Gene Therapy Versus Common Care for Eligible Individuals With Sickle Cell Disease in the United States : A Cost-Effectiveness Analysis

BACKGROUND

Sickle cell disease (SCD) and its complications contribute to high rates of morbidity and early mortality and high cost in the United States and African heritage community.

OBJECTIVE

To evaluate the cost-effectiveness of gene therapy for SCD and its value-based prices (VBPs).

DESIGN

Comparative modeling analysis across 2 independently developed simulation models (University of Washington Model for Economic Analysis of Sickle Cell Cure [UW-MEASURE] and Fred Hutchinson Institute Sickle Cell Disease Outcomes Research and Economics Model [FH-HISCORE]) using the same databases.

DATA SOURCES

Centers for Medicare & Medicaid Services claims data, 2008 to 2016; published literature.

TARGET POPULATION

Persons eligible for gene therapy.

TIME HORIZON

Lifetime.

PERSPECTIVE

U.S. health care sector and societal.

INTERVENTION

Gene therapy versus common care.

OUTCOME MEASURES

Incremental cost-effectiveness ratios (ICERs), equity-informed VBPs, and price acceptability curves.

RESULTS OF BASE-CASE ANALYSIS

At an assumed $2 million price for gene therapy, UW-MEASURE and FH-HISCORE estimated ICERs of $193 000 per QALY and $427 000 per QALY, respectively, under the health care sector perspective. Corresponding estimates from the societal perspective were $126 000 per QALY and $281 000 per QALY. The difference in results between models stemmed primarily from considering a slightly different target population and incorporating the quality-of-life (QOL) effects of splenic sequestration, priapism, and acute chest syndrome in the UW model. From a societal perspective, acceptable (>90% confidence) VBPs ranged from $1 million to $2.5 million depending on the use of alternative effective metrics or equity-informed threshold values.

RESULTS OF SENSITIVITY ANALYSIS

Results were sensitive to the costs of myeloablative conditioning before gene therapy, effect on caregiver QOL, and effect of gene therapy on long-term survival.

LIMITATION

The short-term effects of gene therapy on vaso-occlusive events were extrapolated from 1 study.

CONCLUSION

Gene therapy for SCD below a $2 million price tag is likely to be cost-effective when applying a societal perspective at an equity-informed threshold for cost-effectiveness analysis.

PRIMARY FUNDING SOURCE

National Heart, Lung, and Blood Institute.

Long-term survival with sickle cell disease: a nationwide cohort study of Medicare and Medicaid beneficiaries

To our knowledge, we report the first population-based period life table, the expected lifetime survival for Medicare and Medicaid beneficiaries with sickle cell disease (SCD), and the disparities in survival by insurance types in the United States. We constructed a retrospective cohort of individuals with diagnosed SCD receiving common care (any real-world patterns of care except transplant) based on nationwide Medicare and Medicaid claim data (2008-2016), covering beneficiaries in all 50 states. We analyzed lifetime survival probabilities using Kaplan-Meier curves and projected life expectancies at various ages for all, stratified by sex and insurance types. Our analysis included 94 616 individuals with SCD that have not undergone any transplant. Life expectancy at birth was 52.6 years (95% confidence interval: 51.9-53.4). Compared with the adults covered by Medicaid only, those covered by Medicare for disabilities or end-stage renal disease and those dually insured by Medicare and Medicaid had significantly worse life expectancy. Similarly, for beneficiaries aged ≥65 years, these 2 insurance types were associated with significantly shorter life expectancy than those enrolled in Medicare old age and survivor's insurance. Our study underscores the persistent life expectancy shortfall for patients with SCD, the burden of premature mortality during adulthood, and survival disparities by insurance status.

Lifetime medical costs attributable to sickle cell disease among nonelderly individuals with commercial insurance

Sickle cell disease (SCD) is a severe monogenic disease associated with high morbidity, mortality, and a disproportionate burden on Black and Hispanic communities. Our objective was to estimate the total healthcare costs and out-of-pocket (OOP) costs attributable to SCD among commercially insured individuals over their nonelderly lifetimes (0 to 64 years of age). We constructed a retrospective cohort of individuals with diagnosed SCD using Truven Health Marketscan commercial claims data from 2007 through 2018, compared with matched control subjects from the Medical Expenditure Panel Survey. We estimated Kaplan-Meier sample average costs using previously reported survival curves for SCD and control subjects. Individuals with SCD (20 891) and control subjects (33 588) were included in our analysis. The SCD sample had a mean age of 25.7 (standard deviation, 17.4) years; 58.0% were female. Survival-adjusted costs of SCD peaked at age 13 to 24 years and declined at older ages. There was no significant difference in total medical costs or OOP costs between the sexes. SCD-attributable costs over 0 to 64 years of age were estimated to be $1.6 million (95% confidence interval [CI], $1.3M-$1.9M) and $1.7 million (95% CI, $1.4M-$2.1M) for females and males with SCD, respectively. The corresponding OOP estimates were $42 395 (95% CI, $34 756-$50 033) for females and $45 091 (95% CI, $36 491-$53 691) for males. These represent a 907% and 285% increase in total medical and OOP costs over control subjects, respectively. Although limited to the commercially insured population, these results indicate that the direct economic burden of SCD is substantial and peaks at younger ages, suggesting the need for curative and new medical therapies.

Prevalence of comorbidities associated with sickle cell disease among non-elderly individuals with commercial insurance-A retrospective cohort study

Sickle cell disease (SCD) is a severe monogenic disease associated with high morbidity and mortality and a disproportionate burden on Black communities. Few population-based studies have examined the prevalence of comorbidities among persons with SCD. We estimated the prevalence of comorbidities experienced by individuals with SCD enrolled in employer-based health insurance plans in the US over their non-elderly lifetimes (0-64 years of age) with a retrospective cohort design using Truven Health MarketScan commercial claims data from 2007-2018. ICD-9/10 codes were used to identify individuals with SCD using a previously published algorithm. For this cohort, comorbidities associated with SCD were identified across 3 age categories (<18, 18-45, 46-64 years-old), based on the CMS Chronic Comorbidities Warehouse or SCD-specific diagnosis codes, when applicable. The total number of SCD patients available for analysis in each age category was 7,502 (<18 years), 10,183 (18-45 years) and 4,459 (46-64 years). Across all ages, vaso-occlusive pain, infections (non-specific), and fever were the most common comorbidities. Vaso-occlusive pain and infection were the most prevalent conditions for persons age <18- and 18-45-year-olds, while in the 46-54-year-old age group, infection and cardiovascular including pulmonary hypertension were most prevalent. Compared to persons <18 years old, the prevalence of vaso-occlusive pain, fever, and acute chest syndrome claims declined in older populations. The comorbidity burden of SCD is significant across all age groups. SCD patients experience comorbidities of age such as chronic pain, cardio-vascular conditions including pulmonary hypertension and renal disease at far higher rates than the general population. Novel disease modifying therapies in development have the potential to significantly reduce the comorbidity burden of SCD.

