I am interested in using genetic diversity to understand human demographic and evolutionary processes in human populations. In the past, I have worked in immunology, virology, and my PhD dissertation was in using experimentally evolved Drosophila melanogaster to understand genetic basis of complex traits.
Honors & Awards
Predoctoral Fellowship (T32), NIH (2012-2014)
Undergraduate Recruitment Travel Award (55th Annual Drosophila Research Conference), Biological Sciences Division, The University of Chicago (2014)
Undergraduate Recruitment Travel Award (ASHG), Biological Sciences Division, The University of Chicago, Chicago, IL (2014)
Future of Science Fund, Keystone Symposia Travel Award (2013)
PIRE Fellowship, Open Science Data Cloud, The University of Chicago, Chicago, IL (2011-2012)
Travel Award, 17th International HIV Dynamics and Evolution Conference, Monterey, CA (2010)
Symposium Fellow, Evolution: the Molecular Landscape, Cold Spring Harbor Laboratory’s 74th Symposium (2009)
R. Sternheim, Rodkey, and G. Douglass Scholarships,, University of California Berkeley (2004-2006)
Allan R. Brown Outstanding Student Leadership Award, College of San Mateo, San Mateo, CA (2004)
Student Services and Associated Students Scholarships, College of San Mateo, San Mateo, CA (2003-2004)
Doctor of Philosophy, University of Chicago (2015)
Bachelor of Arts, University of California Berkeley (2007)
Associate of Arts, College Of San Mateo (2004)
Carlos Bustamante, Postdoctoral Research Mentor
Gut microbiome transition across a lifestyle gradient in Himalaya.
2018; 16 (11): e2005396
The composition of the gut microbiome in industrialized populations differs from those living traditional lifestyles. However, it has been difficult to separate the contributions of human genetic and geographic factors from lifestyle. Whether shifts away from the foraging lifestyle that characterize much of humanity's past influence the gut microbiome, and to what degree, remains unclear. Here, we characterize the stool bacterial composition of four Himalayan populations to investigate how the gut community changes in response to shifts in traditional human lifestyles. These groups led seminomadic hunting-gathering lifestyles until transitioning to varying levels of agricultural dependence upon farming. The Tharu began farming 250-300 years ago, the Raute and Raji transitioned 30-40 years ago, and the Chepang retain many aspects of a foraging lifestyle. We assess the contributions of dietary and environmental factors on their gut-associated microbes and find that differences in the lifestyles of Himalayan foragers and farmers are strongly correlated with microbial community variation. Furthermore, the gut microbiomes of all four traditional Himalayan populations are distinct from that of the Americans, indicating that industrialization may further exacerbate differences in the gut community. The Chepang foragers harbor an elevated abundance of taxa associated with foragers around the world. Conversely, the gut microbiomes of the populations that have transitioned to farming are more similar to those of Americans, with agricultural dependence and several associated lifestyle and environmental factors correlating with the extent of microbiome divergence from the foraging population. The gut microbiomes of Raute and Raji reveal an intermediate state between the Chepang and Tharu, indicating that divergence from a stereotypical foraging microbiome can occur within a single generation. Our results also show that environmental factors such as drinking water source and solid cooking fuel are significantly associated with the gut microbiome. Despite the pronounced differences in gut bacterial composition across populations, we found little differences in alpha diversity across lifestyles. These findings in genetically similar populations living in the same geographical region establish the key role of lifestyle in determining human gut microbiome composition and point to the next challenging steps of determining how large-scale gut microbiome reconfiguration impacts human biology.
