The amounts and types of human Hb produced at any given age are determined by the selective expression of the individual genes encoding each globin chain. The globin genes of humans are located in two clusters (Fig. 1): α-like genes in approximately 30 kb of DNA on the short arm of chromosome 16 between band p13.2 and the telomere and β-like genes in approximately 70 kb of DNA on the terminal portion of the short arm of chromosome 11 (p15). Each gene shares certain basic organizational features. Each contains three exons separated by two introns. Both introns of the α-gene are small (100 bp to 300 bp); non-α-genes have one small and one large (1000 bp to 1200 bp) intron. The second exon of each globin gene encodes the major components of the heme-binding pocket, and the third encodes the α and non-α contact points.

Fig1. MAPS OF THE Β-LIKE AND Α-LIKE GLOBIN GENE CLUSTERS LOCATED ON CHROMOSOME 11 (A) AND CHROMOSOME 16 (B). Within each gene cluster are pseudogenes, which are remnants of previously expressed globin genes that have become inactivated as a result of mutation. Active genes are shown in red boxes filled with clear introns; inactive or pseudogenes genes are shown in black boxes, and the θ-globin gene is shown as a pink box. Although this gene is transcribed, it is not clear whether it is represented in a cellular protein. The distance between the functional ζ-globin and pseudo-ζ-globin gene is variable because of the presence of repeated elements. E, Enhancer; HS, DNase hypersensitive site; S, silencer.

Flanking each gene at the 5′ and 3′ ends are groups of conserved nucleotides. In conjunction with protein factors, these influence the promotion of gene transcription, ensure the fidelity of the transcript and its translatability, specify sites for the initiation and termination of translation, and improve the stability of the newly synthesized mRNA (Fig. 2). Also encoded within the genes are signals that permit the enzymatic machinery within the nucleus to excise precisely the introns from the mRNA precursor and splice together the exons to form a contiguous “mature” mRNA. The spliced mRNA is transported to the cytoplasm and translated into protein. These conserved signals lie at the junction of the exon and intron and within the introns themselves. They are recognized by small nuclear ribonucleoprotein particles, which participate in the formation of a spliceosome, or splicing complex. Their preservation is critical for the splicing process to occur. When mutations occur within splice signal sites, globin synthesis is often impaired. The 5′ end of the mRNA contains a cap structure, and the 3′ end contains a poly(A) tail. Chapter 4 provides a detailed explanation of the processes of pre-mRNA splicing, capping, polyadenylation, exporting to the cytoplasm and stabilization, and translation.

Fig2. PATHWAY OF GLOBIN BIOSYNTHESIS. Transcription of the globin gene results in a large pre-mRNA molecule containing intervening sequences. During intranuclear processing of this molecule, the intervening sequences are excised and the coding sequences ligated to form a contiguous stretch of RNA, which codes for the globin protein. The message is further processed by the addition of a CAP and a poly(A) tail. The mature message is transported from the nucleus to cytoplasm, where it is translated on poly ribosomes by the addition of activated amino acids to a growing polypeptide chain. Globin acquires heme and α: non-α dimers are formed and a hemoglobin tetramer is assembled. (Reproduced with permission from Steinberg, MH. Hemoglobinopathies and thalassemias. In: Stein JH, ed. Internal Medicine. 4th ed. St. Louis: Mosby-Year Book; 1994:852.)

Conserved nucleotide clusters located 5′ to the coding portion of each globin gene in the aggregate act as promoters (see Fig. 2). Globin promoters are modular. Some modules are located relatively close to the initiation site of mRNA translation, and some are more distally placed. Promoters ultimately form the binding sites for the RNA polymerase complexes that catalyze gene transcription. Mutations within the promoter can affect the level of gene transcription and the amount of globin made. Surrounding and within each gene are other sequence elements that play important roles in its transcriptional regulation (see Fig. 1). These clusters, called enhancers and silencers, may lie within introns or 5′ and 3′ to the coding sequences; in some instances, they are quite remote from the gene. The higher-order structure of DNA in chromatin may permit close approximation of these remote enhancers to the gene during transcription by forming loops that bring otherwise distant sequences together spatially. Enhancers play important roles in the tissue-specific regulation of globin gene expression. Representative regulatory sequences near the globin genes are shown in Fig. 1 (enhancer-like element). DNA elements controlling globin genes are described in more detail later.

The α-like and β-like globin genes are ordered in the 5′ to 3′ direction in the same sequence expressed during embryonic, fetal, and adult development (Fig. 3). The functional significance of this arrangement is unclear. However, evidence suggests that the ordering of the ε, γ, δ, and β genes could be an important factor influencing the ability of each locus to interact with distant control elements at different developmental stages.

Fig3. HEMOGLOBIN (HB) SWITCHING DURING EMBRYONIC, FETAL, AND ADULT DEVELOPMENT. The ζ and ε genes are transcribed during embryonic development and are soon replaced by the fetal γ-globin and adult α-globin gene. At birth, fetal Hb (HbF) forms approximately 75%, and HbA forms 25% of the total. Transcription of the γ gene begins to decrease before birth, and by 6 months of age, this gene is expressed only at very low levels. Expression of the δ-globin gene begins near birth. In adults, HbA makes up approximately 97%, HbA2 approximately 2.5%, and HbF less than 1% of the total. (Reproduced with permission from Steinberg MH. Hemoglobinopathies and thalassemias. In: Stein JH, ed. Internal Medicine. 4th ed. St. Louis: Mosby-Year Book; 1994:852.)