Barriers to hydroxyurea use from the perspectives of providers, individuals with sickle cell disease, and families: Report from a U.S. regional collaborative

Sickle cell disease (SCD) is an inherited blood disorder that affects about 100,000 people in the U.S., primarily Blacks/African-Americans. A multitude of complications negatively impacts quality of life. Hydroxyurea has been FDA approved since 1998 as a disease-modifying therapy for SCD, but is underutilized. Negative and uninformed perceptions of hydroxyurea and barriers to its use hinder adherence and promotion of the medication. As the largest real-world study to date that assessed hydroxyurea use for children and adults with SCD, we gathered and analyzed perspectives of providers, individuals with SCD, and families. Participants provided information about socio-demographics, hospital and emergency admissions for pain, number of severe pain episodes interfering with daily activities, medication adherence, and barriers to hydroxyurea. Providers reported on indications for hydroxyurea, reasons not prescribed, and current laboratory values. We found that hydroxyurea use was reported in over half of eligible patients from this large geographic region in the U.S., representing a range of sickle cell specialty clinical settings and practices. Provider and patient/caregiver reports about hydroxyurea use were consistent with one another; adults 26 years and older were least likely to be on hydroxyurea; and the likelihood of being on hydroxyurea decreased with one or more barriers. Using the intentional and unintentional medication nonadherence framework, we found that, even for patients on hydroxyurea, challenges to taking the medicine at the right time and forgetting were crucial unintentional barriers to adherence. Intentional barriers such as worry about side effects and “tried and it did not work” were important barriers for young adults and adults. For providers, diagnoses other than HgbSS or HgbS-β0 thalassemia were associated with lower odds of prescribing, consistent with evidence-based guidelines. Our results support strengthening provider understanding and confidence in implementing existing SCD guidelines, and the importance of shared decision making. Our findings can assist providers in understanding choices and decisions of families; guide individualized clinical discussions regarding hydroxyurea therapy; and help with developing tailored interventions to address barriers. Addressing barriers to hydroxyurea use can inform strategies to minimize similar barriers in the use of emerging and combination therapies for SCD.

Medical and Non-medical Costs of Sickle Cell Disease and Treatments from a US Perspective: A Systematic Review and Landscape Analysis

BACKGROUND

Sickle cell disease (SCD) is a complex genetic disorder that manifests in infancy and progresses throughout life in the form of acute and chronic complications. As the upfront costs of potentially curative, genetic therapies will likely be high, an assessment and comprehensive characterization of the medical and non-medical cost burden will inform future decision making.

OBJECTIVE

We sought to systematically summarize the existing literature surrounding SCD medical and non-medical costs.

METHODS

We searched MEDLINE and EMBASE (2008-2020) and identified US-based studies that detailed medical or non-medical costs. Eligible studies provided empirical estimates about any aspect of cost or SCD individuals of all ages and their caregivers. Study quality was assessed using the Newcastle-Ottawa Scale, and costs were adjusted to 2019 US$.

RESULTS

Search queries returned 479 studies, with 342 from medical burden searches and 137 from non-medical burden searches, respectively. Herein, we report the results of the 40 studies that contained relevant cost information: 39 detailed medical costs and 1 detailed non-medical costs. Costs were higher for SCD patients when compared with non-SCD individuals (cost difference range: $6636-$63,436 annually). The highest medical cost component for SCD patients was inpatient ($11,978-$59,851 annually), followed by outpatient and then pharmacy. No studies characterized the cost burden throughout the lifetime disease trajectory of an SCD individual, and no studies captured caregiver or productivity costs.

CONCLUSION

Our results reveal an incomplete characterization of medical and non-medical costs within SCD. A deeper understanding of the medical and non-medical cost burden requires completion of additional studies that capture the burden across the patient's lifetime, in addition to expression of the impact of existing and emergent health technologies on disease trajectory.

Development of a conceptual model for evaluating new non-curative and curative therapies for sickle cell disease

BACKGROUND

Sickle cell disease (SCD) is a clinically heterogeneous disease with many acute and chronic complications driven by ongoing vaso-occlusion and hemolysis. It causes a disproportionate burden on Black and Hispanic communities. Our objective was to follow the SMDM/ISPOR Task Force recommendations for good practices and create a conceptual model of the progression of SCD under current clinical practice to inform cost-effectiveness analyses (CEA) of promising curative therapies in the pipeline over a lifetime horizon.

METHODS

We used consultations with experts, providers, and patients to identify acute events and chronic conditions in the conceptual model. We compared our model structure to previous CEA models of interventions for SCD, assessed the prevalence of the identified disease attributes in Medicaid and Medicare claims databases, and identified relevant outcomes following the 2nd Panel in CEA. We determined an appropriate modeling technique and relevant data sources for parameterizing the model.

RESULTS

The conceptual model structure included four dimensions of disease: chronic pain, acute events, chronic conditions, and treatment complications, spanning 26 disease attributes with significant impacts on health-related quality of life and resource. We modeled chronic pain separately to reflect its importance to patients and interaction with all other disease attributes. We identified additional data sources for health state utilities and non-medical costs and benefits of SCD. We will use a microsimulation model with age- and sex-specific transitions between health states predicted by patient demographic characteristics and disease history.

CONCLUSION

Developing the model structure through an explicit process of model conceptualization can increase the transparency and accuracy of results. We will populate the conceptual model with the data sources described and evaluate the cost-effectiveness of curative therapies.

Health State Utilities for Sickle Cell Disease: A Catalog Prepared From a Systematic Review

OBJECTIVES

Sickle cell disease (SCD) is a complex, chronic condition that impairs health-related quality of life of affected individuals and their caregivers. As curative therapies emerge, comprehensive cost-effectiveness models will inform their value. These models will require descriptions of health states and their corresponding utility values that accurately reflect health-related quality of life over the disease trajectory. The objectives of this systematic review were to develop a catalog of health state utility (HSU) values for SCD, identify research gaps, and provide future directions for preference elicitation.

METHODS

Records were identified through searches of PubMed and Embase, Tufts Medical Center Cost-Effectiveness Analysis Registry, reference lists of relevant articles, and consultation with SCD experts (2008-2020). We removed duplicate records and excluded ineligible studies. For included studies, we summarized the study characteristics, methods used for eliciting HSUs, and HSU values.

RESULTS

Five studies empirically elicited utilities using indirect methods (EQ-5D) (n = 3) and Short Form-6 Dimension (n = 2); these represent health states associated with general SCD (n = 1), SCD complications (n = 2), and SCD treatments (n = 3). Additionally, we extracted HSUs from 7 quality-adjusted life-years-based outcome research studies. The HSU among patients with general SCD without specifying complications ranged from 0.64 to 0.887. Only 36% of the HSUs used in the quality-adjusted life-year-based outcomes research studies were derived from individuals with SCD. No study estimated HSUs in caregivers.