View details for PubMedID 30439937
Shared Genetic Signals of Hypoxia Adaptation in Drosophila and in High-Altitude Human Populations
MOLECULAR BIOLOGY AND EVOLUTION
2016; 33 (2): 501-517
The ability to withstand low oxygen (hypoxia tolerance) is a polygenic and mechanistically conserved trait that has important implications for both human health and evolution. However, little is known about the diversity of genetic mechanisms involved in hypoxia adaptation in evolving populations. We used experimental evolution and whole-genome sequencing in Drosophila melanogaster to investigate the role of natural variation in adaptation to hypoxia. Using a generalized linear mixed model we identified significant allele frequency differences between three independently evolved hypoxia-tolerant populations and normoxic control populations for approximately 3,800 single nucleotide polymorphisms. Around 50% of these variants are clustered in 66 distinct genomic regions. These regions contain genes that are differentially expressed between hypoxia-tolerant and normoxic populations and several of the differentially expressed genes are associated with metabolic processes. Additional genes associated with respiratory and open tracheal system development also show evidence of directional selection. RNAi-mediated knockdown of several candidate genes' expression significantly enhanced survival in severe hypoxia. Using genomewide single nucleotide polymorphism data from four high-altitude human populations-Sherpas, Tibetans, Ethiopians, and Andeans, we found that several human orthologs of the genes under selection in flies are also likely under positive selection in all four high-altitude human populations. Thus, our results indicate that selection for hypoxia tolerance can act on standing genetic variation in similar genes and pathways present in organisms diverged by hundreds of millions of years.
View details for DOI 10.1093/molbev/msv248
View details for Web of Science ID 000369993600018
View details for PubMedID 26576852
Whole-Genome Resequencing of Experimental Populations Reveals Polygenic Basis of Egg-Size Variation in Drosophila melanogaster.
Molecular biology and evolution
2015; 32 (10): 2616-2632
Complete genome resequencing of populations holds great promise in deconstructing complex polygenic traits to elucidate molecular and developmental mechanisms of adaptation. Egg size is a classic adaptive trait in insects, birds, and other taxa, but its highly polygenic architecture has prevented high-resolution genetic analysis. We used replicated experimental evolution in Drosophila melanogaster and whole-genome sequencing to identify consistent signatures of polygenic egg-size adaptation. A generalized linear-mixed model revealed reproducible allele frequency differences between replicated experimental populations selected for large and small egg volumes at approximately 4,000 single nucleotide polymorphisms (SNPs). Several hundred distinct genomic regions contain clusters of these SNPs and have lower heterozygosity than the genomic background, consistent with selection acting on polymorphisms in these regions. These SNPs are also enriched among genes expressed in Drosophila ovaries and many of these genes have well-defined functions in Drosophila oogenesis. Additional genes regulating egg development, growth, and cell size show evidence of directional selection as genes regulating these biological processes are enriched for highly differentiated SNPs. Genetic crosses performed with a subset of candidate genes demonstrated that these genes influence egg size, at least in the large genetic background. These findings confirm the highly polygenic architecture of this adaptive trait, and suggest the involvement of many novel candidate genes in regulating egg size.
View details for DOI 10.1093/molbev/msv136
View details for PubMedID 26044351
Global diversity, population stratification, and selection of human copy-number variation
2015; 349 (6253): 1181-?
In order to explore the diversity and selective signatures of duplication and deletion human copy-number variants (CNVs), we sequenced 236 individuals from 125 distinct human populations. We observed that duplications exhibit fundamentally different population genetic and selective signatures than deletions and are more likely to be stratified between human populations. Through reconstruction of the ancestral human genome, we identify megabases of DNA lost in different human lineages and pinpoint large duplications that introgressed from the extinct Denisova lineage now found at high frequency exclusively in Oceanic populations. We find that the proportion of CNV base pairs to single-nucleotide-variant base pairs is greater among non-Africans than it is among African populations, but we conclude that this difference is likely due to unique aspects of non-African population history as opposed to differences in CNV load.
View details for DOI 10.1126/science.aab3761
View details for Web of Science ID 000360968400033
View details for PubMedID 26249230
miR-9a Minimizes the Phenotypic Impact of Genomic Diversity by Buffering a Transcription Factor
2013; 155 (7): 1556-1567
Gene expression has to withstand stochastic, environmental, and genomic perturbations. For example, in the latter case, 0.5%-1% of the human genome is typically variable between any two unrelated individuals. Such diversity might create problematic variability in the activity of gene regulatory networks and, ultimately, in cell behaviors. Using multigenerational selection experiments, we find that for the Drosophila proneural network, the effect of genomic diversity is dampened by miR-9a-mediated regulation of senseless expression. Reducing miR-9a regulation of the Senseless transcription factor frees the genomic landscape to exert greater phenotypic influence. Whole-genome sequencing identified genomic loci that potentially exert such effects. A larger set of sequence variants, including variants within proneural network genes, exhibits these characteristics when miR-9a concentration is reduced. These findings reveal that microRNA-target interactions may be a key mechanism by which the impact of genomic diversity on cell behavior is dampened.
