Genome-wide Analysis of Body Proportion Classifies Height-Associated Variants by Mechanism of Action and Implicates Genes Important for Skeletal Development

Human height is a composite measurement, reflecting the sum of leg, spine, and head lengths. Many common variants influence total height, but the effects of these or other variants on the components of height (body proportion) remain largely unknown. We studied sitting height ratio (SHR), the ratio of sitting height to total height, to identify such effects in 3,545 African Americans and 21,590 individuals of European ancestry. We found that SHR is heritable: 26% and 39% of the total variance of SHR can be explained by common variants in European and African Americans, respectively, and global European admixture is negatively correlated with SHR in African Americans (r² ≈ 0.03). Six regions reached genome-wide significance (p < 5 × 10⁻⁸) for association with SHR and overlapped biological candidate genes, including TBX2 and IGFBP3. We found that 130 of 670 height-associated variants are nominally associated (p < 0.05) with SHR, more than expected by chance (p = 5 × 10⁻⁴⁰). At these 130 loci, the height-increasing alleles are associated with either a decrease (71 loci) or increase (59 loci) in SHR, suggesting that different height loci disproportionally affect either leg length or spine/head length. Pathway analyses via DEPICT revealed that height loci affecting SHR, and especially those affecting leg length, show enrichment of different biological pathways (e.g., bone/cartilage/growth plate pathways) than do loci with no effect on SHR (e.g., embryonic development). These results highlight the value of using a pair of related but orthogonal phenotypes, in this case SHR with height, as a prism to dissect the biology underlying genetic associations in polygenic traits and diseases.     

DNA loops generate intracentromere tension in mitosis

The centromere is the DNA locus that dictates kinetochore formation and is visibly apparent as heterochromatin that bridges sister kinetochores in metaphase. Sister centromeres are compacted and held together by cohesin, condensin, and topoisomerase-mediated entanglements until all sister chromosomes bi-orient along the spindle apparatus. The establishment of tension between sister chromatids is essential for quenching a checkpoint kinase signal generated from kinetochores lacking microtubule attachment or tension. How the centromere chromatin spring is organized and functions as a tensiometer is largely unexplored. We have discovered that centromere chromatin loops generate an extensional/poleward force sufficient to release nucleosomes proximal to the spindle axis. This study describes how the physical consequences of DNA looping directly underlie the biological mechanism for sister centromere separation and the spring-like properties of the centromere in mitosis.     

DNA2 drives processing and restart of reversed replication forks in human cells

Accurate processing of stalled or damaged DNA replication forks is paramount to genomic integrity and recent work points to replication fork reversal and restart as a central mechanism to ensuring high-fidelity DNA replication. Here, we identify a novel DNA2- and WRN-dependent mechanism of reversed replication fork processing and restart after prolonged genotoxic stress. The human DNA2 nuclease and WRN ATPase activities functionally interact to degrade reversed replication forks with a 5′-to-3′ polarity and promote replication restart, thus preventing aberrant processing of unresolved replication intermediates. Unexpectedly, EXO1, MRE11, and CtIP are not involved in the same mechanism of reversed fork processing, whereas human RECQ1 limits DNA2 activity by preventing extensive nascent strand degradation. RAD51 depletion antagonizes this mechanism, presumably by preventing reversed fork formation. These studies define a new mechanism for maintaining genome integrity tightly controlled by specific nucleolytic activities and central homologous recombination factors.     

