Posts tagged genetics
Posts tagged genetics
Although a family history of Alzheimer’s disease is a primary risk factor for the devastating neurological disorder, mutations in only three genes – the amyloid precursor protein and presenilins 1 and 2 – have been established as causative for inherited, early-onset Alzheimer’s, accounting for about half of such cases. Now Massachusetts General Hospital (MGH) researchers have discovered a type of mutation known as copy-number variants (CNVs) – deletions, duplications, or rearrangements of human genomic DNA – in affected members of 10 families with early-onset Alzheimer’s. Notably, different genomic changes were identified in the Alzheimer’s patients in each family.
The study was conducted as part of the Alzheimer’s Genome Project – directed by Rudolph Tanzi, PhD, director of the Genetics and Aging Research Unit at Massachusetts General Hospital (MGH) and a co-discoverer of the first three early-onset genes – and was supported by the Cure Alzheimer’s Fund and the National Institute of Mental Health (NIMH).
“We found that the Alzheimer’s-afflicted members of these families had duplications or deletions in genes with important roles in brain function, while their unaffected siblings had unaltered copies of those genes,” says Basavaraj Hooli, PhD, of the Genetics and Aging Research Unit, MassGeneral Institute for Neurodegenerative Disease, lead author of a report that has been published online in Molecular Psychiatry. “Since our preliminary review of the affected genes has provided strong clues to a range of pathways associated with Alzheimer’s disease and other forms of dementia, we believe that further research into the functional effects of these CNVs will provide new insights into Alzheimer’s pathogenesis.” Hooli is a research fellow in Neurology at Harvard Medical School.
Most studies searching for genes contributing to Alzheimer’s risk have looked for variants in a single nucleotide, and while thousands of such changes have been identified, each appears to have a very small impact on disease risk. Recently research has found that CNVs – in which DNA segments of varying lengths are deleted or duplicated – have a greater impact on genomic diversity than do single-nucleotide changes. This led Tanzi and his team to search for large CNVs in affected members of families with inherited Alzheimer’s disease. “These are the first new early-onset familial Alzheimer’s disease gene mutations to be reported since 1995, when we co-discovered the presenilins. As with those original genes, we hope to use the information gained from studies of the new Alzheimer’s mutations to guide the development of novel therapies aimed at preventing and treating this devastating disease.” Tanzi explains.
The investigators reviewed genomic data from two sources – the NIMH Alzheimer’s Disease Genetics Initiative and the National Cell Repository for Alzheimer’s Disease – and focused on 261 families with at least one member who developed Alzheimer’s before the age of 65. Using a novel algorithm they had developed for analyzing CNVs, the researchers identified deletions or duplications that appeared only in affected members of these families. Two of these families had CNVs that included the well-established amyloid precursor protein gene, but 10 others were found to have novel Alzheimer’s-associated CNVs, with different gene segments being affected in each family.
While none of the novel variants have previously been associated with Alzheimer’s disease, most of them affect genes believed to be essential to normal neuronal function, and several have been previously associated with other forms of dementia. For example, one of the identified CNVs involves deletion of a gene called CHMP2B, mutations of which can cause ALS. In another family, affected members had three copies of the gene MAPT, which encodes the tau protein found in the neurofibrillary tangles characteristic of Alzheimer’s. Mutations in MAPT also cause frontotemporal dementia.
Hooli explains, “Potential clinical application of the findings of this study are not yet clear and require two additional pieces of information: similar studies in larger groups of families with inherited Alzheimer’s to establish the prevalence of these CNVs and whether the presence of one ensures development of the disease, and a better understanding of how these variants affect neuronal pathways leading to the early-onset form of Alzheimer’s disease.”
“In a broader sense,” Tanzi adds, “the advent of affordable, advanced whole-genome sequencing will lead to the identification of novel, rare mutations that lead to many human disorders. In the future, diagnosis and prognosis may rely more on disease genetics than on traditional laboratory results and behavioral effects. If knowing the exact genetic causes of these disorders leads to more effective and efficient treatment strategies targeted to specific defects, the consequences of this approach would be enormous.”
