Complementation of alpha-thalassaemia in alpha-globin knockout mice with a 191 kb transgene containing the human alpha-globin locus

Identification of 129S6/SvEvTac-specific polymorphisms on mouse chromosome 11

Polymorphisms such as single-nucleotide polymorphisms (SNPs) and insertions/deletions (Indels) can be associated with phenotypic traits and be used as markers for disease diagnosis. Identification of these genetic variations within laboratory mice is crucial to improve our understanding of the genetic background of the mice used for research. As part of a positional cloning project, we sequenced six genes (Mettl16, Evi2a, Psmd11, Cct6d, Rffl, and Ap2b1) within a 6.8-Mb domain of mmu chr 11 in the C57BL/6J and 129S6/SvEvTac inbred strains. Although 129S6/SvEvTac is widely used in the mouse community, there is very little current (or projected future) sequence information available for this strain. We identified 6 Indels and 21 novel SNPs and confirmed genotype information for 114 additional SNPs in these 6 genes. Mettl16 and Ap2b1 contained the largest numbers of variants between the C57BL/6J and 129S6/SvEvTac strains. In addition, we found five new SNPs between 129S6/SvEvTac and 129S1/SvImJ within the Ap2b1 locus. Although we did not detect differences between C57BL/6J and 129S6/SvEvTac within Evi2a, this locus contains a relatively high SNP density compared with the surrounding sequence. Our study highlights the genetic differences among three inbred mouse strains (C57BL/6J, 129S6/SvEvTac, and 129S1/SvImJ) and provides valuable sequence information that can be used to track alleles in genomics-based studies.

Integration of functional bacterial artificial chromosomes into human cord blood-derived multipotent stem cells

Stem cells from a patient with a genetic disease could be used for cell therapy if it were possible to insert a functional copy of the defective gene. In this study, we investigate the transfection and subsequent integration of large genomic fragments into human cord blood-derived multipotent stem cells. We describe for the first time the creation of clonal stem cells carrying a human bacterial artificial chromosome (BAC) containing the Friedreich ataxia locus with an enhanced green fluorescent protein (EGFP) reporter gene fused to exon 5a of the frataxin (FXN) gene. Integration of the BAC into the host cell genome was confirmed by PCR, Southern blot and fluorescent in situ hybridization analysis. Reverse transcription-PCR and flow cytometry confirmed expression of FXN-EGFP. Correct mitochondrial localization of the protein was confirmed using fluorescent microscopy. The transfected stem cells also retained the ability to differentiate into cells from all three germline layers, as demonstrated by the capacity to form neuron-specific beta-tubulin-expressing cells, Alizarin Red S-positive bone-like cells, and epithelial-like cells expressing surfactant protein C. This is the first study to demonstrate that cord blood-derived multipotent stem cells may be useful targets for gene therapy applications using large genomic loci.

Position independent and copy-number-related expression of the bovine neonatal Fc receptor α-chain in transgenic mice carrying a 102 kb BAC genomic fragment

We generated and characterized transgenic mice carrying a 102 kb bovine genomic fragment, encoding the neonatal Fc receptor alpha-chain (bFcRn). FcRn plays a crucial role in the maternal IgG transport and it also regulates the IgG and albumin homeostasis. Some of its functions and transcriptional regulation show species specific differences. The FcRn heterodimer is composed of the alpha-chain and beta-2-microglobulin (beta2 m). A bacterial artificial chromosome containing the bovine FcRn alpha-chain gene (bFCGRT) with its 44 kb 5' and 50 kb long 3' flanking sequences was microinjected into fertilized mouse oocytes. Two of the transgenic lines generated, showed copy number related and integration site independent bFcRn expression. The bFcRn alpha-chain forms a functional receptor with the mouse beta2-microglobulin and extends the half-life of the mouse IgG in transgenic mice. Our results underline the feasibility of creating BAC transgenic mouse models of economically important bovine genes.

Transcription factors in myeloid development: balancing differentiation with transformation

In recent years, great progress has been made in elucidating the progenitor-cell hierarchy of the myeloid lineage. Transcription factors have been shown to be key determinants in the orchestration of myeloid identity and differentiation fates. Most transcription factors show cell-lineage-restricted and stage-restricted expression patterns, indicating the requirement for tight regulation of their activities. Moreover, if dysregulated or mutated, these transcription factors cause the differentiation block observed in many myeloid leukaemias. Consequently, therapies designed to restore defective transcription factor functions are an attractive option in the treatment of myeloid and other human cancers.

HS-48 alone has no enhancement role on the expression of human α-globin gene cluster

To investigate the in vivo function of the newly defined DNase I hypersensitive site HS-48 on the whole human alpha-globin gene cluster, the region containing all the other known 5 hypersensitive sites HS-4 to HS-40 was deleted from a 117 kb bacterial artificial chromosome clone bearing the whole human alpha-globin gene cluster. Transgenic mice were generated from this construct. The RNase protection assays showed that with HS-48 left and all the other 5 hypersensitive sites deleted, the expression of human alpha-like globin genes was completely silenced in embryonic, fetal and adult stages in all tissues. This finding indicates that HS-48 alone has no enhancer activity on the expression of human alpha-like globin genes, and that the region of HS-4 to HS-40 already contains all the upstream cis-elements needed for regulating human alpha-like globin genes.