An unconventional route to becoming a cell biologist

I am honored to be the E. B. Wilson Award recipient for 2015. As we know, it was E. B. Wilson who popularized the concept of a “stem cell” in his book The Cell in Development and Inheritance (1896, London: Macmillan & Co.). Given that stem cell research is my field and that E. B. Wilson is so revered within the cell biology community, I am a bit humbled by how long it took me to truly grasp his vision and imaginative thinking. I appreciate it deeply now, and on this meaningful occasion, I will sketch my rather circuitous road to cell biology.I grew up in a suburb of Chicago. My father was a geochemist, and for everyone whose parents worked at Argonne National Laboratories, Downers Grove was the place to live. My father’s sister was a radiobiologist and my uncle was a nuclear chemist, both at Argonne; they lived in the house next door. Across the street from their house was the Schmidtke’s Popcorn Farm—a great door to knock on at Halloween. The cornfields were also super for playing hide-and-seek, particularly when you happened to be shorter than those Illinois cornstalks.Open in a separate windowElaine FuchsI remember when the first road in the area was paved. It made biking and roller-skating an absolute delight. Fields of butterflies were everywhere, and with development came swamps and ponds filled with pollywogs and local creeks with crayfish. It was natural to become a biologist. When coupled with a family of scientists and a mother active in the Girl Scouts, all the resources were there to make it a perfect path to becoming a scientist.I could hardly wait until I was in junior high school, when I could enter science fairs. You would think that my science-minded family might help me choose and develop a research project. True to their mentoring ethos, they left these decisions to me. My first project was on crayfish behavior. I recorded the response of the crayfish I had caught to “various external stimuli.” At the end of this assault, I dissected the crayfish and, using “comparative anatomy,” attempted to identify all the parts. The second project was no gentler. I focused on tadpole metamorphosis and the effects of thyroid hormone in accelerating development at low concentrations and death at elevated concentrations. Somehow, I ended up going all the way to the state fair, where it became clear that I had serious competition. That experience, however, whetted my appetite to gain more lab experience and to learn to read the literature more carefully.My experience with high school biology prompted me to gravitate toward chemistry, physics, and math. When it came to college, my father told me that if there was a $2000/year (translated in 2015 to be $30,000/year) reason why I should go anywhere besides the University of Chicago (where Argonne scientists received a 50% tuition cut for their children) or the University of Illinois (then $200/year tuition), we could “discuss” it further. Having a sister, father, aunt, and uncle who went to the University of Chicago, I chose the University of Illinois and saved my Dad a bundle of money. At Illinois, I thought I might revisit biology, but my choices for a major were “biology for teachers” or “honors biology.” The first did not interest me; the second seemed intimidating.I enrolled as a chemistry major. Four years went by, during which time I never took a biology class. I enjoyed quantum mechanics, physics, and differential equations, and problem solving became one of my strengths. In the midst of the Vietnam War era, however, Illinois was a hotbed of activity. I was inspired to apply to the Peace Corps, with a backup plan to pursue science that would be more biomedically relevant than quantum mechanics. I was accepted to go to Uganda with the Peace Corps, but with Idi Amin in office, my path to science was clear. Fortunately, the schools I applied to accepted me, even though, in lieu of GRE scores, I had submitted a three-page essay on why I did not think another exam was going to prove anything. I chose Princeton’s biochemistry program. This turned out to be a great, if naïve choice, as only after accepting their offer did I take a biochemistry class to find out what I was getting into. I chose to carry out my PhD with a terrific teacher of intermediary metabolism, Charles Gilvarg, who worked on bacterial cell walls. My thesis project was to tackle how spores break down one cell wall and build another as they transition from quiescence to vegetative growth.By my fourth year of graduate school, I was trained as a chemist and biochemist and was becoming increasingly hooked on biomedical science. I listened to a seminar given by Howard Green, who had developed a method to culture cells from healthy human skin under conditions in which they could be maintained and propagated for hundreds of generations without losing their ability to make tissue. At the time, Howard referred to them as epidermal keratinocytes, but in retrospect, these were the first stem cells ever to be successfully cultured. I was profoundly taken by the system, and Howard’s strength in cell biology inspired me. It was the perfect match for pursuing my postdoctoral research. The time happened to be at the cusp of DNA recombinant technology.At MIT, I learned how to culture these cells. I wanted to determine their program of gene expression and how this changed when epidermal progenitors embark on their terminal differentiation program. While the problem in essence was not so different from that of my graduate work at Princeton, I had miraculously managed to receive my PhD without ever