A study led by researchers at The Ohio State University Comprehensive Cancer Center – Arthur G. James Cancer Hospital and Richard J. Solove Research Institute (OSUCCC – James) has identified an abnormal metabolic pathway that drives cancer-cell growth in a particular glioblastoma subtype. The finding might lead to new therapies for a subset of patients with glioblastoma, the most common and lethal form of brain cancer.
The physician scientists sought to identify glioblastoma subtype-specific cancer stem cells. Genetic analyses have shown that high-grade gliomas can be divided into four subtypes: proneural, neural, classic and mesenchymal.
This study shows that the mesenchymal subtype is the most aggressive subtype, that it has the poorest prognosis among affected patients, and that cancer stem cells isolated from the mesenchymal subtype have significantly higher levels of the enzyme ALDH1A3 compared with the proneural subtype.
The findings, published recently in the Proceedings of the National Academy of Sciences, show that high levels of the enzyme drive tumor growth.
“Our study suggests that ALDH1A3 is a potentially functional biomarker for mesenchymal glioma stem cells, and that inhibiting that enzyme might offer a promising therapeutic approach for high-grade gliomas that have a mesenchymal signature,” says principal investigator Ichiro Nakano, MD, PhD, associate professor of neurosurgery at the OSUCCC – James. “This indicates that therapies for high-grade gliomas should be personalized, that is, based on the tumor subtype instead of applying one treatment to all patients,” he says.
The National Cancer Institute estimates that 23,130 Americans will be diagnosed with brain and other nervous system tumors in 2013, and that 14,000 people will die of these malignancies. Glioblastoma accounts for about 15 percent of all brain tumors, is resistant to current therapies and has a survival as short as 15 months after diagnosis.
Little is known, however, about the metabolic pathways that drive the growth of individual glioblastoma subtypes – knowledge that is crucial for developing novel and effective targeted therapies that might improve treatment for these lethal tumors.
For this study, Nakano and his collaborators used cancer cells from 40 patients with high-grade gliomas, focusing on tumor cells with a stem-cell signature. The researchers then used microarray analysis and pre-clinical animal assays to identify two predominant glioblastoma subtypes, proneural and mesenchymal.
Key technical findings include:
“Overall, our data suggest that a novel signaling mechanism underlies the transformation of proneural glioma stem cells to mesenchymal-like cells and their maintenance as stem-like cells,” Nakano says. Currently, their discoveries are in provision patent application, led by the Technology Licensing Office at University of Pittsburgh.
A new study of the genetic origins of dyslexia and other learning disabilities could allow for earlier diagnoses and more successful interventions, according to researchers at Yale School of Medicine. Many students now are not diagnosed until high school, at which point treatments are less effective.
The study is published online and in the July print issue of the American Journal of Human Genetics. Senior author Dr. Jeffrey R. Gruen, professor of pediatrics, genetics, and investigative medicine at Yale, and colleagues analyzed data from more than 10,000 children born in 1991-1992 who were part of the Avon Longitudinal Study of Parents and Children (ALSPAC) conducted by investigators at the University of Bristol in the United Kingdom.
Gruen and his team used the ALSPAC data to unravel the genetic components of reading and verbal language. In the process, they identified genetic variants that can predispose children to dyslexia and language impairment, increasing the likelihood of earlier diagnosis and more effective interventions.
Dyslexia and language impairment are common learning disabilities that make reading and verbal language skills difficult. Both disorders have a substantial genetic component, but despite years of study, determining the root cause had been difficult.
In previous studies, Gruen and his team found that dopamine-related genes ANKK1 and DRD2 are involved in language processing. In further non-genetic studies, they found that prenatal exposure to nicotine has a strong negative affect on both reading and language processing. They had also previously found that a gene called DCDC2 was linked to dyslexia.