A humanized BAC transgenic/knockout mouse model for HbE/beta-thalassemia

Hemoglobin E (HbE) is caused by a G-->A mutation at codon 26 of the beta-globin gene, which substitutes Glu-->Lys. This mutation gives rise to functional but unstable hemoglobin and activates a cryptic splice site causing mild anemia. HbE reaches a carrier frequency of 60-80% in some Southeast Asian populations. HbE causes serious disease when co-inherited with a beta-thalassemia mutation. In this study, we report the creation and evaluation of humanized transgenic mice containing the beta(E) mutation in the context of the human beta-globin locus. Developmental expression of the human beta(E) locus transgene partially complements the hematological abnormalities in heterozygous knockout mice ((mu)beta(th-3/+)) and rescues the embryonic lethality of homozygous knockout mice ((mu)beta(th-3/th-3)). The phenotype of rescued mice was dependent on the transgene copy number. This mouse model displays hematological abnormalities similar to HbE/beta-thalassemia patients and represent an ideal in vivo model system for pathophysiological studies and evaluation of novel therapies.

A 191-kb genomic fragment containing the human alpha-globin locus can rescue alpha-thalassemic mice

A 191-kb human bacterial artificial chromosome (BAC) containing the human alpha-globin genomic locus was used to generate transgenic mice that express, exclusively, human alpha-globin ((hu)alpha-globin). Expression of (hu)alpha-globin reaches a level of 36% of that of endogenous mouse alpha-globin ((mu)alpha-globin) on a heterozygous mouse alpha-thalassemia background ((mu)alpha-globin knockout, (mu)alpha(+/-)). Hemizygous transgenic mice carrying the (hu)alpha-globin locus on a heterozygous knockout background ((hu)alpha(+/0), (mu)alpha(++/--)) demonstrated complementation of most hematologic parameters. By crossing (hu)alpha(+/0), (mu)alpha(++/--) mice, we were able to generate mice entirely dependent on (hu)alpha-globin synthesis. Breeding and fluorescent in situ hybridization studies demonstrate that only mice homozygous for the transgene were able to rescue embryonic lethal homozygous (mu)alpha-globin knockout embryos ((mu)alpha(--/--)). Adult rescued mice produce hemoglobin at levels similar to wild-type mice, with partial red cell complementation based on mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and red cell distribution width (RDW) measurements. Significant erythrocythemia above wild-type levels seems to be the main compensatory mechanism for the normalization of the hemoglobin levels in the rescued animals. Our studies demonstrate that the (hu)alpha-globin locus in the 191-kb transgene contains all the necessary elements for the regulated expression of (hu)alpha-globin in transgenic mice. This animal model should be valuable for studying the mechanisms regulating (hu)alpha-globin production and for development of therapeutic strategies for beta-thalassemia based on downregulation of alpha-globin expression.

Role of transcription factors C/EBPalpha and PU.1 in normal hematopoiesis and leukemia

Differentiation of hematopoietic stem and progenitor cells is under strict control of a regulatory network orchestrated by lineage-specific transcription factors. A block in normal differentiation is a major contributing factor in the development of solid tumors and leukemias. Cells from patients with acute myeloid leukemia (AML) frequently harbor mutated or dysregulated transcription factor genes, suggesting their involvement in leukemogenesis. As a consequence, these alterations diminish the pool of available molecules of a small number of critical transcription factors, such as CCAAT enhancer binding proteins, PU.1, GATA-1, and AML-1. In this review, we focus on the mechanisms of how this functional pool of transcription factors is maintained during normal and malignant hematopoiesis, including direct protein-protein interactions, competition for DNA binding, and the control of transcription factor genes by proximal and distal regulatory elements. Results of recent studies of mice carrying hypomorphic PU.1 alleles have indicated that reduction in the expression of a single transcription factor is capable of predisposing mice to AML. The implications of these findings for the study of hematopoiesis in the future as well as novel approaches to more disease-specific therapies are discussed.

Transgene copy number-dependent rescue of murine beta-globin knockout mice carrying a 183 kb human beta-globin BAC genomic fragment

We report the generation and characterisation of the first transgenic mice exclusively expressing normal human beta-globin ((hu)beta-globin) from a 183 kb genomic fragment. Four independent lines were generated, each containing 2-6 copies of the (hu)beta-globin locus at a single integration site. Steady state levels of (hu)beta-globin protein were dependent on transgene copy number, but independent of the site of integration. Hemizygosity for the transgene on a heterozygous knockout background ((hu)beta(+/0), (mu)beta(th-3/+)) complemented fully the hematological abnormalities associated with the heterozygous knockout mutation in all four lines. Importantly, the rescue of the embryonic lethal phenotype that is characteristic of homozygosity for the knockout mutation was also demonstrated in two transgenic lines that were homozygous for two copies of the (hu)beta-globin locus, and in one transgenic line, which was hemizygous for six copies of the (hu)beta-globin locus. Our results illustrate the importance of transgene copy number determination and of the hemizygosity/homozygosity status in phenotypic complementation studies of transgenic mice containing large heterologous transgenes. Transgenic mouse colonies with 100% (hu)beta-globin production from the intact (hu)beta-globin locus have been established and will be invaluable in comparative and gene therapy studies with mouse models containing specific beta-thalassemia mutations in the (hu)beta-globin locus.

Ham-Wasserman Lecture

Over the past fifty years, many advances in our understanding of the general principles controlling gene expression during hematopoiesis have come from studying the synthesis of hemoglobin. Discovering how the α and β globin genes are normally regulated and documenting the effects of inherited mutations which cause thalassemia have played a major role in establishing our current understanding of how genes are switched on or off in hematopoietic cells. Previously, nearly all mutations causing thalassemia have been found in or around the globin loci, but rare inherited and acquired trans-acting mutations are being found with increasing frequency. Such mutations have demonstrated new mechanisms underlying human genetic disease. Furthermore, they are revealing new pathways in the regulation of globin gene expression which, in turn, may eventually open up new avenues for improving the management of patients with common types of thalassemia.