having isolated protein, RNA, or DNA. Working in a quintessential cell biology lab and tackling a molecular biology question necessitated venturing outside the confines of the Green lab and beyond the boundaries of my expertise. Fortunately, this was easy at MIT. Richard Hynes, Bob Horvitz, Bob Weinberg, and Graham Walker were all assistant professors, and their labs were very helpful, as were those of David Baltimore and Phil Sharp, a mere walk across the street. On the floor of my building, Steve Farmer, Avri Ben Ze’ev, Gideon Dreyfuss, and Ihor Lemischka were in Sheldon Penman’s lab just down the hall, and they were equally interested in mRNA biology, providing daily fuel for discussions. Uttam Rhajbandary’s and Gobind Khorana’s labs were also on the same floor, making it easy to learn how to make oligo(dT)-Sepharose to purify my mRNAs. Vernon Ingram’s lab was also on the same floor, so learning to make rabbit reticulocyte lysates to translate my mRNAs was also possible. Howard bought a cryostat, so I could section human skin and separate the layers. And as he was already working with clinicians at Harvard to apply his ability to create sheets of epidermal cells for the treatment of burn patients, I had access to the leftover scraps of human tissue that were also being used in these operations.The three years of my postdoc were accompanied by three Fuchs and Green papers. The first showed that epidermal keratinocytes spend most of their time expressing a group of keratin proteins with distinct sequences. The second showed that these keratins were each encoded by distinct mRNAs. The third showed that, as epidermal keratinocytes commit to terminally differentiate, they switch off expression of basal keratins (K5 and K14) and switch on the expression of suprabasal keratins (K1 and K10). That paper also revealed that different stratified tissues express the same basal keratins but distinct sets of suprabasal keratins. I am still very proud of these accomplishments, and my MIT experience made me thirst to discover more about the epidermis and its stem cells.My first and only real job interview came during my second year of postdoc, at a time when I was not looking for a job. I viewed the opportunity, initiated by my graduate advisor, as a free trip home to visit my parents and my trial run to prepare me for future searching. I was thrilled when this interview materialized into an offer to join the faculty, for which the University of Chicago extended my start time to allow me to complete my three years with Howard.Times have clearly changed, and it is painful to see talented young scientists struggle so much more today. That said, I have never looked ahead very far, and having a lack of expectations or worry is likely to be as helpful today as it was then. I am sure it is easier said than done, but this has also been the same for my science. I have always enjoyed the experiments and the joy of discovery. There was no means to an end other than to contemplate what the data meant in a broader scope.I arrived at the University of Chicago with a well-charted route. My aim was to make a cDNA library and clone and characterize the sequences and genes for the differentially expressed keratins I had identified when I was at MIT. It was three months into my being at Chicago when my chair lined up some interviews for me to hire a technician. I was so immersed in my science that I did not want to take time to hire anyone. I hired the first technician I interviewed. Fortunately, it worked out. However, I turned graduate students away the first year, preferring to carry out the experiments with my technician and get results. After publishing two more papers—one on the existence of two types of keratins that were differentially expressed as pairs and the other on signals that impacted the differential expression of these keratin pairs, I decided to accept a student, who analyzed the human keratin genes. My first postdoc was a fellow grad student with me at Princeton; she studied signaling and keratin gene expression. My second postdoc was initiated by my father, who chatted with him at the elevator when I was moving into my apartment. He set up DNA sequencing and secondary-structure prediction methods, and the lab stayed small, focused, and productive.I was fascinated by keratins, how they assembled into a network of intermediate filaments (Ifs). When thalassemias and sickle cell anemia turned out to be due to defects in globin genes, I began to wonder whether there might be human skin disorders with defective keratin genes. I had no formal training in genetics, and there were no hints of what diseases to focus on. Thus, rather than using positional cloning to identify a gene mutation associated with a particular disease, we took a reverse approach: we first identified the key residues for keratin filament assembly. After discovering that mutations at these sites acted dominant negatively, we engineered transgenic mice harboring our mutant keratin genes and then diagnosed the mouse pathology. Our diagnoses, first for our K14 mutations and then for our K10 mutations, turned out to be correct: on sequencing the keratins from humans with epidermolysis bullosa simplex (EBS), we found K14 or K5 mutations; similarly, we found K1 or K10 mutations in affected, but not in unaffected, members of families with epidermolytic hyperkeratosis (EH). Both are autosomal-dominant disorders in which patients have skin blistering or degeneration upon mechanical stress. Without a proper keratin network, the basal (EBS) or suprabasal (EH) cells could not withstand pressure. Ironically, family