Global predictions and tests of erythroid regulatory regions.
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Animals↗
Biomedical subjects
R C Hardison
Publications and source records attributed to R C Hardison.
Association between divergence and interspersed repeats in mammalian noncoding genomic DNA.
The amount of noncoding genomic DNA sequence that aligns between human and mouse varies substantially in different regions of their genomes, and the amount of repetitive DNA also varies. In this report, we show that divergence in noncoding nonrepetitive DNA is strongly correlated with the amount of repetitive DNA in a region. We investigated aligned DNA in four large genomic regions with finished human sequence and almost or completely finished mouse sequence. These regions, totaling 5.89 Mb of DNA, are on different chromosomes and vary in their base composition. An analysis based on sliding windows of 10 kb shows that the fraction of aligned noncoding nonrepetitive DNA and the fraction of repetitive DNA are negatively correlated, both at the level of an entire region and locally within it. This conclusion is strongly supported by a randomization study, in which repetitive elements are removed and randomly relocated along the sequences. Thus, regions of noncoding genomic DNA that accumulated fewer point mutations since the primate-rodent divergence also suffered fewer retrotransposition events. These results indicate that some regions of the genome are more "flexible" over the time scale of mammalian evolution, being able to accommodate many point mutations and insertions, whereas other regions are more "rigid" and accumulate fewer changes. Stronger conservation is generally interpreted as indicating more extensive or more important function. The evidence presented here of correlated variation in the rates of different evolutionary processes across noncoding DNA must be considered in assessing such conservation for evidence of selection.
Animals↗
New views of evolution and regulation of vertebrate beta-like globin gene clusters from an orphaned gene in marsupials.
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Animals↗
Databases of human hemoglobin variants and other resources at the globin gene server.
Building on the pioneering efforts of Professor Huisman, several different databases of hemoglobin variants have been developed, each with progressively increased capacity for sophisticated queries and prompt updating. These resources are reviewed in the context of a larger plan for providing related resources on hemoglobins, benign and pathological variation in these proteins and the genes that encode them, and the regulation of the globin genes.
Databases, Nucleic Acid↗
Sequences flanking hypersensitive sites of the beta-globin locus control region are required for synergistic enhancement.
The major distal regulatory sequence for the beta-globin gene locus, the locus control region (LCR), is composed of multiple hypersensitive sites (HSs). Different models for LCR function postulate that the HSs act either independently or synergistically. To test these possibilities, we have constructed a series of expression cassettes in which the gene encoding the enhanced green fluorescent protein (EGFP) is under the control of DNA fragments containing single and multiple HSs of the LCR. LCR DNA fragments containing only the minimal region needed for position-independent expression (HS cores) or containing cores plus flanking sequences (HS units) were compared to ascertain whether conserved sequences between the HS cores contributed to enhancement. Expression of these constructs was measured after targeted integration into three defined loci in murine erythroleukemia cells using recombinase-mediated cassette exchange. At all three marked loci, synergistic enhancement of expression was observed in cassettes containing a combination of HS2, HS3, and HS4 units. In contrast, HS2, HS3, and HS4 cores (without flanking sequences) give an activity equivalent to the sum of the activities of the individual HS cores. These data suggest a model in which an HS core plus flanking regions, bound by specific proteins, forms a structure needed for interaction with other HS units to confer strong enhancement by the LCR. The three targeted integration sites differ substantially in their permissivity for expression, but even the largest LCR construct tested could not overcome these position effects to confer equal expression at all three sites.
Binding Sites↗
Conserved noncoding sequences are reliable guides to regulatory elements.
A 'working draft' of the human genome sequence is now available. Comparisons with the sequences of mouse and other species will be a powerful approach to identifying functional segments of the noncoding regions, such as gene regulatory elements. However, the choice of a species for most effective comparison differs among various loci.
Animals↗
Access to a syllabus of human hemoglobin variants (1996) via the World Wide Web.
Information on mutations in human hemoglobin is important in many efforts, including understanding the pathophysiology of hemoglobin diseases, developing therapies, elucidating the dynamics of sequence alterations inhuman populations, and dissecting the details of protein structure/function relationships. Currently, information is available on a large number of mutations and variants, but is distributed among thousands of papers. In an effort to organize this voluminous data set, two Syllabi have been prepared compiling succinct information on human hemoglobin abnormalities. In both of these, each entry provides amino acid and/or DNA sequence alterations, hematological and clinical data, methodology used for characterization, ethnic distribution, and functional properties and stability of the hemoglobin, together with appropriate literature references. A Syllabus of Human Hemoglobin Variants (1996) describes 693 abnormal hemoglobins resulting from alterations in the alpha-, beta-, gamma-, and delta-globin chains, including special abnormalities such as double mutations, hybrid chains, elongated chains, deletions, and insertions. We have converted this resource to an electronic form that is accessible via the World Wide Web at the Globin Gene Server (http://globin.cse.psu.edu). Hyperlinks are provided from each entry in the tables of variants to the corresponding full description. In addition, a simple query interface allows the user to find all entries containing a designated word or phrase. We are in the process of converting A Syllabus of Thalassemia Mutations (1997) to a similar electronic format.