CONCLUSIONS

There is a dearth of literature of HSUs for use in SCD models. Future empirical studies should elicit a comprehensive set of HSUs from individuals with SCD and their caregivers.

Enhancers predominantly regulate gene expression during differentiation via transcription initiation

Gene transcription occurs via a cycle of linked events, including initiation, promoter-proximal pausing, and elongation of RNA polymerase II (Pol II). A key question is how transcriptional enhancers influence these events to control gene expression. Here, we present an approach that evaluates the level and change in promoter-proximal transcription (initiation and pausing) in the context of differential gene expression, genome-wide. This combinatorial approach shows that in primary cells, control of gene expression during differentiation is achieved predominantly via changes in transcription initiation rather than via release of Pol II pausing. Using genetically engineered mouse models, deleted for functionally validated enhancers of the α- and β-globin loci, we confirm that these elements regulate Pol II recruitment and/or initiation to modulate gene expression. Together, our data show that gene expression during differentiation is regulated predominantly at the level of initiation and that enhancers are key effectors of this process.

Newborn Screening Practices and Alpha-Thalassemia Detection - United States, 2016

Alpha-thalassemia comprises a group of inherited disorders in which alpha-hemoglobin chain production is reduced. Depending on the genotype, alpha-thalassemia results in moderate to profound anemia, hemolysis, growth delays, splenomegaly, and increased risk for thromboembolic events; certain patients might require chronic transfusions. Although alpha-thalassemia is not a core condition of the United States Recommended Uniform Screening Panel* for state newborn screening programs, methodologies used by some newborn screening programs to detect sickle cell disease, which is a core panel condition, also detect a quantitative marker of alpha-thalassemia, hemoglobin (Hb) Bart's, an abnormal type of hemoglobin. The percentage of Hb Bart's detected correlates with alpha-thalassemia severity. The Association of Public Health Laboratories' Hemoglobinopathy Workgroup conducted a survey of state newborn screening programs' alpha-thalassemia screening methodologies and reporting and follow-up practices. Survey findings indicated that 41 of 44 responding programs (93%) report some form of alpha-thalassemia results and 57% used a two-method screening protocol. However, the percentage of Hb Bart's used for thalassemia classification, the types of alpha-thalassemia reported, and the recipients of this information varied widely. These survey findings highlight the opportunity for newborn screening programs to revisit their policies as they reevaluate their practices in light of the recently released guideline from the Clinical and Laboratory Standards Institute (CLSI) on Newborn Screening for Hemoglobinopathies (1). Although deferring to local programs for policies, the report used a cutoff of 25% Hb Bart's in its decision tree, a value many programs do not use. Standardization of screening and reporting might lead to more timely diagnoses and health care services and improved outcomes for persons with a clinically significant alpha-thalassemia.

A comparative encyclopedia of DNA elements in the mouse genome

The laboratory mouse shares the majority of its protein-coding genes with humans, making it the premier model organism in biomedical research, yet the two mammals differ in significant ways. To gain greater insights into both shared and species-specific transcriptional and cellular regulatory programs in the mouse, the Mouse ENCODE Consortium has mapped transcription, DNase I hypersensitivity, transcription factor binding, chromatin modifications and replication domains throughout the mouse genome in diverse cell and tissue types. By comparing with the human genome, we not only confirm substantial conservation in the newly annotated potential functional sequences, but also find a large degree of divergence of sequences involved in transcriptional regulation, chromatin state and higher order chromatin organization. Our results illuminate the wide range of evolutionary forces acting on genes and their regulatory regions, and provide a general resource for research into mammalian biology and mechanisms of human diseases.

Conservation of trans-acting circuitry during mammalian regulatory evolution

The basic body plan and major physiological axes have been highly conserved during mammalian evolution, yet only a small fraction of the human genome sequence appears to be subject to evolutionary constraint. To quantify cis- versus trans-acting contributions to mammalian regulatory evolution, we performed genomic DNase I footprinting of the mouse genome across 25 cell and tissue types, collectively defining ∼8.6 million transcription factor (TF) occupancy sites at nucleotide resolution. Here we show that mouse TF footprints conjointly encode a regulatory lexicon that is ∼95% similar with that derived from human TF footprints. However, only ∼20% of mouse TF footprints have human orthologues. Despite substantial turnover of the cis-regulatory landscape, nearly half of all pairwise regulatory interactions connecting mouse TF genes have been maintained in orthologous human cell types through evolutionary innovation of TF recognition sequences. Furthermore, the higher-level organization of mouse TF-to-TF connections into cellular network architectures is nearly identical with human. Our results indicate that evolutionary selection on mammalian gene regulation is targeted chiefly at the level of trans-regulatory circuitry, enabling and potentiating cis-regulatory plasticity.

Mouse genomic footprinting reveals conservation of transcription factor (TF) recognition repertoires and trans-regulatory circuitry despite massive turnover of DNA elements that contact TFs in vivo.

Having generated genomic DNase I footprinting data of the mouse genome across 25 cell and tissue types, these authors use these data to quantify cis-versus-trans regulatory contributions to mammalian regulatory evolution. They describe more than 600 motifs that collectively are over 95% similar to that recognized in vivo by human transcription factors (TFs). Despite substantial turnover of the cis-regulatory landscape around each TF gene, nearly half of all pairwise regulatory interactions connecting mouse TF genes have been maintained in orthologous human cell types through evolutionary innovation of TF recognition sequences. Conservation between mouse and human TF regulatory networks is particularly similar at the highest organization level. The work was performed as part of the mouse ENCODE project.

Mouse regulatory DNA landscapes reveal global principles of cis-regulatory evolution

To study the evolutionary dynamics of regulatory DNA, we mapped >1.3 million deoxyribonuclease I-hypersensitive sites (DHSs) in 45 mouse cell and tissue types, and systematically compared these with human DHS maps from orthologous compartments. We found that the mouse and human genomes have undergone extensive cis-regulatory rewiring that combines branch-specific evolutionary innovation and loss with widespread repurposing of conserved DHSs to alternative cell fates, and that this process is mediated by turnover of transcription factor (TF) recognition elements. Despite pervasive evolutionary remodeling of the location and content of individual cis-regulatory regions, within orthologous mouse and human cell types the global fraction of regulatory DNA bases encoding recognition sites for each TF has been strictly conserved. Our findings provide new insights into the evolutionary forces shaping mammalian regulatory DNA landscapes.

A functional element necessary for fetal hemoglobin silencing

BACKGROUND

An improved understanding of the regulation of the fetal hemoglobin genes holds promise for the development of targeted therapeutic approaches for fetal hemoglobin induction in the β-hemoglobinopathies. Although recent studies have uncovered trans-acting factors necessary for this regulation, limited insight has been gained into the cis-regulatory elements involved.