View details for DOI 10.1016/j.cell.2013.10.057
View details for Web of Science ID 000328693300012
View details for PubMedID 24360277
Associations between Antibodies to a Panel of Plasmodium falciparum Specific Antigens and Response to Sub-Optimal Antimalarial Therapy in Kampala, Uganda
2012; 7 (12)
Antibodies are important in the control of blood stage Plasmodium falciparum infection. It is unclear which antibody responses are responsible for, or even associated with protection, partly due to confounding by heterogeneous exposure. Assessment of response to partially effective antimalarial therapy, which requires the host to assist in clearing parasites, offers an opportunity to measure protection independent of exposure.A cohort of children aged 1-10 years in Kampala, Uganda were treated with amodiaquine+sulfadoxine-pyrimethamine for uncomplicated malaria. Serum samples from the time of malaria diagnosis and 14 days later were analyzed for total IgG to 8 P. falciparum antigens using a quantitative indirect ELISA. Associations between antibody levels and risk of treatment failure were estimated using Cox proportional hazard regression.Higher levels of antibodies to apical membrane antigen 1 (AMA-1), but to none of the other 7 antigens were significantly associated with protection against treatment failure (HR 0.57 per 10-fold increase in antibody level, CI 0.41-0.79, p = 0.001). Protection increased consistently across the entire range of antibody levels.Measurement of antibody levels to AMA-1 at the time of malaria may offer a quantitative biomarker of blood stage immunity to P. falciparum, a tool which is currently lacking.
View details for DOI 10.1371/journal.pone.0052571
View details for Web of Science ID 000312694300092
View details for PubMedID 23285095
Tim-3 marks human natural killer cell maturation and suppresses cell-mediated cytotoxicity
14th International Congress of Immunology
AMER SOC HEMATOLOGY. 2012: 3734–43
Natural killer (NK) cells are innate lymphocytes that play an important role against viral infections and cancer. This effect is achieved through a complex mosaic of inhibitory and activating receptors expressed by NK cells that ultimately determine the magnitude of the NK-cell response. The T-cell immunoglobulin- and mucin domain-containing (Tim)-3 receptor was initially identified as a T-helper 1-specific type I membrane protein involved in regulating T-cell responses. Human NK cells transcribe the highest amounts of Tim-3 among lymphocytes. Tim-3 protein is expressed on essentially all mature CD56(dim)CD16(+) NK cells and is expressed heterogeneously in the immature CD56(bright)CD16(-) NK-cell subset in blood from healthy adults and in cord blood. Tim-3 expression was induced on CD56(bright)CD16(-) NK cells after stimulation with IL-15 or IL-12 and IL-18 in vitro, suggesting that Tim-3 is a maturation marker on NK cells. Whereas Tim-3 has been used to identify dysfunctional T cells, NK cells expressing high amounts of Tim-3 are fully responsive with respect to cytokine production and cytotoxicity. However, when Tim-3 was cross-linked with antibodies it suppressed NK cell-mediated cytotoxicity. These findings suggest that NK-cell responses may be negatively regulated when NK cells encounter target cells expressing cognate ligands of Tim-3.
View details for DOI 10.1182/blood-2011-11-392951
View details for Web of Science ID 000305282100017
View details for PubMedID 22383801
An Aboriginal Australian Genome Reveals Separate Human Dispersals into Asia
2011; 334 (6052): 94-98
We present an Aboriginal Australian genomic sequence obtained from a 100-year-old lock of hair donated by an Aboriginal man from southern Western Australia in the early 20th century. We detect no evidence of European admixture and estimate contamination levels to be below 0.5%. We show that Aboriginal Australians are descendants of an early human dispersal into eastern Asia, possibly 62,000 to 75,000 years ago. This dispersal is separate from the one that gave rise to modern Asians 25,000 to 38,000 years ago. We also find evidence of gene flow between populations of the two dispersal waves prior to the divergence of Native Americans from modern Asian ancestors. Our findings support the hypothesis that present-day Aboriginal Australians descend from the earliest humans to occupy Australia, likely representing one of the oldest continuous populations outside Africa.
