T H E   N I H   C A T A L Y S T     N O V E M B E R  –  D E C E M B E R  2006



Munira A. Basrai

Munira A. Basrai received her Ph.D. from the University of Tennessee, Knoxville, in 1992. She pursued her postdoctoral studies at The Johns Hopkins School of Medicine, Baltimore. She joined NCI in 1998 as a tenure-track investigator in the Genetics Branch and is currently a senior investigator in that branch.

Aneuploidy is a hallmark of cancer cells. Defects in mechanisms that ensure high-fidelity chromosome transmission contribute to aneuploidy.

Our research focuses on defining the molecular determinants of chromosome segregation and cell-cycle checkpoint responses in the budding yeast, Saccharomyces cerevisiae, and the human orthologs of these determinants.

The high degree of conservation between yeast and human genes makes S. cerevisiae an attractive model system to elucidate how the failure of chromosome segregation mechanisms may give rise to diseases in humans.

We have two research projects: 1) mechanism of faithful chromosome transmission and cell-cycle checkpoint function, and 2) identification and characterization of small open reading frames (sORFs).

Our major research efforts are focused on the first project, in which we have used genetic screens and a colony color assay for chromosome loss to establish that mutations or deletions in S. cerevisiae SPT4 and NUP170 lead to defects in chromosome-transmission fidelity and integrity of the kinetochore (centromere DNA and associated proteins).

We determined that Spt4p is a novel component of centromeric and heterochromatic chromatin and is required for localization of Cse4p, the evolutionarily conserved centromeric histone H3 variant (CENP-A) in S. cerevisiae. Our results have shown that restricting the localization of Cse4p to centromeric DNA is essential for high-fidelity chromosome transmission.Overexpression and mislocalization of CENP-A has been observed in colorectal cancer cell lines.

In collaborative efforts with NCI’s Natasha Caplen and Anna Roschke, we are doing RNAi experiments to determine if human SPT4 plays a role in genome stability and CENP-A localization in mammalian cells.

In a cross-species approach, we have shown that the yeast mutant phenotypes are functionally complemented by a human homolog of SPT4.

Our current research is focused on understanding how SPT4, CSE4, and other factors contribute to chromosome- transmission fidelity in both yeast and humans, with the goal of understanding chromosomal aneuploidy, which is observed in essentially all sporadic tumors.

In addition to the kinetochore, checkpoints regulate progression through mitosis by halting the cell cycle in response to defective kinetochore function.

In collaboration with Richard Wozniak, University of Alberta, Edmonton, and Forrest Spencer, Johns Hopkins School of Medicine in Baltimore, we established that the evolutionarily conserved Nup170p is a specialized component of the nucleopore complex (NPC), with roles in kinetochore function and checkpoint regulation via its association with Mad1p.

Our studies demonstrate that the Nup170p complex associates with spindle checkpoint proteins Mad1p and Mad2p in S. cerevisiae. Similar observations have since been made by other investigators in other systems, including humans.

Our novel findings that S. cerevisiae Mad1p and Mad2p are localized to the NPC prompted us to investigate the localization of another spindle checkpoint protein, Bub3p. We designed a novel genetically engineered reporter strain and showed preferential enrichment of Bub3p at defective kinetochores.

Enrichment of a spindle checkpoint protein at kinetochores upon checkpoint activation had not previously been reported in S. cerevisiae.

We are currently investigating the molecular mechanisms for spindle checkpoint activation and how cells resume cell-cycle progression when the checkpoint activation response is fulfilled.

The second project is a pioneering effort aimed at defining and characterizing previously nonannotated sORFs (<100 a.a.) in S. cerevisiae. We undertook the first functional studies of sORFs in any system and showed that there may be at least 299 sORFs in S. cerevisiae.

In collaboration with Jef Boeke (Johns Hopkins), Ronald Davis (Stanford University, Stanford, Calif.), and Michael Snyder (Yale University, New Haven, Conn.), we made gene deletion strains for 148 sORFs. About 75 percent of the sORFs are evolutionarily conserved, and several of the sORFs are required for genome stability.

As the databases expand, we propose to establish the presence of additional sORFs and investigate their molecular role in both protein coding and noncoding functions.