Computer Communication Networks↗
Restoration of the CCAAT box or insertion of the CACCC motif activates [corrected] delta-globin gene expression.
Hemoglobin A2 (HbA2), which contains delta-globin as its non-alpha-globin, represents a minor fraction of the Hb found in normal adults. It has been shown recently that HbA2 is as potent as HbF in inhibiting intracellular deoxy-HbS polymerization, and its expression is therefore relevant to sickle cell disease treatment strategies. To elucidate the mechanisms responsible for the low-level expression of the delta-globin gene in adult erythroid cells, we first compared promoter sequences and found that the delta-globin gene differs from the beta-globin gene in the absence of an erythroid Krüppel-like factor (EKLF) binding site, the alteration of the CCAAT box to CCAAC, and the presence of a GATA-1 binding site. Second, serial deletions of the human delta-globin promoter sequence fused to a luciferase (LUC) reporter gene were transfected into K562 cells. We identified both positive and negative regulatory regions in the 5' flanking sequence. Furthermore, a plasmid containing a single base pair (bp) mutation in the CCAAC box of the delta promoter, restoring the CCAAT box, caused a 5.6-fold and 2.4-fold (P < .05) increase of LUC activity in transfected K562 cells and MEL cells, respectively, in comparison to the wild-type delta promoter. A set of substitutions that create an EKLF binding site centered at -85 bp increased the expression by 26.8-fold and 6.5-fold (P < .05) in K562 and MEL cells, respectively. These results clearly demonstrate that the restoration of either an EKLF binding site or the CCAAT box can increase delta-globin gene expression, with potential future clinical benefit.
Adult↗
Long human-mouse sequence alignments reveal novel regulatory elements: a reason to sequence the mouse genome.
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Agammaglobulinaemia Tyrosine Kinase↗
CpG islands from the alpha-globin gene cluster increase gene expression in an integration-dependent manner.
In contrast to other globin genes, the human and rabbit alpha-globin genes are expressed in transfected erythroid and nonerythroid cells in the absence of an enhancer. This enhancer-independent expression of the alpha-globin gene requires extensive sequences not only from the 5' flanking sequence but also from the intragenic region. However, the features of these internal sequences that are responsible for their positive effect are unclear. We tested several possible determinants of this activity. One possibility is that a previously identified array of discrete binding sites for known and potential regulatory proteins within the alpha-globin gene comprise an intragenic enhancer specific for the alpha-globin promoter, but directed rearrangements of the sequences show that this is not the case. Alternatively, the promoter may extend into the gene, with the function of the discrete binding sites being dependent on maintenance of their proper positions and orientations relative to the 5' flanking sequence. However, the positive effects observed in gene fusions do not localize to a discrete region of the alpha-globin gene and the results of internal deletions and point mutations argue against a required role of the targeted discrete binding sites. A third possibility is that the CpG island, which includes both the 5' flanking and intragenic regions associated with the positive activity, may itself have a more general effect on expression in transfected cells. Indeed, we show that the size of the CpG island in constructs correlates with the level of gene expression. Furthermore, the alpha-globin promoter is more active in the context of a previously inactive CpG island than in an A+T-rich context, showing that the CpG island provides an environment more permissive for expression. These effects are seen only after integration, suggesting a possible mechanism at the level of chromatin structure.
Animals↗
Sequences within and flanking hypersensitive sites 3 and 2 of the beta-globin locus control region required for synergistic versus additive interaction with the epsilon-globin gene promoter.
The locus control region is required for high-level, position-independent expression of mammalian beta-globin genes. It is marked by five major DNase hypersensitive sites (HSs) in a 16 kb region of chromatin, and the protein-DNA complexes that form these HSs may interact in a holocomplex that carries out the full function of the locus control region. Previous studies showed that a large rabbit DNA fragment containing both HS2 and HS3 in their native sequence context and spacing produced a much larger increase in expression of a linked reporter gene than the sum of the largest effects observed with DNA fragments containing HS2 or HS3 individually. To test whether this reflected a synergistic interaction between the 200-400 bp cores of the HSs or if this effect required additional sequences outside the cores, combinations of different restriction fragments containing HS2 or HS3 were tested for their ability to increase the expression of a hybrid epsilon-globin-luciferase reporter gene in transfected K562 cells. The results show that the human HS2 and HS3 cores do not interact either additively or synergistically with the reporter gene when juxtaposed, and separation by spacer DNA has little effect on their function. Fragments of human DNA containing cores plus flanking sequences for HS3 or HS2 show an additive effect in combination, whereas homologous fragments of rabbit DNA containing HS3 and HS2 interact synergistically. At least part of this difference localizes to the rabbit DNA fragment containing HS3, which can interact synergistically with the human DNA fragment containing HS2. The region 5' to the HS3 core plays a role both in the cooperative interaction observed with the rabbit DNA fragment and the domain-opening observed with the human DNA. A minor DNase HS maps to this region, and the pattern of sequence conservation is consistent with some difference in function between species.