View details for DOI 10.1126/science.1211177
View details for Web of Science ID 000295580300047
View details for PubMedID 21940856
Human Endogenous Retrovirus K106 (HERV-K106) Was Infectious after the Emergence of Anatomically Modern Humans
2011; 6 (5)
HERV-K113 and HERV-K115 have been considered to be among the youngest HERVs because they are the only known full-length proviruses that are insertionally polymorphic and maintain the open reading frames of their coding genes. However, recent data suggest that HERV-K113 is at least 800,000 years old, and HERV-K115 even older. A systematic study of HERV-K HML2 members to identify HERVs that may have infected the human genome in the more recent evolutionary past is lacking. Therefore, we sought to determine how recently HERVs were exogenous and infectious by examining sequence variation in the long terminal repeat (LTR) regions of all full-length HERV-K loci. We used the traditional method of inter-LTR comparison to analyze all full length HERV-Ks and determined that two insertions, HERV-K106 and HERV-K116 have no differences between their 5' and 3' LTR sequences, suggesting that these insertions were endogenized in the recent evolutionary past. Among these insertions with no sequence differences between their LTR regions, HERV-K106 had the most intact viral sequence structure. Coalescent analysis of HERV-K106 3' LTR sequences representing 51 ethnically diverse individuals suggests that HERV-K106 integrated into the human germ line approximately 150,000 years ago, after the emergence of anatomically modern humans.
View details for DOI 10.1371/journal.pone.0020234
View details for Web of Science ID 000291006500044
View details for PubMedID 21633511
HTLV-1 Tax Specific CD8+ T Cells Express Low Levels of Tim-3 in HTLV-1 Infection: Implications for Progression to Neurological Complications
PLOS NEGLECTED TROPICAL DISEASES
2011; 5 (4)
The T cell immunoglobulin mucin 3 (Tim-3) receptor is highly expressed on HIV-1-specific T cells, rendering them partially "exhausted" and unable to contribute to the effective immune mediated control of viral replication. To elucidate novel mechanisms contributing to the HTLV-1 neurological complex and its classic neurological presentation called HAM/TSP (HTLV-1 associated myelopathy/tropical spastic paraparesis), we investigated the expression of the Tim-3 receptor on CD8(+) T cells from a cohort of HTLV-1 seropositive asymptomatic and symptomatic patients. Patients diagnosed with HAM/TSP down-regulated Tim-3 expression on both CD8(+) and CD4(+) T cells compared to asymptomatic patients and HTLV-1 seronegative controls. HTLV-1 Tax-specific, HLA-A*02 restricted CD8(+) T cells among HAM/TSP individuals expressed markedly lower levels of Tim-3. We observed Tax expressing cells in both Tim-3(+) and Tim-3(-) fractions. Taken together, these data indicate that there is a systematic downregulation of Tim-3 levels on T cells in HTLV-1 infection, sustaining a profoundly highly active population of potentially pathogenic T cells that may allow for the development of HTLV-1 complications.
View details for DOI 10.1371/journal.pntd.0001030
View details for Web of Science ID 000289937400005
View details for PubMedID 21541358
IL-2 Immunotherapy to Recently HIV-1 Infected Adults Maintains the Numbers of IL-17 Expressing CD4+ T (T(H)17) Cells in the Periphery
JOURNAL OF CLINICAL IMMUNOLOGY
2010; 30 (5): 681-692
Little is known about the manipulation of IL-17 producing CD4+ T cells (T(H)17) on a per-cell basis in humans in vivo. Previous studies on the effects of IL-2 on IL-17 secretion in non-HIV models have shown divergent results. We hypothesized that IL-2 would mediate changes in IL-17 levels among recently HIV-1-infected adults receiving anti-retroviral therapy. We measured cytokine T cell responses to CD3/CD28, HIV-1 Gag, and CMV pp65 stimulation, and changes in multiple CD4+ T cell subsets. Those who received IL-2 showed a robust expansion of naive and total CD4+ T cell counts and T-reg counts. However, after IL-2 treatment, the frequency of T(H)17 cells declined, while counts of T(H)17 cells did not change due to an expansion of the CD4+ naïve T cell population (CD27+CD45RA+). Counts of HIV-1 Gag-specific T cells declined modestly, but CMV pp65 and CD3/CD28 stimulated populations did not change. Hence, in contrast with recent studies, our results suggest IL-2 is not a potent in vivo regulator of T(H)17 cell populations in HIV-1 disease. However, IL-2-mediated T-reg expansions may selectively reduce responses to certain antigen-specific populations, such as HIV-1 Gag.