Dennis Drayna

Dennis Drayna received his Ph.D. from Harvard University in Cambridge, Mass., in 1981. After 15 years in the California biotechnology industry, he came to NIH in 1996 under the Visiting Investigator Program at NHGRI. In 1997, he joined NIDCD, where he is currently the acting chief of the Section on Systems Biology of Communication Disorders in the Laboratory of Molecular Genetics.

Over the past 20 years, the goal of my research has been to understand how genetic variation in humans contributes to disease.

Although the disorders we study are quite diverse, our projects are unified by a common set of technologies and by the statistical analysis methods and intellectual framework of human genetics.

We use traditional genetic-linkage methods in families as well as population-based association studies, with the goal of identifying specific genetic variants in individual genes that underlie complex disorders, that is, disorders with both genetic and nongenetic causes.

Stuttering, a common speech disorder that can have profound quality-of-life and economic consequences, is a major focus of our lab. Many studies have supported the view that this disorder can have genetic underpinnings, and about half of all affected individuals have a family history of stuttering.

However, the disorder does not display a clear pattern of inheritance, and it has many characteristics, such as a high rate of spontaneous recovery in children, that make genetic analysis difficult.

To overcome these difficulties, we have worked in two specialized populations. The first of these is centered in Pakistan, where traditional marriage patterns, involving unions between cousins, prevail.

Such inbreeding can increase the frequency of some types of genetic disorders, and we have identified many highly inbred families with a high density of individuals who stutter.

Our studies have shown that a gene on chromosome 12 appears to be responsible in a significant fraction of these families, and we are currently focused on identifying that gene.

We have also discovered several families in the Republic of Cameroon, in equatorial West Africa, in which stuttering is transmitted as an apparently simple autosomal dominant trait. Several of these families are quite large, containing over 100 individuals, roughly half of whom stutter as adults.

Our preliminary results suggest a gene on chromosome 1 is responsible in at least one of these families, and we are currently narrowing down the location on the chromosome in which this gene resides.

My lab is also studying deficits in the sense of taste. We previously focused on the inability to taste the substance phenylthiocarbamide (PTC), which is intensely bitter to three-quarters of the world’s population (including myself), but essentially tasteless to the remainder of the population.

This bitter-taste deficit has served for many decades as a classroom example of a human Mendelian trait. We discovered that this deficit is caused by alterations in the bitter-taste receptor gene T2R38.

Remarkably, the non-taster allele represents almost half of all the copies of the gene in humans worldwide. We showed that this is due to balancing natural selection, which maintains both the taster and non-taster alleles at high frequency.

Because our sense of bitter taste serves to protect us from toxic substances produced in plants (which are typically bitter), the selective force that maintains the non-taster allele is something of a puzzle.

We’ve hypothesized that the non-taster form of this taste receptor serves as a perfectly functional receptor for some other toxic bitter substance not yet identified.

We have also performed population genetic studies of variation in all of the human bitter- and sweet-taste receptor genes and shown that these genes are unusually polymorphic in their coding sequences.

We are currently exploring how this genetic variation affects taste perception of a wide variety of substances.

Mario Roederer

Mario Roederer received his Ph.D. in biology in 1988 in the laboratory of Robert Murphy at Carnegie Mellon in Pittsburgh. He did his postdoctoral training with Leonard Herzenberg at Stanford University, Stanford, Calif., and became a tenure-track investigator at the Vaccine Research Center, NIAID, in 2000. He is currently a senior investigator, leading the ImmunoTechnology Section in the Laboratory of Immunology, VRC.

A major goal of our laboratory is to identify the types of immune responses that may predict vaccine-generated protection or elucidate the pathogenesis of infections.

To this end, we are working on technology development (multicolor flow cytometry), human immunology (HIV disease and vaccines), and nonhuman primate models of HIV.

The original impetus for much of this work came from a study involving HIV-infected individuals in whom we characterized a selective loss of naïve CD4 and CD8 T cells during chronic disease. This fundamental change in the immune system had been largely unrecognized because of the inadequacy of extant technology to discriminate T-cell subsets.