METHODS

We identified three families with unusual patterns of hemoglobin expression, suggestive of deletions in the locus of the β-globin gene (β-globin locus). We performed array comparative genomic hybridization to map these deletions and confirmed breakpoints by means of polymerase-chain-reaction assays and DNA sequencing. We compared these deletions, along with previously mapped deletions, and studied the trans-acting factors binding to these sites in the β-globin locus by using chromatin immunoprecipitation.

RESULTS

We found a new (δβ)(0)-thalassemia deletion and a rare hereditary persistence of fetal hemoglobin deletion with identical downstream breakpoints. Comparison of the two deletions resulted in the identification of a small intergenic region required for γ-globin (fetal hemoglobin) gene silencing. We mapped a Kurdish β(0)-thalassemia deletion, which retains the required intergenic region, deletes other surrounding sequences, and maintains fetal hemoglobin silencing. By comparing these deletions and other previously mapped deletions, we elucidated a 3.5-kb intergenic region near the 5' end of the δ-globin gene that is necessary for γ-globin silencing. We found that a critical fetal hemoglobin silencing factor, BCL11A, and its partners bind within this region in the chromatin of adult erythroid cells.

CONCLUSIONS

By studying three families with unusual deletions in the β-globin locus, we identified an intergenic region near the δ-globin gene that is necessary for fetal hemoglobin silencing. (Funded by the National Institutes of Health and others.).

Cohesin mediates chromatin interactions that regulate mammalian β-globin expression

The β-globin locus undergoes dynamic chromatin interaction changes in differentiating erythroid cells that are thought to be important for proper globin gene expression. However, the underlying mechanisms are unclear. The CCCTC-binding factor, CTCF, binds to the insulator elements at the 5' and 3' boundaries of the locus, but these sites were shown to be dispensable for globin gene activation. We found that, upon induction of differentiation, cohesin and the cohesin loading factor Nipped-B-like (Nipbl) bind to the locus control region (LCR) at the CTCF insulator and distal enhancer regions as well as at the specific target globin gene that undergoes activation upon differentiation. Nipbl-dependent cohesin binding is critical for long-range chromatin interactions, both between the CTCF insulator elements and between the LCR distal enhancer and the target gene. We show that the latter interaction is important for globin gene expression in vivo and in vitro. Furthermore, the results indicate that such cohesin-mediated chromatin interactions associated with gene regulation are sensitive to the partial reduction of Nipbl caused by heterozygous mutation. This provides the first direct evidence that Nipbl haploinsufficiency affects cohesin-mediated chromatin interactions and gene expression. Our results reveal that dynamic Nipbl/cohesin binding is critical for developmental chromatin organization and the gene activation function of the LCR in mammalian cells.

Comprehensive mapping of long-range interactions reveals folding principles of the human genome

We describe Hi-C, a method that probes the three-dimensional architecture of whole genomes by coupling proximity-based ligation with massively parallel sequencing. We constructed spatial proximity maps of the human genome with Hi-C at a resolution of 1 megabase. These maps confirm the presence of chromosome territories and the spatial proximity of small, gene-rich chromosomes. We identified an additional level of genome organization that is characterized by the spatial segregation of open and closed chromatin to form two genome-wide compartments. At the megabase scale, the chromatin conformation is consistent with a fractal globule, a knot-free, polymer conformation that enables maximally dense packing while preserving the ability to easily fold and unfold any genomic locus. The fractal globule is distinct from the more commonly used globular equilibrium model. Our results demonstrate the power of Hi-C to map the dynamic conformations of whole genomes.

Comprehensive mapping of long-range interactions reveals folding principles of the human genome

We describe Hi-C, a method that probes the three-dimensional architecture of whole genomes by coupling proximity-based ligation with massively parallel sequencing. We constructed spatial proximity maps of the human genome with Hi-C at a resolution of 1 megabase. These maps confirm the presence of chromosome territories and the spatial proximity of small, gene-rich chromosomes. We identified an additional level of genome organization that is characterized by the spatial segregation of open and closed chromatin to form two genome-wide compartments. At the megabase scale, the chromatin conformation is consistent with a fractal globule, a knot-free, polymer conformation that enables maximally dense packing while preserving the ability to easily fold and unfold any genomic locus. The fractal globule is distinct from the more commonly used globular equilibrium model. Our results demonstrate the power of Hi-C to map the dynamic conformations of whole genomes.

Developmental and species-divergent globin switching are driven by BCL11A

The contribution of changes in cis-regulatory elements or trans-acting factors to interspecies differences in gene expression is not well understood. The mammalian beta-globin loci have served as a model for gene regulation during development. Transgenic mice containing the human beta-globin locus, consisting of the linked embryonic (epsilon), fetal (gamma) and adult (beta) genes, have been used as a system to investigate the temporal switch from fetal to adult haemoglobin, as occurs in humans. Here we show that the human gamma-globin (HBG) genes in these mice behave as murine embryonic globin genes, revealing a limitation of the model and demonstrating that critical differences in the trans-acting milieu have arisen during mammalian evolution. We show that the expression of BCL11A, a repressor of human gamma-globin expression identified by genome-wide association studies, differs between mouse and human. Developmental silencing of the mouse embryonic globin and human gamma-globin genes fails to occur in mice in the absence of BCL11A. Thus, BCL11A is a critical mediator of species-divergent globin switching. By comparing the ontogeny of beta-globin gene regulation in mice and humans, we have shown that alterations in the expression of a trans-acting factor constitute a critical driver of gene expression changes during evolution.

The locus control region is required for association of the murine beta-globin locus with engaged transcription factories during erythroid maturation

We have examined the relationship between nuclear localization and transcriptional activity of the endogenous murine beta-globin locus during erythroid differentiation. Murine fetal liver cells were separated into distinct erythroid maturation stages by fluorescence-activated cell sorting, and the nuclear position of the locus was determined at each stage. We find that the beta-globin locus progressively moves away from the nuclear periphery with increasing maturation. Contrary to the prevailing notion that the nuclear periphery is a repressive compartment in mammalian cells, beta(major)-globin expression begins at the nuclear periphery prior to relocalization. However, relocation of the locus to the nuclear interior with maturation is accompanied by an increase in beta(major)-globin transcription. The distribution of nuclear polymerase II (Pol II) foci also changes with erythroid differentiation: Transcription factories decrease in number and contract toward the nuclear interior. Moreover, both efficient relocalization of the beta-globin locus from the periphery and its association with hyperphosphorylated Pol II transcription factories require the locus control region (LCR). These results suggest that the LCR-dependent association of the beta-globin locus with transcriptionally engaged Pol II foci provides the driving force for relocalization of the locus toward the nuclear interior during erythroid maturation.