View details for DOI 10.1007/s10875-010-9432-3
View details for Web of Science ID 000281693700010
View details for PubMedID 20571894
A novel human CD4(+) T-cell inducer subset with potent immunostimulatory properties
EUROPEAN JOURNAL OF IMMUNOLOGY
2010; 40 (1): 134-141
The complexity of immunoregulation has focused attention on the CD4+ T "suppressor" regulatory cell (Treg), which helps maintain balance between immunity and tolerance. An immunoregulatory T-cell population that upon activation amplifies cellular immune responses was described in murine models more than 30 years ago; however, no study has yet identified a naturally occurring T "inducer" cell type. Here, we report that the ectoenzyme CD39/NTPDase1 (ecto-nucleoside triphosphate diphosphohydrolase 1) helps to delineate a novel population of human "inducer" CD4+ T cells (Tind) that significantly increases the proliferation and cytokine production of responder T cells in a dose-dependent manner. Furthermore, this unique Tind subset produces a distinct repertoire of cytokines in comparison to the other CD4+ T-cell subsets. We propose that this novel CD4+ T-cell population counterbalances the suppressive activity of suppressor Treg in peripheral blood and serves as a calibrator of immunoregulation.
View details for DOI 10.1002/eji.200939258
View details for Web of Science ID 000274041800016
View details for PubMedID 19877008
Lower numbers of circulating natural killer T (NK T) cells in individuals with human T lymphotropic virus type 1 (HTLV-1) associated neurological disease
CLINICAL AND EXPERIMENTAL IMMUNOLOGY
2009; 158 (3): 294-299
Human T lymphotropic virus type 1 (HTLV-1) infects 10-20 million people worldwide. The majority of infected individuals are asymptomatic; however, approximately 3% develop the debilitating neurological disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). There is also currently no cure, vaccine or effective therapy for HTLV-1 infection, and the mechanisms for progression to HAM/TSP remain unclear. NK T cells are an immunoregulatory T cell subset whose frequencies and effector functions are associated critically with immunity against infectious diseases. We hypothesized that NK T cells are associated with HAM/TSP progression. We measured NK T cell frequencies and absolute numbers in individuals with HAM/TSP infection from two cohorts on two continents: São Paulo, Brazil and San Francisco, CA, USA, and found significantly lower levels when compared with healthy subjects and/or asymptomatic carriers. Also, the circulating NK T cell compartment in HAM/TSP subjects is comprised of significantly more CD4(+) and fewer CD8(+) cells than healthy controls. These findings suggest that lower numbers of circulating NK T cells and enrichment of the CD4(+) NK T subset are associated with HTLV-1 disease progression.
View details for DOI 10.1111/j.1365-2249.2009.04019.x
View details for Web of Science ID 000271311200006
View details for PubMedID 19778295
Cross-Sectional Dating of Novel Haplotypes of HERV-K 113 and HERV-K 115 Indicate These Proviruses Originated in Africa before Homo sapiens
MOLECULAR BIOLOGY AND EVOLUTION
2009; 26 (11): 2617-2626
The human genome, human endogenous retroviruses (HERV), of which HERV-K113 and HERV-K115 are the only known full-length proviruses that are insertionally polymorphic. Although a handful of previously published papers have documented their prevalence in the global population; to date, there has been no report on their prevalence in the United States population. Here, we studied the geographic distribution of K113 and K115 among 156 HIV-1+ subjects from the United States, including African Americans, Hispanics, and Caucasians. In the individuals studied, we found higher insertion frequencies of K113 (21%) and K115 (35%) in African Americans compared with Caucasians (K113 9% and K115 6%) within the United States. We also report the presence of three single nucleotide polymorphism sites in the K113 5' long terminal repeats (LTRs) and four in the K115 5' LTR that together constituted four haplotypes for K113 and five haplotypes for K115. HERV insertion times can be estimated from the sequence differences between the 5' and 3' LTR of each insertion, but this dating method cannot be used with HERV-K115. We developed a method to estimate insertion times by applying coalescent inference to 5' LTR sequences within our study population and validated this approach using an independent estimate derived from the genetic distance between K113 5' and 3' LTR sequences. Using our method, we estimated the insertion dates of K113 and K115 to be a minimum of 800,000 and 1.1 million years ago, respectively. Both these insertion dates predate the emergence of anatomically modern Homo sapiens.
