its remarkable potency in C. elegans, all pointed toward an active organismal response to the double-stranded RNA. What we wanted to do immediately, upon realizing that there was an active response in the organism, was to find the genes in the animal that encode that response. Therefore, we set out to use the powerful genetics of C. elegans to look for mutant strains defective in RNAi. We imagined that these mutants would define genes required for the recognition of the foreign double-stranded RNA, genes required for the transport of the silencing signal from cell to cell, genes required for the amplification of silencing, and genes required for the silencing apparatus itself.

Hiroaki was able to identify numerous mutants. Some of these lacked the RNAi response and had no other obvious phenotypes, like rde-1 and rde-4. However, some of his mutants had additional defects, including a very striking phenotype in which the transposons, which are selfreplicating DNA elements present in the genomes of all organisms, became hyperactive, causing mutations by jumping from place to place in the genome. In addition, these same mutants had a reduced tendency to silence transgenes in the germline (transgenes are genes that are experimentally introduced into the organism). In normal worms, transgenes have the vexing property of becoming silent after introduction into the animal experimentally. The same mutants with activated transposons also exhibited activation of transgenes in the germline. These observations suggested that the normal physiological function of RNAi might be to defend cells against the potentially damaging effects of transposons and other foreign genetic elements (perhaps including viruses)…..The rde- 1 and rde-4 mutants, as I indicated earlier, had no other phenotypes. They Figure 7. Hiroaki Tabara 249 were strongly deficient in RNAi in response to double-stranded RNA, but the transposon silencing and the transgene silencing mechanisms were still functioning in these mutant strains. These observations indicated to us, even at that very early stage of our analysis, the existence of some additional, very interesting complexity.

Hiroaki cloned the rde-1 gene and showed that it encodes a highly conserved protein that we now refer to as an Argonaute protein [6]. RDE-1 was an interesting protein for a couple of reasons. It had highly conserved domains found in related genes in organisms as diverse as plants and humans, and yet nothing was known about the enzymatic activities or the biological functions of these domains. This was a very exciting time in the laboratory. We at last had a gene that we knew was involved in the mechanism. Furthermore, previous work on one gene closely related to RDE-1 from Drosophila had linked this gene family to an epigenetic silencing pathway in the fruit fly [7, 8], and work in plants had linked a member of the family to the control of development [9]. Very shortly after our paper was published, Carlo Cogoni and Giuseppe Macino [10] published a very nice paper implicating an RDE-1 family member in silencing triggered by the introduction of a transgene in the fungus Neurospora. So from these findings in other organisms, and from Hiroaki’s genetics, we hypothesized that there may be other types of triggers that initiate related silencing pathways either through natural developmental mechanisms or in response to transposons and transgenes.

…, the lin-4 gene appeared to encode two RNA products: an ~70 nucleotide-long RNA capable of forming a double-stranded RNA molecule with a hairpin-like structure, and a single-stranded 22 nucleotide RNA that appeared to be derived from this longer RNA (Figure 10). This short RNA was capable of binding directly to sites in the transcript of the lin-14 gene, a gene that is negatively regulated by lin-4 during the normal course of worm development.

Even before we identified RDE-1, we were interested in the possibility of a relationship between the RNAi pathway and the lin-4 pathway. Indeed, Hiroaki had raised the concern that RNAi-defective mutants could be hard to recover as viable strains since they might also cause disruption of the lin-4 pathway. Making all of these possibilities even more exciting – while we were conducting genetic screens for RNAi deficient strains, beautiful work was published by Hamilton and Baulcombe [12], linking small RNAs of ~21 nucleotides to viral gene silencing in plants, and by Gary Ruvkun’s lab, identifying a second lin-4- like worm gene, let-7 [13]. Whereas lin-4 was a worm-specific gene, it turned out that the let-7 gene had homologs in every animal, including humans. Remarkably, e­ve­ry single nucleotide in the twenty-one nucleotide mature let-7 RNA products from the worm and human were identical to each other. The conservation of let-7 initiated a gold rush to find small RNA encoding genes, now referred to as micro-RNA genes, in the genomes of numerous organisms.