Flanking HS-62.5 and 3' HS1, and regions upstream of the LCR, are not required for beta-globin transcription

The locus control region (LCR) was thought to be necessary and sufficient for establishing and maintaining an open beta-globin locus chromatin domain in the repressive environment of the developing erythrocyte. However, deletion of the LCR from the endogenous locus had no significant effect on chromatin structure and did not silence transcription. Thus, the cis-regulatory elements that confer the open domain remain unidentified. The conserved DNaseI hypersensitivity sites (HSs) HS-62.5 and 3'HS1 that flank the locus, and the region upstream of the LCR have been implicated in globin gene regulation. The flanking HSs bind CCCTC binding factor (CTCF) and are thought to interact with the LCR to form a "chromatin hub" involved in beta-globin gene activation. Hispanic thalassemia, a deletion of the LCR and 27 kb upstream, leads to heterochromatinization and silencing of the locus. Thus, the region upstream of the LCR deleted in Hispanic thalassemia (upstream Hispanic region [UHR]) may be required for expression. To determine the importance of the UHR and flanking HSs for beta-globin expression, we generated and analyzed mice with targeted deletions of these elements. We demonstrate deletion of these regions alone, and in combination, do not affect transcription, bringing into question current models for the regulation of the beta-globin locus.

Deletion of the core region of 5' HS2 of the mouse beta-globin locus control region reveals a distinct effect in comparison with human beta-globin transgenes

The beta-globin locus control region (LCR) is a large DNA element that is required for high-level expression of beta-like globin genes from the endogenous mouse locus or in transgenic mice carrying the human beta-globin locus. The LCR encompasses 6 DNaseI hypersensitive sites (HSs) that bind transcription factors. These HSs each contain a core of a few hundred base pairs (bp) that has most of the functional activity and exhibits high interspecies sequence homology. Adjoining the cores are 500- to 1000-bp "flanks" with weaker functional activity and lower interspecies homology. Studies of human beta-globin transgenes and of the endogenous murine locus show that deletion of an entire HS (core plus flanks) moderately suppresses expression. However, human transgenes in which only individual HS core regions were deleted showed drastic loss of expression accompanied by changes in chromatin structure. To address these disparate results, we have deleted the core region of 5'HS2 from the endogenous murine beta-LCR. The phenotype was similar to that of the larger 5'HS2 deletion, with no apparent disruption of chromatin structure. These results demonstrate that the greater severity of HS core deletions in comparison to full HS deletions is not a general property of the beta-LCR.

Proximity among distant regulatory elements at the beta-globin locus requires GATA-1 and FOG-1

Recent evidence suggests that long-range enhancers and gene promoters are in close proximity, which might reflect the formation of chromatin loops. Here, we examined the mechanism for DNA looping at the beta-globin locus. By using chromosome conformation capture (3C), we show that the hematopoietic transcription factor GATA-1 and its cofactor FOG-1 are required for the physical interaction between the beta-globin locus control region (LCR) and the beta-major globin promoter. Kinetic studies reveal that GATA-1-induced loop formation correlates with the onset of beta-globin transcription and occurs independently of new protein synthesis. GATA-1 occupies the beta-major globin promoter normally in fetal liver erythroblasts from mice lacking the LCR, suggesting that GATA-1 binding to the promoter and LCR are independent events that occur prior to loop formation. Together, these data demonstrate that GATA-1 and FOG-1 are essential anchors for a tissue-specific chromatin loop, providing general insights into long-range enhancer function.

A complex chromatin landscape revealed by patterns of nuclease sensitivity and histone modification within the mouse beta-globin locus

In order to create an extended map of chromatin features within a mammalian multigene locus, we have determined the extent of nuclease sensitivity and the pattern of histone modifications associated with the mouse beta-globin genes in adult erythroid tissue. We show that the nuclease-sensitive domain encompasses the beta-globin genes along with several flanking olfactory receptor genes that are inactive in erythroid cells. We describe enhancer-blocking or boundary elements on either side of the locus that are bound in vivo by the transcription factor CTCF, but we found that they do not coincide with transitions in nuclease sensitivity flanking the locus or with patterns of histone modifications within it. In addition, histone hyperacetylation and dimethylation of histone H3 K4 are not uniform features of the nuclease-sensitive mouse beta-globin domain but rather define distinct subdomains within it. Our results reveal a complex chromatin landscape for the active beta-globin locus and illustrate the complexity of broad structural changes that accompany gene activation.

The beta -globin locus control region (LCR) functions primarily by enhancing the transition from transcription initiation to elongation

To investigate the molecular basis of beta-globin gene activation, we analyzed factor recruitment and histone modification at the adult beta-globin gene in wild-type (WT)/locus control region knockout (DeltaLCR) heterozygous mice and in murine erythroleukemia (MEL) cells. Although histone acetylation and methylation (Lys 4) are high before and after MEL differentiation, recruitment of the erythroid-specific activator NF-E2 to the promoter and preinitiation complex (PIC) assembly occur only after differentiation. We reported previously that targeted deletion of the LCR reduces beta-globin gene expression to 1%-4% of WT without affecting promoter histone acetylation. Here, we report that NF-E2 is recruited equally efficiently to the adult beta-globin promoters of the DeltaLCR and WT alleles. Moreover, the LCR deletion reduces PIC assembly only twofold, but has a dramatic effect on Ser 5 phosphorylation of RNA polymerase II and transcriptional elongation. Our results suggest at least three distinct stages in beta-globin gene activation: (1) an LCR-independent chromatin opening stage prior to NF-E2 recruitment to the promoter and PIC assembly; (2) an intermediate stage in which NF-E2 binding (LCR-independent) and PIC assembly (partially LCR-dependent) occur; and (3) an LCR-dependent fully active stage characterized by efficient pol II elongation. Thus, in its native location the LCR functions primarily downstream of activator recruitment and PIC assembly.

Targeted deletion of 5'HS1 and 5'HS4 of the beta-globin locus control region reveals additive activity of the DNaseI hypersensitive sites

The mammalian beta-globin locus is a multigenic, developmentally regulated, tissue-specific locus from which gene expression is regulated by a distal regulatory region, the locus control region (LCR). The functional mechanism by which the beta-globin LCR stimulates transcription of the linked beta-like globin genes remains unknown. The LCR is composed of a series of 5 DNaseI hypersensitive sites (5'HSs) that form in the nucleus of erythroid precursors. These HSs are conserved among mammals, bind transcription factors that also bind to other parts of the locus, and compose the functional components of the LCR. To test the hypothesis that individual HSs have unique properties, homologous recombination was used to construct 5 lines of mice with individual deletions of each of the 5'HSs of the endogenous murine beta-globin LCR. Here it is reported that deletion of 5'HS1 reduces expression of the linked genes by up to 24%, while deletion of 5'HS4 leads to reductions of up to 27%. These deletions do not perturb the normal stage-specific expression of genes from this multigenic locus. In conjunction with previous studies of deletions of the other HSs and studies of deletion of the entire LCR, it is concluded that (1) none of the 5'HSs is essential for nearly normal expression; (2) none of the HSs is required for proper developmental expression; and (3) the HSs do not appear to synergize either structurally or functionally, but rather form independently and appear to contribute additively to the overall expression from the locus.