In this new study, Gruen and colleagues looked deeper within the DCDC2 gene to pinpoint the specific parts of the gene that are responsible for dyslexia and language impairment. They found that some variants of a gene regulator called READ1 (regulatory element associated with dyslexia1) within the DCDC2 gene are associated with problems in reading performance while other variants are strongly associated with problems in verbal language performance.
Gruen said these variants interact with a second dyslexia risk gene called KIAA0319. “When you have risk variants in both READ1 and KIAA0319, it can have a multiplier effect on measures of reading, language, and IQ,” he said. “People who have these variants have a substantially increased likelihood of developing dyslexia or language impairment.”
“These findings are helping us to identify the pathways for fluent reading, the components of those pathways; and how they interact,” said Gruen. “We now hope to be able to offer a pre-symptomatic diagnostic panel, so we can identify children at risk before they get into trouble at school. Almost three-quarters of these children will be reading at grade level if they get early intervention, and we know that intervention can have a positive lasting effect.”
In the first prospective study of its kind, Seaver Autism Center researchers at the Icahn School of Medicine at Mount Sinai provide new evidence of the severity of intellectual, motor, and speech impairments in a subtype of autism called Phelan-McDermid Syndrome (PMS). The data are published online in the June 11 issue of the journal Molecular Autism.
Mutation or deletion of a gene known as SHANK3 is one of the more common single-gene causes of autism spectrum disorders and is critical to the development of PMS, a severe type of autism. To date, clinicians have relied on case studies and retrospective reviews of medical records to understand the features of this disorder and how the clinical presentation relates to the extent of the genetic changes in the SHANK3 region. In the first systematic and comprehensive prospective trial, researchers led by Alex Kolevzon, MD, Clinical Director of the Seaver Autism Center, under the direction of Joseph Buxbaum, PhD, Director of the Seaver Autism Center, enrolled 32 participants with SHANK3 deletions to comprehensively assess their clinical symptoms and examine how the size of the SHANK3 deletion correlated to those symptoms.
“Previous studies have not utilized prospective assessments to understand Phelan-McDermid Syndrome, and the prevalence of autism spectrum disorder has never been examined using gold-standard instruments” said Dr. Kolevzon. “There is no established standard for assessing this type of autism, and our study provides important guidance in developing such a standard.”
Of the 32 patients enrolled, 84 percent met criteria for an autism spectrum disorder. Seventy-seven percent of patients exhibited severe to profound intellectual disability, with 19 percent using some form of verbal communication. Other common features included low muscle tone, gait disturbance, and seizures. The researchers also found that patients who had larger SHANK3 deletions had more severe disease.
“Our findings provide additional evidence of the significant impairment associated with SHANK3 deficiency,” said Dr. Kolevzon. “Also, knowing how large the deletion of the SHANK3 gene is may have important implications for medical monitoring and individualizing treatment plans. Results also provide much-needed guidance in developing a standardized methodology for evaluating the features of this disorder.”
Many of the patients who participated in this study were next enrolled in a clinical trial at Mount Sinai evaluating Insulin-Like Growth Factor-1 (IGF-1), a commercially available compound for growth deficiency that is known to promote nerve cell survival as well as synaptic maturation and plasticity. The primary aim of the study is to target core features of PMS, including social withdrawal and language impairment, which will be measured using both behavioral and objective assessments. The clinical studies with IGF-1 were supported by studies in a genetically modified mouse with a mutation in SHANK3. These studies, carried out by Dr. Ozlem Bozdagi of the Seaver Autism Center, carefully examined brain function in the mice when SHANK3 was mutated, and provided preclinical evidence for a beneficial effect of IGF-1. These studies were reported the April 27th issue of Molecular Autism (1, 2).
“The Seaver Autism Center has the unique capacity to evaluate autism spectrum disorders on both the molecular level and the clinical level,” said Dr. Buxbaum. “This capability puts us in a unique position to see the entire picture—the connection between genetics and behavior in these disorders—and to develop new treatments and better tailor existing ones for these children.”