….activities present in Drosophila cells could process double-stranded RNA into tiny RNAs approximately 21 nucleotides long. Tom Tuschl and colleagues were the first to show that these small RNAs could silence gene expression in vertebrate cells [16]. Thus genetic studies in worms had identified small RNAs as silencing agents beginning in 1993, experimental studies of virus infections in plants identified small RNAs accumulating in infected plants, biochemical studies in fly extracts identified small RNAs in extracts, and finally experimental studies identified silencing activity in cellular assays with vertebrate cells. But were these small RNA molecules only similar in size, or did their similarity extend to mechanism?

Alla’s work provided an answer. When Alla knocked out alg-1 and alg-2, she observed a phenotype that was very similar to that observed when you knock out let-7. To confirm this connection we collaborated with Gary Ruvkun and Amy Pasquinelli, who had recently developed probes for following the processing of the lin-4 and let-7 precursor RNAs into their mature 21 nucleotide RNAs. In wild-type animals, the precursor forms are barely detectable. However, we found that, after inactivation of alg-1 and -2, this precursor accumulates to high levels while the product, the mature twenty-one nucleotide RNA, is greatly diminished [17] (Figure 11).

We also looked at the involvement of Dicer in this process. Dicer was identified by Greg Hannon’s lab as a nuclease required for processing long do­uble-stranded RNA into approximately 21-nucleotide fragments in Drosophila cells. We were able to show, as did several other groups [18, 19], that when you knock out Dicer you also see defects in the processing of these micro­RNAs (Figure 11). With these findings, the first link was established between RNA interference and a natural developmental mechanism for regulating gene expression. This was extremely exciting, and we envisioned a model (Figure 12), in which the RNAi and microRNA pathways utilized different members of the RDE-1 family and converged on Dicer. Downstream of Dicer these pathways appeared to diverge again, through the action of unknown effectors that direct different types of silencing, including mRNA destruction, transcriptional silencing and inhibition of translation. And yet, we still had not identified the RDE-1 family member involved in transposon and transgene silencing.

At that time we thought of the RDE-1 family members (also known as Argonaute proteins) as initiators of the silencing pathways. Genetic stu­dies had placed RDE-1 at an upstream step in the pathway and, as I showed you, ALG-1 and -2 are required for processing the microRNA precursors. However, there was mounting evidence that these proteins might also function downstream in the silencing step. Definitive support for this idea came from Greg Hannon’s group through a collaboration with Ji-Joon Song and Leemor Joshua-Tor at Cold Spring Harbor [20]. They showed that Argonaute proteins have structural similarity to an enzyme domain that can cut RNA, and they presented a model for how Argonaute proteins can bind the ends of the short RNAs and utilize the sequence information to find and destroy target mRNAs in the cell. These studies demonstrated that Argonaute proteins represent the long sought “slicer” activity (or the cop) that lies at the heart of the RNA-induced silencing pathway. We were surprised to learn that RDE-1 was probably the slicer enzyme because our genetic studies had placed RDE-1 activity at an upstream step in the pathway. However, we realized that this observation could be explained if Argonautes function more than once during RNAi in C. elegans. For example (Figure 13), we imagined that RDE-1 could function along with small RNAs derived from processing of the trigger dsRNA in an initial round of target mRNA cleavage. The cleaved target mRNA could then serve as a template for an RNA-dependent RNA polymerase that produces new siRNAs that could, in turn, interact with other Argonautes to mediate efficient silencing of the gene.

The last concept I want to discuss relates to the question of how RNAi can interact with chromatin to silence genes, and the potential importance of this mechanism for gene regulation during both development and evolution. As indicated earlier in my talk, many of the genes involved in RNAi are also required for the silencing of transgenes in the germline.