The murine beta-globin locus control region regulates the rate of transcription but not the hyperacetylation of histones at the active genes

Locus control regions (LCRs) are defined by their ability to confer high-level tissue-specific expression to linked genes in transgenic assays. Previously, we reported that, at its native site, the murine beta-globin LCR is required for high-level beta-globin gene expression, but is not required to initiate an open chromatin conformation of the locus. To further investigate the mechanism of LCR-mediated transcriptional enhancement, we have analyzed allele-specific beta-globin expression and the pattern of histone acetylation in the presence and absence of the LCR. In single cells from mice heterozygous for a deletion of the LCR, beta-globin expression from the LCR-deleted allele is consistently low ( approximately 1-4% of wild type). Thus, the endogenous LCR enhances globin gene expression by increasing the rate of transcription from each linked allele rather than by increasing the probability of establishing transcription per se. Furthermore, in erythroid cells from mice homozygous for the highly expressing wild-type beta-globin locus, hyperacetylation of histones H3 and H4 is localized to the LCR and active genes. In mice homozygous for the LCR deletion reduced histone hyperacetylation is observed in LCR proximal sequences; however, deletion of the LCR has no effect on the localized hyperacetylation of the genes. Together, our results suggest that, in its native genomic context, the LCR follows the rate model of enhancer function, and that the developmentally specific hyperacetylation of the globin genes is independent of both the rate of transcription and the presence of the LCR.

Comparative structural and functional analysis of the olfactory receptor genes flanking the human and mouse beta-globin gene clusters

By sequencing regions flanking the beta-globin gene complex in mouse (Hbbc) and human (HBBC), we have shown that the beta-globin gene cluster is surrounded by a larger cluster of olfactory receptor genes (ORGs). To facilitate sequence comparisons and to investigate the regulation of ORG expression, we have mapped 5' sequences of mRNA from olfactory epithelium encoding beta-globin-proximal ORGs. We have found that several of these genes contain multiple noncoding exons that can be alternatively spliced. Surprisingly, the only common motifs found in the promoters of these genes are a "TATA" box and a purine-rich motif. Sequence comparisons between human and mouse reveal that most of the conserved regions are confined to the coding regions and transcription units of the genes themselves, but a few blocks of conserved sequence also are found outside of ORG transcription units. The possible influence of beta-globin regulatory sequences on ORG expression in olfactory epithelium was tested in mice containing a deletion of the endogenous beta-globin locus control region, but no change in expression of the neighboring ORGs was detected. We evaluate the implications of these results for possible mechanisms of regulation of ORG transcription.

Independent formation of DnaseI hypersensitive sites in the murine beta-globin locus control region

Mammalian beta-globin loci are composed of multiple orthologous genes whose expression is erythroid specific and developmentally regulated. The expression of these genes both from the endogenous locus and from transgenes is strongly influenced by a linked 15-kilobase region of clustered DNaseI hypersensitive sites (HSs) known as the locus control region (LCR). The LCR encompasses 5 major HSs, each of which is highly homologous among humans, mice, and other mammals. To analyze the function of individual HSs in the endogenous murine beta-globin LCR, we have used homologous recombination in embryonic stem cells to produce 5 mouse lines, each of which is deficient for 1 of these major HSs. In this report, we demonstrate that deletion of the conserved region of 5'HS 1, 2, 3, 4, or 5/6 abolishes HS formation at the deletion site but has no influence on the formation of the remaining HSs in the LCR. Therefore, in the endogenous murine locus, there is no dominant or initiating site whose formation must precede the formation of the other HSs. This is consistent with the idea that HSs form autonomously. We discuss the implications of these findings for current models of beta-globin regulation.

Beta-globin gene switching and DNase I sensitivity of the endogenous beta-globin locus in mice do not require the locus control region

We have generated mice with a targeted deletion of the beta-globin locus control region (LCR). Mice homozygous for the deletion die early in embryogenesis but can be rescued with a YAC containing the human beta-globin locus. After germline passage, deletion of the LCR leads to a severe reduction in expression of all mouse beta-like globin genes, but no alteration in the developmental specificity of expression. Furthermore, a DNase I-sensitive "open" chromatin conformation of the locus is established and maintained. Thus, the dominant role of the LCR in the native locus is to confer high-level transcription, and elements elsewhere in the locus are sufficient to establish and maintain an open conformation and to confer developmentally regulated globin gene expression.

Deletions within the mouse beta-globin locus control region preferentially reduce beta(min) globin gene expression

The mouse beta-globin gene cluster is regulated, at least in part, by a locus control region (LCR) composed of several developmentally stable DNase I hypersensitive sites located upstream of the genes. In this report, we examine the level of expression of the beta(min) and beta(maj) genes in adult mice in which HS2, HS3, or HS5,6 has been either deleted or replaced by a selectable marker via homologous recombination in ES cells. Primer extension analysis of RNA extracted from circulating reticulocytes and HPLC analysis of globin chains from peripheral red blood cells revealed that all mutations that reduce the overall output of the locus preferentially decrease beta(min) expression over beta(maj). The implications of these findings for the mechanism by which the LCR controls expression of the beta(maj) and beta(min) promoters are discussed.

Analysis of mammalian cis-regulatory DNA elements by homologous recombination

The use of homologous recombination to modify and thereby functionally analyze cis-regulatory DNA elements in mammalian cells has become an important approach in mammalian gene expression research. We have emphasized the necessity of designing a system that allows the removal of selectable markers used in targeting and facilitates the further modification of the region under study. To perform these tasks, we presently favor making an initial HR-mediated replacement of the entire element under study with an active positive selectable marker in combination with either an inactive second positive selectable marker or an active negative selectable marker. The plug and socket system, in which an inactive selectable marker is complemented by HR, is the most dependable and well-characterized option for making secondary modifications. However, the double-replacement system has certain advantages, and the recently developed RMCE approach, which allows replacement of a negative selectable marker by site-specific recombinase-mediated insertion without using a positive selectable marker, will likely prove very valuable in future experiments. Each of the systems, or combinations thereof, should be considered in light of the specifics of any given experiment to select the most appropriate option. Although the emphasis of this article has been the analysis of cis-acting regulatory elements involved in transcription, these same approaches can be used to analyze other regulatory elements (e.g., origins of replication) and to make multiple subtle mutations in polypeptides.