View details for DOI 10.1093/molbev/msp180
View details for Web of Science ID 000271422300020
View details for PubMedID 19666991
Interleukin-10-secreting T cells define a suppressive subset within the HIV-1-specific T-cell population
EUROPEAN JOURNAL OF IMMUNOLOGY
2009; 39 (5): 1280-1287
Recent studies have indicated that Treg contribute to the HIV type 1 (HIV-1)-related immune pathogenesis. However, it is not clear whether T cells with suppressive properties reside within the HIV-1-specific T-cell population. Here, PBMC from HIV-1-infected individuals were stimulated with a 15-mer Gag peptide pool, and HIV-1-specific T cells were enriched by virtue of their secretion of IL-10 or IFN-gamma using immunomagnetic cell-sorting. Neither the IL-10-secreting cells nor the IFN-gamma-secreting cells expressed the Treg marker FOXP3, yet the IL-10-secreting cells potently suppressed anti-CD3/CD28-induced CD4(+) as well as CD8(+) T-cell proliferative responses. As shown by intracellular cytokine staining, IL-10- and IFN-gamma-producing T cells represent distinct subsets of the HIV-1-specific T cells. Our data collectively suggest that functionally defined HIV-1-specific T-cell subsets harbor potent immunoregulatory properties that may contribute to HIV-1-associated T-cell dysfunction.
View details for DOI 10.1002/eji.200839002
View details for Web of Science ID 000266137700010
View details for PubMedID 19384871
Tim-3 expression defines a novel population of dysfunctional T cells with highly elevated frequencies in progressive HIV-1 infection
JOURNAL OF EXPERIMENTAL MEDICINE
2008; 205 (12): 2763-2779
Progressive loss of T cell functionality is a hallmark of chronic infection with human immunodeficiency virus 1 (HIV-1). We have identified a novel population of dysfunctional T cells marked by surface expression of the glycoprotein Tim-3. The frequency of this population was increased in HIV-1-infected individuals to a mean of 49.4 +/- SD 12.9% of CD8(+) T cells expressing Tim-3 in HIV-1-infected chronic progressors versus 28.5 +/- 6.8% in HIV-1-uninfected individuals. Levels of Tim-3 expression on T cells from HIV-1-infected inviduals correlated positively with HIV-1 viral load and CD38 expression and inversely with CD4(+) T cell count. In progressive HIV-1 infection, Tim-3 expression was up-regulated on HIV-1-specific CD8(+) T cells. Tim-3-expressing T cells failed to produce cytokine or proliferate in response to antigen and exhibited impaired Stat5, Erk1/2, and p38 signaling. Blocking the Tim-3 signaling pathway restored proliferation and enhanced cytokine production in HIV-1-specific T cells. Thus, Tim-3 represents a novel target for the therapeutic reversal of HIV-1-associated T cell dysfunction.
View details for DOI 10.1084/jem.20081398
View details for Web of Science ID 000261295300011
View details for PubMedID 19001139
Suppression of HIV-1 plasma viral load below detection preserves IL-17 producing T cells in HIV-1 infection
2008; 22 (8): 990-992
IL-17 is proinflammatory cytokine secreted by a unique CD4+ T (Th17) cell subset and proposed to play a role in host defense. We hypothesized that Th17 cells are lost in HIV-1 infection. HIV-1-infected children with plasma viremia below 50 copies/ml had IL-17 production, whereas those with detectable viremia had minimal secretion. These results imply viral-mediated destruction or impairment of Th17 cells and argue for complete suppression of viremia for reconstitution of Th17 cells.
View details for Web of Science ID 000256358300011
View details for PubMedID 18453860