The recognition that T cells were far more complex than envisioned led us to develop the technology—the hardware, software, and chemistry—we needed to identify and characterize these cells.

The current incarnation of that technology includes our 18-color flow cytometers (sorters and analyzers), which accord us the ability to discriminate as many as 18 different cell-associated markers.

Not only can we identify fine T-cell subsets (for example, by differentiation or activation) but we can also simultaneously interrogate different functions, such as cytokine and chemokine profile, on a cell-by-cell basis.

Perhaps not surprising, we found that immune responses to antigenic challenge are highly complex, with a dozen or more functionally defined subsets of CD4 or CD8 T cells, each associated, for example, with a unique combination of cytokines.

In collaboration with other sections of the Laboratory of Immunology, we are identifying selective subsets of these functional states that are associated with good clinical prospects—indeed, the "best" T cells appear to be those that simultaneously make many functions at once. We are now focusing on understanding what makes these cells different from other antigen-specific T cells.

Finally, working with the nonhuman primate model for HIV, we recently demonstrated that the acute phase of SIV infection is accompanied by an enormous destruction of the memory CD4 compartment, a destruction that predicts subsequent progression during chronic disease.

Furthermore, we showed that vaccination against SIV could ameliorate this destruction, resulting in significantly increased life expectancy for the animals.

We are now seeking to identify which vaccine-induced T-cell responses accounted for the protection during acute infection and to identify the mechanism of this protection.

Our continuing efforts include developing new models for understanding the generation and efficacy of vaccine-induced immune responses and expanding our arsenal of research tools.

Yun Wang

Yun Wang received an M.D. degree in 1979 from the National Defense Medical Center in Taiwan and a Ph.D. in pharmacology from the University of Colorado in 1986. He came to the Molecular Neuropsychiatry Branch, NIDA, in 1997 and is now chief of the Neural Protection and Regeneration Section.

Our laboratory uses animal models of neurodegenerative disorders to identify and study genes and compounds with neural protection and repair capabilities.

In particular, we are focusing on three models:

Acute and chronic methamphetamine (Meth) exposure (drug toxicity)

6-Hydroxydopamine–lesioning of substantia nigra (Parkinson’s disease)

Middle cerebral artery (MCA) occlusion (stroke).

We also use primary neuronal cultures derived from mouse and rat embryonic tissue for in vitro work.

We target three common pathways of degeneration and death: free radical toxicity, excitotoxicity, and apoptosis.

In the past few years, we have demonstrated the protective and regenerative properties of TGF-b family members, in particular, glial cell line–derived neurotrophic factor (GDNF).

For example, we reported that GDNF receptor–a 1 (GFRa-1) mRNA is upregulated in brain after cerebral ischemia and that intracerebral administration of GDNF potently protects against cerebral infarction induced by MCA occlusion in rodents.

In addition, we found that GDNF counters the increase in nitric oxide that accompanies MCA occlusion and subsequent reperfusion. This neuroprotective effect is greatly suppressed in GFRa-1+/- animals.

These data provided the first evidence that GDNF has a neuroprotective effect in the context of brain ischemia.

We also found that intranigral transplantation of fetal ventral mesencephalic tissue and nigrostriatal bridge administration of GDNF restore striatal dopamine input in hemiparkinsonian rats.

These results indicate that combinations of trophic factors and fetal nigral bridge transplants can restore the nigrostriatal dopamine pathway in Parkinsonian rats and thus may have clinical implications for fetal transplant surgery in human Parkinson’s disease.

Finally, our laboratory found that Meth potentiates ischemic brain injury in mice. We found that pretreatment with Meth increases ischemia-induced cerebral infarction, potentiates the expression of p53 mRNA in the ischemic mouse brain, and decreases GDNF levels in ischemic striatum; conversely, intracerebral administration of GDNF before ischemia protects against Meth toxicity.

In sum, our data indicate that Meth can exacerbate ischemic insults in brain through the inhibition of GDNF-mediated pathways.

We are currently studying the protective and regenerative mechanisms of bone morphogenetic proteins, purinergic compounds, and antioxidants after methamphetamine intoxication and in parkinsonian animals.

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