Findings may have implications for treating compulsive behavior associated with psychiatric disease and eating disorders
What started as an experiment to probe brain circuits involved in compulsive behavior has revealed a surprising connection with obesity.
The University of Iowa-led researchers bred mice missing a gene known to cause obesity, and suspected to also be involved in compulsive behavior, with a genetic mouse model of compulsive grooming. The unexpected result was offspring that were neither compulsive groomers nor obese.
The study, published the week of June 10 in the online early edition of the Proceedings of the National Academy of Sciences (PNAS), suggests that the brain circuits that control obsessive-compulsive behavior are intertwined with circuits that control food intake and body weight. The findings have implications for treating compulsive behavior, which is associated with many forms of psychiatric disease, including obsessive-compulsive disorder (OCD), Tourette syndrome, and eating disorders.
UI neuro-psychiatrists Michael Lutter, M.D., Ph.D. and Andrew Pieper, M.D., Ph.D. led the study. The team also included researchers from Stanford University School of Medicine, University of Texas Southwestern Medical Center, Beth Israel Deaconess Medical Center, and Harvard Medical School.
Lutter, an assistant professor of psychiatry, and Pieper, an associate professor of psychiatry and neurology at the UI Carver College of Medicine, both recently arrived at the UI and use mouse models in their laboratories to study human disorders and conditions.
Pieper is interested in compulsive behavior. His mouse model of compulsivity lacks a brain protein called SAPAP3. These mice groom themselves excessively to the point of lesioning their skin, and their compulsive behavior can be effectively treated by fluoxetine, a drug that is commonly used to treat OCD in people.
Lutter works with a mouse that genetically mimics an inherited form of human obesity. This mouse lacks a brain protein known a MC4R. Mutations in the MC4R gene are the most common single-gene cause of morbid obesity and over-eating in people.
“I study MC4R signaling pathways and their involvement in the development of obesity,” Lutter explains. “I’m also interested in how these same molecules affect mood and anxiety and reward, because it’s known that there is a connection between depression and anxiety and development of obesity.”
An old study hinted that in addition to its role in food intake and obesity, MC4R might also play a role in compulsive behavior, which got Lutter and Pieper thinking of ways to test the possible interaction.
“We knew in one mouse you could stimulate excessive grooming through this MC4R pathway and in another mouse a different pathway (SAPAP3) caused compulsive grooming,” Lutter says. “So, we decided to breed the two mice together to see if it would have an effect on compulsive grooming.”
The experiment proved their original hypothesis—knocking out the MC4R protein in the OCD mouse normalized grooming behavior in the animals. In addition, chemically blocking MC4R in the OCD mice also eliminated compulsive grooming. The rescued behavior is mirrored by normalization of a particular pattern of brain cell communication linked to compulsive behavior.
However, the breeding experiment revealed another totally unexpected result. Loss of the SAPAP3 protein from the mice that were obese due to lack of MC4R produced mice of normal weight.
“We had this other, completely shocking finding—we completely rescued body weight and food intake in the double null mouse,” Lutter says. “So, not only were we affecting the brain regions involved in grooming and behavior, but we also affected the brain regions involved in food intake and body weight.”
Although obesity and obsessive-compulsive behavior may seem unrelated, Lutter suggests that the connection may be rooted in the evolutionary need to eat safe, clean food in times of a food abundance, and to lessen this drive when food is scarce.
“Food safety has been an issue through the entire course of human evolution—refrigeration is a relatively recent invention,” he says. “Obsessive behavior, or fear of contamination, may be an evolutionary protection against eating rotten food.”
Oils and fats have lots of calories and nutrients but they also spoil much more easily than less nutrient- and calorie-dense foods like potatoes, onions, or apples.
“I think this circuit that we have uncovered is probably involved in determining whether or not people should eat calorically dense foods,” he says.