Conservation of sequence and structure flanking the mouse and human beta-globin loci: the beta-globin genes are embedded within an array of odorant receptor genes

In mouse and human, the beta-globin genes reside in a linear array that is associated with a positive regulatory element located 5' to the genes known as the locus control region (LCR). The sequences of the mouse and human beta-globin LCRs are homologous, indicating conservation of an essential function in beta-globin gene regulation. We have sequenced regions flanking the beta-globin locus in both mouse and human and found that homology associated with the LCR is more extensive than previously known, making up a conserved block of approximately 40 kb. In addition, we have identified DNaseI-hypersensitive sites within the newly sequenced regions in both mouse and human, and these structural features also are conserved. Finally, we have found that both mouse and human beta-globin loci are embedded within an array of odorant receptor genes that are expressed in olfactory epithelium, and we also identify an olfactory receptor gene located 3' of the beta-globin locus in chicken. The data demonstrate an evolutionarily conserved genomic organization for the beta-globin locus and suggest a possible role for the beta-globin LCR in control of expression of these odorant receptor genes and/or the presence of mechanisms to separate regulatory signals in different tissues.

Antihyperalgesic effects of infection with a preproenkephalin-encoding herpes virus

To test the utility of gene therapeutic approaches for the treatment of pain, a recombinant herpes simplex virus, type 1, has been engineered to contain the cDNA for an opioid peptide precursor, human preproenkephalin, under control of the human cytomegalovirus promoter. This virus and a similar recombinant containing the Escherichia coli lacZ gene were applied to the abraded skin of the dorsal hindpaw of mice. After infection, the presence of beta-galactosidase in neuronal cell bodies of the relevant spinal ganglia (lacZ-containing virus) and of human proenkephalin (preproenkephalin-encoding virus) in the central terminals of these neurons indicated appropriate gene delivery and expression. Baseline foot withdrawal responses to noxious radiant heat mediated by Adelta and C fibers were similar in animals infected with proenkephalin-encoding and beta-galactosidase-encoding viruses. Sensitization of the foot withdrawal response after application of capsaicin (C fibers) or dimethyl sulfoxide (Adelta fibers) observed in control animals was reduced or eliminated in animals infected with the proenkephalin-encoding virus for at least 7 weeks postinfection. Hence, preproenkephalin cDNA delivery selectively blocked hyperalgesia without disrupting baseline sensory neurotransmission. This blockade of sensitization was reversed by administration of the opioid antagonist naloxone, apparently acting in the spinal cord. The results demonstrate that the function of sensory neurons can be selectively altered by viral delivery of a transgene. Because hyperalgesic mechanisms may be important in establishing and maintaining neuropathic and other chronic pain states, this approach may be useful for treatment of chronic pain and hyperalgesia in humans.

Description and targeted deletion of 5' hypersensitive site 5 and 6 of the mouse beta-globin locus control region

The most upstream hypersensitive site (HS) of the beta-globin locus control region (LCR) in humans (5' HS 5) and chickens (5' HS 4) can act as an insulating element in some gain of function assays and may demarcate a beta-globin domain. We have mapped the most upstream HSs of the mouse beta-globin LCR and sequenced this region. We find that mice have a region homologous to human 5' HS 5 that is associated with a minor HS. In addition we map a unique HS upstream of 5' HS 5 and refer to this novel site as mouse 5' HS 6. We have also generated mice containing a targeted deletion of the region containing 5' HS 5 and 6. We find that after excision of the selectable marker in vivo, deletion of 5' HS 5 and 6 has a minimal effect on transcription and does not prevent formation of the remaining LCR HSs. Taken together these findings suggest that the most upstream HSs of the mouse beta-globin LCR are not necessary for maintaining the beta-globin locus in an active configuration or to protect it from a surrounding repressive chromatin environment.

The beta-globin LCR is not necessary for an open chromatin structure or developmentally regulated transcription of the native mouse beta-globin locus

The murine beta-globin locus control region (LCR) was deleted from its native chromosomal location. The approximately 25 kb deletion eliminates all sequences and structures homologous to those defined as the human LCR. In differentiated ES cells and erythroleukemia cells containing the LCR-deleted chromosome, DNasel sensitivity of the beta-globin domain is established and maintained, developmental regulation of the locus is intact, and beta-like globin RNA levels are reduced 5%-25% of normal. Thus, in the native murine beta-globin locus, the LCR is necessary for normal levels of transcription, but other elements are sufficient to establish the open chromatin structure, transcription, and developmental specificity of the locus. These findings suggest a contributory rather than dominant function for the LCR in its native location.

Reduced beta-globin gene expression in adult mice containing deletions of locus control region 5' HS-2 or 5' HS-3

To gain insights into the functions of individual DNA'se hypersensitive sites within the beta globin locus control region (LCR), we deleted the endogenous 5' HS-2 and HS-3 regions from the mouse germline using homologous recombination techniques. We demonstrated that the deletion of either murine 5' HS-2 or 5' HS-3 reduced the expression of the embryonic epsilon y and beta h1 globin genes minimally in yolk sac-derived erythrocytes, but that both knockouts reduced the output of the adult beta (beta-Major + beta-Minor) globin genes by approximately 30% in adult erythrocytes. When the selectable marker PGK-Neo cassette was retained within either the HS-2 or HS-3 region, a much more severe reduction in globin gene expression was observed at all developmental stages. PGK-Neo was shown to be expressed in an erythroid-specific fashion when it was retained in the HS-3 position. These results show that neither 5' HS-2 nor HS-3 is required for the activity of embryonic globin genes, nor are these sites required for correct developmental switching. However, each site is required for approximately 30% of the total LCR activity associated with adult beta-globin gene expression in adult red blood cells. Each site therefore contains some non-redundant information that contributes to adult globin gene function.

Herpes virus-mediated preproenkephalin gene transfer to the amygdala is antinociceptive

To evaluate the role of the amygdala in pain modulation and opioid-mediated antinociception, a recombinant, replication-defective herpes virus carrying the human preproenkephalin cDNA was injected bilaterally into the rat amygdala. Four days after gene delivery nociceptive behavior was assessed by the formalin test. Rats infected with the virus expressing preproenkephalin showed a selective, naloxone-reversible abolition of phase 2 flinching behavior compared to rats infected with a control virus. The results implicate the amygdala in the control of pain and in opioid analgesia and demonstrate the use of recombinant herpes viruses as tools for studying gene function in specific neural pathways of the central nervous system.

Issues in cytogenetic biological dosimetry: emphasis on radiation environments in space

Issues central to the reliability of cytogenetic biodosimetry are presented. Although it now appears that cytogenetic biodosimetry can be used reliably to reconstruct radiation dose after acute, uniform whole-body exposure (albeit within certain dose ranges), additional data are required to fully validate cytogenetic biodosimetry for the protracted and complex exposure conditions in space. Approaches are presented that could be used to obtain the data necessary for validation. It also appears that the use of dicentric aberrations for biodosimetry on missions lasting several months or more may not be reliable because of the large variability observed among individuals in the rate of loss of cells with dicentrics, making back-extrapolations uncertain. This may be testable, however, by comparing the results for dicentrics and translocations obtained by biodosimetry with the radiation dosimetry obtained on board the spacecraft. In addition to these and several other issues, estimates are provided of the current limitations of detection of dicentrics and reciprocal translocations.