….Beautiful direct evidence for a link between RNAi and chromatin silencing has more recently come from work in the fission yeast S. pombe, where a complex containing an Argonaute protein and known chromatin interacting factors has been shown to interact directly with silenced genes in the nucleus [24]. To explain how RNAi could regulate DNA directly, I have to tell you a little bit about the physiological nature of DNA inside cells. Your DNA isn’t just lying around by itself. The unit of packaging for DNA is a protein structure called the nucleosome. The DNA wraps around the nucleosome twice, and the nucleosomes are in turn wrapped up and packaged into even thicker fibers. Chromosomes are composed of these protein/DNA fibers, also referred to as chromatin. Partly, what’s achieved by packaging the DNA into chromatin is a silencing effect. Structural stu­dies of the nucleosome core suggest that short protein tails stick out past the DNA in such a way that they are readily accessible for modification [25].




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RNA polymerase – molecular basis for DNA transcription

Larry H. Bernstein, MD, FCAP, Curator

Leaders in Pharmaceutical Intelligence

Series E: 2; 3.1

Roger Kornberg, MD
Nobel Prize in Chemistry
Stanford University

Son of Arthur Kornberg, who received the Nobel Prize for DNA polymerase, Roger Kornberg spent decades on the problem of transcription of the genetic code in eukaryotic cells. Roger Kornberg made several contributions to the understanding of the transcription model including – recognition of the nucleosomal structure of DNA, characterization of the chromatin modifying factors, and discovering the bridging factor that mediates transcriptional activation (called Mediator). The three types of RNA are termed mRNA, tRNA, and rRNA. Kornberg recognized that chromatin consists of nucleosomes arranged along DNA in the form of beads on a string. He used electron crystallography to determine that lateral diffusion in molecules tethered to the bilayer to pack into two-dimensional crystals suitable for crystallography.   Using yeast, Kornberg identified the role of RNA polymerase II and other proteins in transcribing DNA, and he created three-dimensional images of the protein cluster using X-ray crystallography. Polymerase II is used by all organisms with nuclei, including humans, to transcribe DNA.

While a graduate student working with Harden McConnell at Stanford in the late 1960s, he discovered the “flip-flop” and lateral diffusion of phospholipids in bilayer membranes. While a postdoctoral fellow working with Aaron Klug and Francis Crick at the MRC in the 1970s, Kornberg discovered the nucleosome as the basic protein complex packaging chromosomal DNA in the nucleus of eukaryotic cells (chromosomal DNA is often termed “Chromatin” when it is bound to proteins in this manner, reflecting Walther Flemming‘s discovery that certain structures within the cell nucleus would absorb dyes and become visible under a microscope).[10] Within the nucleosome, Kornberg found that roughly 200 bp of DNA are wrapped around an octamer of histone proteins.

Kornberg’s research group at Stanford later succeeded in the development of a faithful transcription system from baker’s yeast, a simple unicellular eukaryote, which they then used to isolate in a purified form all of the several dozen proteins required for the transcription process. Through the work of Kornberg and others, it has become clear that these protein components are remarkably conserved across the full spectrum of eukaryotes, from yeast to human cells.

Using this system, Kornberg made the major discovery that transmission of gene regulatory signals to the RNA polymerase machinery is accomplished by an additional protein complex that they dubbed Mediator.[11] As noted by the Nobel Prize committee, “the great complexity of eukaryotic organisms is actually enabled by the fine interplay between tissue-specific substances, enhancers in the DNA and Mediator. The discovery of Mediator is therefore a true milestone in the understanding of the transcription process.”[12]

Kornberg took advantage of expertise with lipid membranes gained from his graduate studies to devise a technique for the formation of two-dimensional protein crystals on lipid bilayers. These 2D crystals could then be analyzed using electron microscopy to derive low-resolution images of the protein’s structure. Eventually, Kornberg was able to use X-ray crystallography to solve the 3-dimensional structure of RNA polymerase at atomic resolution.[13][14] He extended these studies to obtain structural images of RNA polymerase associated with accessory proteins.[15] Through these studies, Kornberg created an actual picture of how transcription works at a molecular level.