Lutter suggests that slight perturbations in this system might lead, on one hand, to disorders that link anxiety and obsessive behavior to limited food selection or intake, such as anorexia nervosa, Tourette syndrome, or OCD, and on the other hand, to obesity, where people over-consume high-fat foods and may have decreased obsessive behavior and anxiety.
“The next step will be to determine how these two pathways communicate with one another, in hopes of identifying new ways to develop drugs to treat either of these disorders,” says Pieper.
There are a variety of factors that determine the number of years a person goes to school – personality, finances, life circumstances, country of origin and social norms. One factor that may be less obvious, however, is genetics. Around 40 percent of the variance in educational attainment can be explained by a person’s DNA, according to previous research. Now a new study is the first to identify specific genes that influence educational achievement.
This research falls under the category of social-science genetics, a topic that includes everything from genes for political affiliation to genes for criminality. Previous studies in the field, however, have found relatively weak associations between specific gene variants and behavior, since behavior is influenced by the accumulation of small effects from many genes.
To counteract that problem, this study was especially large – 125,000 Caucasian people from the United States, Australia, and 13 European countries. Researchers took blood samples and asked participants how many years of schooling they’d completed and whether or not they’d graduated from college. The researchers converted the answers to an international educational standard to allow accurate comparisons between countries.
Then, delving into subjects’ DNA, researchers found three mutations (called SNPs) at specific positions on the genome that were strongly associated with educational outcome – one that corresponded to years of schooling and two that corresponded to college completion. The mutations were found within genes believed to be associated with health, learning, memory and brain-cell mechanics, the researchers report today in Science.
Each mutation contributed only a small amount. In terms of the years of schooling, one copy of the SNP meant that an individual completed 1 month of additional schooling. (Each person can have up to two copies of an individual SNP, one from mom and one from dad.)
For college completion, the most indicative SNP corresponded to a 1.8 percentage-point rise in the likelihood of graduating from college. If a person had two copies of this SNP, then, their likelihood would rise by 3.6 points.
These are small effects but meaningful because they held up on such a large scale. The findings support the general consensus that our behavioral traits are influenced by a large number of genes, each of small effect. Overall, each SNP in this study altered educational attainment by only about 0.02%. In comparison, for a complex physical trait like human height, a single SNP can influence the outcome by 0.4%.
Ongoing genetic research keeps reinforcing this idea that genes aren’t destiny – there’s no gene for graduating college. But it’s good to keep in mind that genes are part of the list of contributors that make us who we are.
Researchers at the MRC Laboratory of Molecular Biology in the United Kingdom have determined the crystal structure of Parkin, a protein found in cells that when mutated can lead to a hereditary form of Parkinson’s disease. The results, which are published in The EMBO Journal, define the position of many of the mutations linked to hereditary Parkinson’s disease and explain how these alterations may affect the stability and function of the protein. The findings may in time reveal how the activity of Parkin is affected in patients with this rare but debilitating type of Parkinson’s disease.
Parkinson’s disease is a progressive neurodegenerative disease that affects more than seven million people worldwide. Most cases of the disease occur in older individuals and are sporadic (non-familial), but around 15% of patients develop symptoms early in life because of inherited mutations in a limited number of disease genes. Why Parkin mutations are especially detrimental in nerve cells is not fully understood, but previous research indicates that Parkin regulates the function of mitochondria, the organelles that generate energy in the cell. Some disease mutations in the PARKIN gene can be easily explained since they lead to loss or instability of the Parkin protein, but many others are more difficult to understand.