Animals↗
A brief history of hemoglobins: plant, animal, protist, and bacteria.
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Animals↗
Chaining multiple-alignment blocks.
We derive a time-efficient method for building a multiple alignment consisting of a highest-scoring chain of "blocks," i.e., short gap-free alignments. Besides executing faster than a general-purpose multiple-alignment program, the method may be particularly appropriate when discovery of blocks meeting a certain criterion is the main reason for aligning the sequences. Utility of the method is illustrated by locating a chain of "phylogenetic footprints" (specifically, exact matches of length 6 or more) in the 5'-flanking regions of six mammalian epsilon-globin genes.
Algorithms↗
Recent developments in linear-space alignment methods: a survey.
A dynamic-programming strategy for sequence alignment first proposed in 1975 by Dan Hirschberg can be adapted to yield a number of extremely space-efficient algorithms. Specifically, these algorithms align two sequences using only "linear space," i.e., an amount of computer memory that is proportional to the sum of the lengths of the two sequences being aligned. This paper begins by reviewing the basic idea, as it applies to the global (i.e., end-to-end) alignment of two DNA or protein sequences. Three of our recent extensions of the technique are then outlined. The first extension computes an optimal alignment subject to the constraint that each position, i, of the first sequence must be aligned somewhere between positions L[i] and U[i] of the second sequence, for given values of L and U. The second finds all aligned position pairs (i.e., potential columns of the alignment) that occur in an alignment whose score exceeds a given threshold. The third treats the case where each of the two sequences is allowed to be an alignment (e.g., a sequence of aligned pairs), using a sensitive scoring scheme. We also describe two linear-space methods for computing k best local (i.e., involving only a part of each sequence) alignments, where k > or = 1. One is a linear-space version of the algorithm of Waterman and Eggert (1987), and the other is based on the strategy proposed by Wilbur and Lipman (1983). Finally, we describe programs that implement various combinations of these techniques to provide a multisequence alignment method that is especially suited to handling a few very long sequences. The utility of these programs is illustrated by analysis of the locus control region of the beta-like globin gene cluster of several mammals.
Algorithms↗
Constructing aligned sequence blocks.
This paper presents an efficient method for constructing aligned blocks (i.e., gap-free multiple alignments) from a set of pairwise alignments. The method is more sensitive than some earlier block-constructing methods for detecting conserved sequence regions. The technique is applied to analyze conserved regions in protein prenyltransferases and to detect regulatory elements in the 5' flank of the beta-globin gene.
Algorithms↗
Upstream regulatory region of the human embryonic beta-like globin gene, epsilon.
The sequence of the human embryonic beta-like globin gene (epsilon) upstream regulatory region has been reported previously. In the course of our own work, we found a significant number of discrepancies between our sequence and the data base sequence, which we show here to contain large clusters of errors within functional epsilon-globin regulatory domains.
Base Sequence↗
Constrained sequence alignment.
This paper presents a dynamic programming algorithm for aligning two sequences when the alignment is constrained to lie between two arbitrary boundary lines in the dynamic programming matrix. For affine gap penalties, the algorithm requires only O(F) computations time and O(M+N) space, when F is the area of the feasible region and M and N are the sequence lengths. The result extends to concave gap penalities, with somewhat increased time and space bounds.
Algorithms↗
Locating well-conserved regions within a pairwise alignment.
Within a single alignment of two DNA sequences or two protein sequences, some regions may be much better conserved than others. Such strong conservation may reveal a region that possesses an important function. When alignments are so long that it is infeasible, or at least undesirable, to inspect them in complete detail, it is helpful to have an automatic process that computes information about the varying degree of conservation along the alignment and displays the information in a graphical representation that is readily assimilated. This paper presents methods for computing several such 'robustness measures' at each position of a given alignment. These methods are all very space-efficient; they use only space proportional to the sum of the two sequence lengths. To illustrate their effectiveness, one of the methods is used to locate particularly well-conserved regions in the beta-globin gene locus control region and in the 5' flank of the gamma-globin gene.
Algorithms↗