“I measured the molecular weight of the purified H3/H4 preparation by equilibrium ultracentrifugation, while Jean Thomas offered to analyze the material by chemical cross-linking. Both methods showed unequivocally that H3 and H4 were in the form of a double dimer, an (H3)2(H4)2 tetramer (Kornberg and Thomas, 1974). I pondered this result for days, and came to the following conclusions (Kornberg, 1974). First, the exact equivalence of H3 and H4 in the tetramer implied that the differences in relative amounts of the histones from various sources measured in the past must be due to experimental error. This and the stoichiometry of the tetramer implied a unit of structure in chromatin based on two each of the four histones, or an (H2A)2(H2B)2(H3)2(H4)2 octamer. Second, since chromatin from all sources contains roughly one of each histone for every 100 bp of DNA, a histone octamer would be associated with 200 bp of DNA. Finally, the (H3)2(H4)2 tetramer was reminiscent of hemoglobin, an a2b2 tetramer. The X-ray structures of hemoglobin and other oligomeric proteins available at the time were compact, with no holes through which a molecule the size of DNA might pass. Rather, the DNA in chromatin must be wrapped on the outside of the histone octamer.

As I turned these ideas over in mind, it struck me how I might explain the results of Hewish and Burgoyne. What if their sedimentation coefficient of unit length DNA fragments was measured under neutral rather than alkaline conditions? Then the DNA would have been double stranded and about 250 bp in length. Allowing for the approximate nature of the result, the correspondence with my prediction of 200 bp was electrifying. Then I recalled a reference near the end of the Hewish and Burgoyne paper to a report of a similar pattern of DNA fragments by Williamson. I rushed to the library and found that Williamson had obtained a ladder of DNA fragments from the cytoplasm of necrotic cells and measured the unit size by sedimentation under neutral conditions: the result was 205 bp! … with colleagues in Cambridge, I proved the existence of the histone octamer and the equivalence of the 200 bp unit with the particle seen in the electron microscope (Kornberg, 1977). This chapter of the chromatin story concluded with the X-ray crystal structure determination of the particle, now known as the nucleosome, showing a histone octamer surrounded by DNA, in near atomic detail (Luger et al., 1997).

I had decided to pursue the function rather than the structure of the nucleosome, and was joined in this by Yahli Lorch, who became my lifelong partner in chromatin research, and also my partner in life. We investigated the consequences of the nucleosome for transcription. It was believed that histones are generally inhibitory to transcription. We found, to the contrary, that RNA polymerases are capable of reading right through a nucleosome. Coiling of promoter DNA in a nucleosome, however, abolished initiation by RNA polymerase II (pol II) (Lorch et al., 1987). This finding, together with genetic studies of Michael Grunstein and colleagues, identified a regulatory role of the nucleosome in transcription. It has since emerged that nucleosomes play regulatory roles in a wide range of chromosomal transactions. A whole new field has emerged, one of the most active in bioscience today. It involves a bewildering variety of posttranslational modifications of the histones, and a protein machinery of great complexity for applying, recognizing, and removing these modifications.”


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Reporter and Curator: Dr. Sudipta Saha, Ph.D.

Targeting a protein important for chromatin organization could be a new strategy for male birth control. Proper regulation of chromatin dynamics is critical for proper sperm development, and mice with alterations in a protein that is central to chromatin organization are infertile. Now, scientists show that treating mice with a drug known to inhibit that protein impedes sperm development and renders the animals infertile—but halting treatment allows sperm production to restart and mice to sire normal litters.

The results, published in Cell, suggest that targeting this protein could produce a safe, reversible method for non-hormonal male contraception—a long-sought goal that has so far failed to materialize as an option alongside condoms and vasectomies.