Around 50% of cases of familial recessive Parkinson’s disease are caused by mutations in the PARKIN gene, which encodes a protein that belongs to the RBR ubiquitin ligase enzyme family. Enzymes in this family couple other proteins in the cell to a molecule called ubiquitin, a step that can alter the function or stability of these target proteins. To understand how Parkin and other RBR ubiquitin ligase enzymes achieve this, EMBO Young Investigator David Komander and his coworker Tobias Wauer crystallized a form of human Parkin and used X-ray diffraction patterns to determine how the Parkin protein chain folds into a three-dimensional structure. Their experiments revealed an in-built control mechanism for Parkin activity, which is lost in the presence of some of the mutations responsible for Parkinson’s disease. Wauer and Komander pinpointed amino acids of Parkin with key functions in ubiquitin ligase activity that are sensitive to blocking by reagents previously characterized in their laboratory. “This sensitivity to inhibitors that were developed for a very different class of enzymes is particularly exciting,” Komander remarked. “We could also show that these inhibitors affect related RBR ubiquitin ligases such as HOIP, which is important for inflammatory immune responses.”
The crystal structure of Parkin is already revealing some of the secrets of this molecule, which under the right conditions can protect cells from the damage that arises during Parkinson’s disease. “In time the structure may also allow development of other compounds that alter Parkin activity, which could serve as ways to limit the progression and impact of Parkinson’s disease,” concluded Komander.
Study in Mice Points to a Synergistic Relationship Between Lead Exposure and Schizophrenia Gene
Mice engineered with a human gene for schizophrenia and exposed to lead during early life exhibited behaviors and structural changes in their brains consistent with schizophrenia. Scientists at Columbia University’s Mailman School of Public Health and the Johns Hopkins University School of Medicine say their findings suggest a synergistic effect between lead exposure and a genetic risk factor, and open an avenue to better understanding the complex gene-environment interactions that put people at risk for schizophrenia and other mental disorders.
Results appear online in Schizophrenia Bulletin.
Going back to 2004, work by scientists at the Mailman School suggested a connection between prenatal lead exposure in humans and increased risk for schizophrenia later in life. But a big question remained: How could lead trigger the disease? Based on his own research, Tomás R. Guilarte, PhD, senior author of the new study, believed the answer was in the direct inhibitory effect of lead on the N-methyl-D-aspartate receptor (NMDAR), a synaptic connection point important to brain development, learning, and memory. His research in rodents found that exposure to lead blunted the function of the NMDAR. The glutamate hypothesis of schizophrenia postulates that a deficit in glutamate neurotransmission and specifically hypoactivity of the NMDAR can explain a significant portion of the dysfunction in schizophrenia.
In the new study, Dr. Guilarte, professor and chair of the department of Environmental Health Sciences at the Mailman School, and his co-investigators focused on mice engineered to carry the mutant form of Disrupted-in-Schizophrenia-1 (DISC1), a gene that is a risk factor for the disease in humans. Beginning before birth, half of the mutant DISC1 mice were fed a diet with lead, and half were given a normal diet. A second group of normal mice not expressing the mutant DISC1 gene were also split into the two feeding groups. All mice were put through a battery of behavioral tests and their brains were examined using MRI.
Mutant mice exposed to lead and given a psychostimulant exhibited elevated levels of hyperactivity and were less able to suppress a startle in response to a loud noise after being given an acoustic warning. Their brains also had markedly larger lateral ventricles—empty spaces containing cerebrospinal fluid—compared with other mice. These results mirror what is known about schizophrenia in humans.
While the role of genes in schizophrenia and mental disorders is well established, the effect of toxic chemicals in the environment is only just beginning to emerge. The study’s results focus on schizophrenia, but implications could be broader.
“We’re just scratching the surface,” says Dr. Guilarte. “We used lead in this study, but there are other environmental toxins that disrupt the function of the NMDAR.” One of these is a family of chemicals in air pollution called polycyclic aromatic hydrocarbons or PAHs. “Similarly, any number of genes could be in play,” adds Dr. Guilarte, noting that DISC1 is among many implicated in schizophrenia.