sizes of all but the mildest forms of these disorders were small, meaning that the disorders were not amenable to positional cloning. But the beauty of this approach is that once we had made the connection to the diseases, we understood their underlying biology. In addition, the IF genes are a superfamily of more than 100 genes differentially expressed in nearly all tissues of the body. Once we had established EBS as the first IF gene disorder, the pathology and biology set a paradigm for a number of diseases of other tissues that turned out to be due to defects in other IF genes.Fortunately, I had students, Bob Vassar (professor, Northwestern University) and Tony Letai (associate professor, Harvard Medical School), and a postdoc, Pierre Coulombe (chair, Biochemistry and Molecular Biology, Johns Hopkins University), who jumped into this fearless venture with me. We had to go off campus to learn transgenic technology. I had never worked with mice before. When Bob returned to campus with transgenic expertise, we hired and trained Linda Degenstein, whose love for animal science was unparalleled. Pierre’s prior training in electron microscopy was instrumental in multiple ways. Additionally, I was not a dermatologist and had no access to human patients. Fortunately Amy Paller, MD, at Northwestern volunteered to work with us.The success of this project attests to an important recipe: 1) Pursue a question you are passionate about. 2) In carrying out rigorous, well-controlled experiments, each new finding should build upon the previous ones. 3) If you have learned to be comfortable with being uncomfortable, then you will not be afraid to chart new territory when the questions you are excited to answer take an unanticipated turn. 4) Science does not operate in a vacuum. Interact well with your lab mates and take an interest in their science as well as your own. And wherever you embark upon a pathway in which the lab’s expertise is limited, do not hesitate to reach out broadly to other labs and universities.I have followed this recipe now for more than three decades, and it seems to work pretty well. A lab works only when its students and postdocs are interactive, naturally inquisitive, and freely share their ideas and findings. I have been blessed to have a number of such individuals in my lab over the years. When push comes to shove, I am always inclined first to shave from the “brilliant” category and settle for smart, nice people who are passionate and interactive about science and original and unconventional in their thinking.So what questions have I been most passionate about? I have always been fascinated with how tissues form during development, how they are maintained in the adult, and how tissue biology goes awry in human disorders, particularly cancers. I first began to think about this problem during my days at Princeton. I also developed a dogma back then that I still hold: to understand malignancies, one must understand what is normal before one can appreciate what is abnormal. I think this is why I have spent so much of my life focusing on normal tissue morphogenesis, despite my passion for being at the interface with medicine. And because skin has so many amazingly interesting complexities, and because it is a great system to transition seamlessly between a culture dish and an animal, I have never found a reason to choose any other tissue over the one I chose many years back.I will not dwell on the various facets of skin biology we have tackled over the years. Our initial work on keratins was to obtain markers for progenitors and their differentiating lineages. This interest broadened to understanding how proliferative progenitors form cytoskeletal networks and how the cytoskeleton makes dynamic rearrangements during tissue morphogenesis.From the beginning, the lab has also been fascinated by how tissue remodeling occurs in response to environmental signals. Indeed, signals from the microenvironment trigger changes in chromatin dynamics and gene expression within tissue stem cells. Ultimately, this leads to changes in proteins and factors that impact on cell polarity, spindle orientation, asymmetric versus symmetric fate specifications, and ultimately, the balance between proliferation and differentiation.The overarching theme of my lab over these decades is clear, namely, to understand the signals that unspecified progenitors receive that instruct them to generate a stratified epidermis, make hair follicles, or make sweat and sebaceous glands. And if we can understand how this happens, then how are stem cells born, and how do they replace dying cells or regenerate tissue after injury? And, finally, how does this process change during malignant progression or in other aberrant skin conditions?In tackling tissue morphogenesis, I have had to forgo knowing the details of each tree and instead focus on the forest. There are many times when I stand back and can only admire those who are able to dissect beautiful cellular mechanisms with remarkable precision. But I crossed that bridge some years ago in tackling a problem that mandates an appreciation of nearly all the topics covered in Bruce Alberts’ textbook Molecular Biology of the Cell. I am now settled comfortably with the uncomfortable, and the problem of tissue morphogenesis in normal biology and disease continues to keep me more excited about each year’s research than I was the previous year. Perhaps the difference between my days as a student, postdoc, and assistant professor and now is that my joy and excitement is as strong for those I mentor and have mentored as it is for myself.     