Hormonal male contraception methods are already well-established. Male hormonal contraception works at least as well as a typical female oral contraceptive pill. But such contraceptives still have some significant hurdles to overcome before making it to market.

First, the strategy, which involves administering a hormone (usually a progestin) to halt production of testosterone and thus inhibit sperm development, does not suppress sperm production enough in every man. It also requires dosing with enough exogenous testosterone to maintain libido and muscle mass, but there’s currently no cheap and easily applied testosterone on the market. Furthermore, hormone-based male contraception can cause side effects. Unlike side effects for the female hormonal contraception, these can’t be balanced against the risks of pregnancy, which are often higher, noted John Amory at the University of Washington. Because men don’t run the same medical risks of pregnancy, there’s a higher bar for ensuring that contraception administered to healthy men doesn’t carry risks. Finally, despite worldwide surveys suggesting public receptiveness to a male contraceptive pill, pharmaceutical companies no longer fund development of such drugs.

Some of these issues have spurred researchers to look for a non-hormonal way to temporarily induce infertility in men, which should cause with fewer side effects and be more appealing to pharma. Amory’s work, for example, has shown that a compound that targets the retinoic acid pathway of sperm development reversibly inhibits sperm production. The drug’s potential is hamstrung by the fact that men taking the drug can’t consume alcohol without nausea—a side effect he’s currently working to circumvent.

The current study builds on previous work by Debra Wolgemuth at Columbia University showing that BRDT—a testes-specific member of a family of bromodomain-containing proteins, which are important for regulating chromatin organization in various tissues—was critical for normal sperm development in mice. Truncating BRDT has an amazing effect on haploid sperm development. Removing the first bromodomain results in production of a shortened protein and, consequently, the aberrant organization and packaging of DNA in the sperm cells produced. Spermatids fail to elongate normally in mutant mice, resulting in decreased sperm production, misshapen sperm, and infertility.

In order to test the possibility that a BRDT-inhibiting drug, JQ1, might have potential as a male contraceptive, Martin Matzuk of Baylor College of Medicine and his collaborators injected male mice daily with the drug, and examined their testis volume. This volume, which reflects the amount of sperm in the testes, dropped by 60 percent over the 6 weeks of treatment. The sperm count of these mice was nearly 90 percent lower than in control mice, and sperm motility also plunged in JQ1-treated mice, collectively resulting in infertility. Though JQ1 is known to inhibit related proteins expressed elsewhere in the body, the mice seemed to have no other effects from JQ1 treatment, and normal hormone levels in treated mice suggested that infertility wasn’t the result of a hormone imbalance.

A closer look at sperm generation in JQ1-treated mice suggested that sperm development was primarily blocked after the sperm cells had undergone meiosis, but before they began the process of elongating—a similar stage to that seen in BRDT-mutant mice. Importantly, the mice regained the ability to sire pups after several weeks off the drug.

The reversibility of the treatment is likely attributable to the fact that the researchers are targeting sperm cells midway through development, rather than accessory cells that support sperm development from stem cells, noted Michael Griswold, who studies sperm cell development at Washington State University, but did not participate in the study. It’s “a great place to inhibit, because you don’t get sperm cells, but you don’t affect stem cells, which makes [the treatment] reversible,” he explained.

Whether JQ1 acts by primarily targeting BRDT and derailing chromatin organization or whether it inhibits other family members expressed during sperm development remains unclear. Matzuk and his colleagues examined gene expression in JQ1-treated and control mice, and saw decreased expression of many genes important for meiosis, suggesting that JQ1 may be working by affecting transcription of a suite of important genes for spermatogensis. Also, because JQ1 also inhibits BRDT-related proteins, researchers need to be watchful for long-term side effects not detected in the current study, Matzuk noted. Going forward, it will be important to design drugs that selectively target BDRT.

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