Future research may reveal to what extent schizophrenia is determined by environmental versus genetic factors or their interactions, and what other mental problems might be in the mix. One ongoing study by Dr. Guilarte is looking at whether lead exposure alone can contribute to deficits of one specialized type of neuron called parvalbumin-positive GABAergic interneuron that is known to be affected in the brain of schizophrenia patients. Scientists are also interested to establish the critical window for exposure—whether in utero or postnatal, or both.
“The animal model provides a way forward to answer important questions about the physiological processes underlying schizophrenia,” says Dr. Guilarte.
The St. Jude Children’s Research Hospital – Washington University Pediatric Cancer Genome Project has identified mutations responsible for more than half of a subtype of childhood brain tumor that takes a high toll on patients. Researchers also found evidence the tumors are susceptible to drugs already in development.
The study focused on a family of brain tumors known as low-grade gliomas (LGGs). These slow-growing cancers are found in about 700 children annually in the U.S., making them the most common childhood tumors of the brain and spinal cord. For patients whose tumors cannot be surgically removed, the long-term outlook remains bleak due to complications from the disease and its ongoing treatment. Nationwide, surgery alone cures only about one-third of patients.
Using whole genome sequencing, researchers identified genetic alterations in two genes that occurred almost exclusively in a subtype of LGG termed diffuse LGG. This subtype cannot be cured surgically because the tumor cells invade the healthy brain. Together, the mutations accounted for 53 percent of the diffuse LGG in this study. Researchers also demonstrated that one of the mutations, which had not previously been linked to brain tumors, caused tumors when introduced into the glial brain cells of mice.
The findings appear in the April 14 advance online edition of the scientific journal Nature Genetics.
“This subtype of low-grade glioma can be a nasty chronic disease, yet prior to this study we knew almost nothing about its genetic alterations,” said David Ellison, M.D., Ph.D., chair of the St. Jude Department of Pathology and the study’s corresponding author. The first author is Jinghui Zhang, Ph.D., an associate member of the St. Jude Department of Computational Biology.
The Pediatric Cancer Genome Project is using next-generation whole genome sequencing to determine the complete normal and cancer genomes of children and adolescents with some of the least understood and most difficult to treat cancers. Scientists believe that studying differences in the 3 billion chemical bases that make up the human genome will provide the scientific foundation for the next generation of cancer care.
“We were surprised to find that many of these tumors could be traced to a single genetic alteration,” said co-author Richard K. Wilson, Ph.D., director of The Genome Institute at Washington University School of Medicine in St. Louis. “This is a major pathway through which low-grade gliomas develop and it provides new clues to explore as we search for better treatments.”
The study involved whole genome sequencing of 39 paired tumor and normal tissue samples from 38 children and adolescents with different subtypes of LGG and related tumors called low-grade glioneuronal tumors (LGGNTs). Although many cancers develop following multiple genetic abnormalities, 62 percent of the 39 tumors in this study stemmed from a single genetic alteration.
Previous studies have linked LGGs to abnormal activation of the MAPK/ERK pathway. The pathway is involved in regulating cell division and other processes that are often disrupted in cancer. Until now, however, the genetic alterations involved in driving this pathway were unknown for some types of LGG and LGGNT.
This study linked activation in the pathway to duplication of a key segment of the FGFR1 gene, which investigators discovered in brain tumors for the first time. The segment is called a tyrosine kinase domain. It functions like an on-off switch for several cell signaling pathways, including the MAPK/ERK pathway. Investigators also demonstrated that experimental drugs designed to block activity along two altered pathways worked in cells with theFGFR1 tyrosine kinase domain duplication. “The finding suggests a potential opportunity for using targeted therapies in patients whose tumors cannot be surgically removed,” Ellison said.
Researchers also showed that the FGFR1 abnormality triggered an aggressive brain tumor in glial cells from mice that lacked the tumor suppressor gene Trp53.
Whole-genome sequencing found previously undiscovered rearrangements in the MYB and MYBL1 genes in diffuse LGGs. These newly identified abnormalities were also implicated in switching on the MAPK/ERK pathway.