Infestation of arboreal nests of coatis by triatomine species,vectors of Trypanosoma cruzi,in a large Neotropical wetland

The coati (Nasua nasua, Carnivora) is a medium‐sized mammal common in the Pantanal of Brazil. Unlike most mammals, coatis construct arboreal nests used for resting and reproduction. In this region, the coati is an important host of Trypanosoma cruzi, the causative agent of Chagas disease. There are two possible routes through coatis can be infected by T. cruzi: the oral route or the vectorial route. However, the relative importance of each of these routes in the infection of coatis and its role in the sylvatic cycle of the parasite are unknown. Our objectives were to investigate: (i) whether coati nests were infested by triatomine bugs, (ii) what species were frequent in the nests, (iii) whether the triatomines in nests were infected by T. cruzi, and (iv) what were the food resources of these triatomines. Eight of the 24 nests sampled were infested with triatomines, a total of 37 specimens of at least two species (Rhodnius stali and Triatoma sordida). In one nest, R. stali and T. sordida co‐occurred and both fed on multiple resources, including coatis. This is the first report of triatomines occurring in arboreal nests of coatis. The co‐occurrence of two different genera of triatomine vectors and coatis within the limited space of the coati nests provide multiple opportunities for the exchange of the protozoan parasite through both the vectorial and oral transmission routes.     

Antiinflammatory Activity of a Novel Folic Acid Targeted Conjugate of the mTOR Inhibitor Everolimus

Folate receptor (FR)-β has been identified as a promising target for antimacrophage and antiinflammatory therapies. In the present study, we investigated EC0565, a folic acid–derivative of everolimus, as a FR-specific inhibitor of the mammalian target of rapamycin (mTOR). Because of its amphiphilic nature, EC0565 was first evaluated for water solubility, critical micelle formation, stability in culture and FR-binding specificity. Using FR-expressing macrophages, the effect of EC0565 on mTOR signaling and cellular proliferation was studied. The pharmacokinetics, metabolism and bioavailability of EC0565 were studied in normal rats. The in vivo activity of EC0565 was assessed in rats with adjuvant arthritis, a “macrophage-rich” model with close resemblance to rheumatoid arthritis. EC0565 forms micellar aggregates in physiological buffers and demonstrates good water solubility as well as strong multivalent FR-binding capacity. EC0565 inhibited mTOR signaling in rat macrophages at nanomolar concentrations and induced G0/G1 cell cycle arrest in serum-starved RAW264.7 cells. Subcutaneously administered EC0565 in rats displayed good bioavailability and a relatively long half-life (~12 h). When given at 250 nmol/kg, EC0565 selectively inhibited proliferating cell nuclear antigen expression in thioglycollate-stimulated rat peritoneal cells. With limited dosing regimens, the antiarthritic activity of EC0565 was found superior to that of etanercept, everolimus and a nontargeted everolimus analog. The in vivo activity of EC0565 was also comparable to that of a folate-targeted aminopterin. Folate-targeted mTOR inhibition may be an effective way of suppressing activated macrophages in sites of inflammation, especially in nutrient-deprived conditions, such as in the arthritic joints. Further investigation and improvement upon the physical and biochemical properties of EC0565 are warranted.