In vivo transgene activation from an HSV-based gene therapy vector by GAL4:vp16

Herpes simplex virus type 1 (HSV-1) has many attributes which make it attractive as a base for the development of vectors for the delivery of transgenes to the nervous system. In this report we describe the adaptation of the bipartite GAL4:VP16 transactivation system to replication-deficient HSV vectors. We demonstrate that the recombinant transactivator GAL4:VP16 produced from a replication-deficient HSV vector is capable of activating transcription of a reporter gene using a synthetic promoter consisting of GAL4 binding sites and the TATA box of the adenovirus E1b gene. Activation by vector produced GAL:VP16 was demonstrated with the recombinant promoter/reporter gene cassette in the infected cell chromosome, in the genome of a second virus infecting the same cells and with a single vector engineered to produce both GAL4:VP16 transactivator and to contain a recombinant promoter/reporter gene cassette. Furthermore, the double recombinant virus also produced the reporter gene product in neurons after direct intracranial inoculation into rat hippocampus. This system may be used to extend and improve promoter function in HSV gene transfer vectors in vivo.

Analysis of mice containing a targeted deletion of beta-globin locus control region 5' hypersensitive site 3

To examine the function of murine beta-globin locus region (LCR) 5' hypersensitive site 3 (HS3) in its native chromosomal context, we deleted this site from the mouse germ line by using homologous recombination techniques. Previous experiments with human 5' HS3 in transgenic models suggested that this site independently contains at least 50% of total LCR activity and that it interacts preferentially with the human gamma-globin genes in embryonic erythroid cells. However, in this study, we demonstrate that deletion of murine 5' HS3 reduces expression of the linked embryonic epsilon y- and beta H 1-globin genes only minimally in yolk sac-derived erythroid cells and reduces output of the linked adult beta (beta major plus beta minor) globin genes by approximately 30% in adult erythrocytes. When the selectable marker PGK-neo cassette was left within the HS3 region of the LCR, a much more severe phenotype was observed at all developmental stages, suggesting that PGK-neo interferes with LCR activity when it is retained within the LCR. Collectively, these results suggest that murine 5' HS3 is not required for globin gene switching; importantly, however, it is required for approximately 30% of the total LCR activity associated with adult beta-globin gene expression in adult erythrocytes.

HSV as a gene transfer vector for the nervous system

Gene therapy for diseases of the nervous system requires vectors capable of delivering the therapeutic gene into postmitotic cells in vivo. Herpes simplex virus type 1 is a neurotropic virus that naturally establishes latency in neurons of the peripheral nervous system. Replication defective HSV vectors have been developed; these are deleted for at least one essential immediate early regulatory gene, rendering the virus less cytotoxic, incapable of reactivation, but still capable of establishing latency. Foreign genes can be vigorously expressed from an HSV-based vector in a transient manner in brain and other tissues. Long-term but weak foreign gene expression may be achieved in the nervous system by exploiting the transcriptional control mechanisms of the natural viral latency active promoter. To meet the needs of specific applications, either highly active long-term or regulatable transgene expression will be needed, requiring further studies in order to design the appropriate latency-based promoter systems.

Cytogenetics research in radiation biology

Radiation cytogenetics goes back approximately six decades and not only contributed to the earliest development of radiobiology, but continues to contribute today. Contributions on three levels are outlined here. Early contributions to radiobiological theory include the nature of dose-effect curves, dose-rate and fractionation effects, and linear energy transfer (LET) effects. Understanding of the roles of aberrations in endpoints such as cell killing, mutation and carcinogenesis have more recently contributed to unraveling mechanisms in these important radiobiological effects. Finally, the study of various details of classical radiation cytogenetics, such as half chromatid exchange or sister chromatid union, has contributed to our current understanding of cytogenetic phenomena on the molecular level.

Spontaneous and induced micronuclei and UDS in peripheral lymphocytes

We have examined the distributions both of spontaneous and X-ray-induced micronuclei (MN) and of spontaneous and UV-induced unscheduled DNA synthesis (autoradiographic grains; UDS) in cultures of peripheral blood lymphocytes from normal, healthy human volunteers. While the spontaneous MN and UDS do not differ significantly from the expected Poisson distributions, both the induced MN and UDS are strongly overdistributed (i.e., variance much greater than mean). This not only must be allowed for in statistical tests used for population monitoring, but offers suggestive evidence that there are large differences in radiation response from sample to sample, some of which may reflect true differences among normal, healthy subjects.

Chromosomal aberration types in cells at the second division after irradiation in G1 or G2

Cultures of JU-56 cells were irradiated in either G1 or G2 and examined either in their first post-irradiation metaphase (in diploids) or in their second post-irradiation metaphase (in colcemid-induced tetraploids). The timing of fixation, together with tritiated thymidine pulse labelling, allowed selection for scoring only metaphases of cells that were in the G1 or G2 phase of the cell cycle during irradiation. With G1 irradiation it was found that many of the aberrations observed at the first division were not present (as derivatives) at the second division, and also that new aberrations were found at the second division, which were not derived from aberrations at the first division. Clonogenic survival was also measured in populations of cells irradiated in G1. It was found that cells containing chromosomal aberrations at the first division were not numerous enough to explain lack of survival. When frequencies of aberrations following G2 irradiation scored at the second division were compared with those scored at the first, there was a significant increase in dicentrics as compared with their progenitor asymmetrical chromatid interchanges, and of mirror-image dicentrics as compared with their progenitor sister unions. A substantial number of sister unions were also observed at the second division. We conclude that some aberrations are lost during the interphase between the first and the second post-irradiation metaphase and that new chromosomal aberrations arise during the second post-irradiation interphase.

Time sequence of events leading to chromosomal aberration formation

Investigations have been carried out which have measured the influence of the repair polymerases on the yield of different types of chromosomal aberrations. The studies were mainly concerned with the effect of inhibiting the polymerases on the yield of aberrations. The polymerases fill in single strand regions, and the fact that their inhibition affects the yield of aberrations suggests that single strand lesions are influential in aberration formation. The results indicate that-- 1. There are two actions of polymerases in clastogenesis. One is in their involvement in a G2 repair system, in which the pair of chromatids is concerned, and which does not yield aberrations unless the inhibition is still operating when the cells enter mitosis. The second also operates in G1 and S, and is such that when repair is inhibited, further damage accrues. 2. The second action is affected by inhibiting polymerase but operates even when the repair enzymes are active. 3. The production of chromosomal exchanges involves a series of reactions, some of which are reversible. 4. The time span over which the reactions occur is much longer than has been envisaged previously (e.g., most of a cell cycle).