Researchers checked an additional 100 LGGs and LGGNTs for the same FGFR1, MYB and MYBL1 mutations. Overall, MYB was altered in 25 percent of the diffuse LGGs, and 24 percent had alterations in FGFR1. Researchers also turned up numerous other mutations that occurred in just a few tumors. The affected genes included BRAF, RAF1, H3F3A, ATRX, EP300, WHSC1 and CHD2.
“The Pediatric Cancer Genome Project has provided a remarkable opportunity to look at the genomic landscape of this disease and really put the alterations responsible on the map. We can now account for the genetic errors responsible for more than 90 percent of low-grade gliomas,” Ellison said. “The discovery that FGFR1 and MYB play a central role in childhood diffuse LGG also serves to distinguish the pediatric and adult forms of the disease.”
In one of the first successful attempts at genetically engineering mosquitoes, HHMI researchers have altered the way the insects respond to odors, including the smell of humans and the insect repellant DEET. The research not only demonstrates that mosquitoes can be genetically altered using the latest research techniques, but paves the way to understanding why the insect is so attracted to humans, and how to block that attraction.
“The time has come now to do genetics in these important disease-vector insects. I think our new work is a great example that you can do it,” says Leslie Vosshall, an HHMI investigator at The Rockefeller University who led the new research, published May 29, 2013 in the journal Nature.
In 2007, scientists announced the completion of the full genome sequence of Aedes aegypti, the mosquito that transmits dengue and yellow fever. A year later, when Vosshall became an HHMI investigator, she shifted the focus of her lab from Drosophila flies to mosquitoes with the specific goal of genetically engineering the insects. Studying mosquitoes appealed to her because of their importance as disease carriers, as well as their unique attraction to humans.
Vosshall’s first target: a gene called orco, which her lab had deleted in genetically engineered flies 10 years earlier. “We knew this gene was important for flies to be able to respond to the odors they respond to,” says Vosshall. “And we had some hints that mosquitoes interact with smells in their environment, so it was a good bet that something would interact with orco in mosquitoes.”
Vosshall’s team turned to a genetic engineering tool called zinc-finger nucleases to specifically mutate the orco gene in Aedes aegypti. They injected the targeted zinc-finger nucleases into mosquito embryos, waited for them to mature, identified mutant individuals, and generated mutant strains that allowed them to study the role of orco in mosquito biology. The engineered mosquitoes showed diminished activity in neurons linked to odor-sensing. Then, behavioral tests revealed more changes.
When given a choice between a human and any other animal, normal Aedes aegypti will reliably buzz toward the human. But the mosquitoes with orco mutations showed reduced preference for the smell of humans over guinea pigs, even in the presence of carbon dioxide, which is thought to help mosquitoes respond to human scent. “By disrupting a single gene, we can fundamentally confuse the mosquito from its task of seeking humans,” says Vosshall. But they don’t yet know whether the confusion stems from an inability to sense a “bad” smell coming from the guinea pig, a “good” smell from the human, or both.
Next, the team tested whether the mosquitoes with orco mutations responded differently to DEET. When exposed to two human arms—one slathered in a solution containing 10 percent DEET, the active ingredient in many bug repellants, and the other untreated—the mosquitoes flew equally toward both arms, suggesting they couldn’t smell the DEET. But once they landed on the arms, they quickly flew away from the DEET-covered one. “This tells us that there are two totally different mechanisms that mosquitoes are using to sense DEET,” explains Vosshall. “One is what’s happening in the air, and the other only comes into action when the mosquito is touching the skin.” Such dual mechanisms had been discussed but had never been shown before.
Vosshall and her collaborators next want to study in more detail how the orco protein interacts with the mosquitoes’ odorant receptors to allow the insects to sense smells. “We want to know what it is about these mosquitoes that makes them so specialized for humans,” she says. “And if we can also provide insights into how existing repellants are working, then we can start having some ideas about what a next-generation repellant would look like.”