The α-like and β-like gene clusters probably are the result of an ancient duplication of a primordial globin gene that existed early in the history of vertebrates, approximately 500 million years ago. Each gene cluster probably developed from the duplication of ancestral genes and subsequent divergence through eons of evolution. Within the α-like gene cluster, the ζ-globin gene (HBZ) is expressed only very early in embryogenesis and participates in the formation of embryonic Hbs. A μ, α-like globin gene (HBM), originally considered a pseudogene (ψα2), codes for a 141 amino acid α-globin–like chain, is expressed in erythroid cells in a highly regulated fashion; however, an associated protein has not been found.

The α-globin genes (HBA2, HBA1) are duplicated, and their encoded amino acid sequences are identical; therefore, only a single α-globin polypeptide results. Minor differences within the second intervening sequence and the 3′ flanking regions of the α-globin gene permit identification of transcripts from each gene. The 5′ or α2-gene is expressed more efficiently than the 3′ or α1-gene, so abnormalities of this gene are more likely to be clinically apparent. Both clusters contain genes that are actively transcribed, as well as pseudogenes whose defective structures prohibit expression at any time.

The gene 3′ to the α1-gene is the Θ-gene (HBQ1), a somewhat mysterious element of the α-gene cluster. Although Θ-gene transcripts are found in fetal tissue and adult erythroid marrow, it is unclear whether this gene’s translation product is able to participate in the formation of a functional tetramer. The Θ-globin protein has been found in vivo, but deletion of the Θ-globin gene does not appear to have any implications for developing fetuses. In vitro, Θ-globin mRNA is correctly spliced, and Θ-globin cDNA can direct the syn thesis of a translatable mRNA and a Θ-globin protein.

The β-like–globin gene cluster consists of the embryonic ε-gene (HBE), transcribed only during the first 6 to 11 weeks of life; the duplicated γ-globin genes (HBG2, HBG1) that code for the dominant non-α-globin of fetal life; and the δ- (HBD) and β-globin (HBB) genes that code for the Hbs of adults. The coding sequences of the two γ-globin genes are identical, except at codon 136, where the 5′ or Gγ-gene codes for glutamic acid; the 3′ or Aγ-gene encodes an alanine residue. These genes are unequally expressed during fetal development. A switch in their relative rates of expression leads to a similar disparity between the amounts of Gγ and Aγ chains in adults. Although the Gγ/Aγ switch is interesting from the standpoint of the control of gene expression, it is of little clinical importance. HbF in fetuses and adults contains a mixture of Gγ and Aγ chains; the functional qualities of these Hbs are identical.

The δ- and β-globin genes are probably the result of a duplication event that occurred more than 40 million years ago. The β-globin gene has become the predominant gene, coding for most non-α globin chains of adults. The δ-globin gene promoter has undergone mutation in several critical areas, and its expression is greatly cur tailed. Its product, a minor fraction of adult Hb (HbA2 ), has become functionally insignificant by virtue of its very low level in the erythrocyte. It is likely that the δ-globin gene is a “pseudogene in evolution.” HbA2 is clinically useful, however, for characterizing hemoglobinopathies such as β-thalassemia.

The pseudogenes dispersed within both globin gene clusters provide interesting glimpses into the evolutionary history of globin genes. Pseudogenes are inactive remnants of previously expressed genes. As a result of relaxed selection, their mutation rates are higher than those of surrounding active genes. Because of this, the expression of the δ-globin gene might be totally abolished as it acquires an inactivating mutation.

The expression of the human globin genes is highly regulated. Until recently, globin was thought to be synthesized in only one tis sue—erythroid cells—and only during a narrowly defined stage of erythroid progenitor cell differentiation—the 5 to 7 days that commence with the proerythroblast stage and end when the enucleated reticulocyte loses the last traces of its RNA. Recently, however, evidence has been found for the expression of small amounts of α-globin in the myoendothelial junctions of the endothelium. Its presence there may be involved in NO regulation of vascular tone. Within the confines of these tissue-specific and differentiation stage-specific boundaries, the globin genes are extraordinarily active in erythroblasts. By the late normoblast and reticulocyte stages, 90% to 95% of all protein synthesis in these cells is globin synthesis.

Individual globin genes are expressed at different levels in developing erythroblasts of human embryos, fetuses, and “adults” (i.e., 37 to 38 weeks of gestation and beyond). Different subsets of α-genes and non-α-genes are expressed and silenced at each developmental stage. Moreover, the overall balance of non-α-globin, α-globin, and heme production is maintained throughout each of these complex switching events. The complex mechanisms ensuring the proper tissue-specific, differentiation stage-specific, and ontologic stage-specific expression are incompletely defined. However, much information about relevant DNA control elements and transcription factors is emerging. These topics are discussed after a review of the ontogeny of Hb.

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العتبة العباسية المقدسة تشهد تشييع جثمان المحقق الشيخ محمد رضا المامقاني

قسم الشؤون الفكرية يختتم برنامج (فتية سيد الماء) لرعاية الأيتام بنسخته الثانية

لتطوير المهارات الإدارية.. العتبة العباسية المقدسة تطلق الدورة التعليمية الأولى لمسؤولي وحداتها

قسم التطوير يقيم دورة تدريبية عن الظهور الإعلامي لملاكاته

المزيد

الآخبار الصحية

دراسة تكشف "السبب الخفي" وراء ترك التمارين الرياضية

كشف التوقيت "المثالي" لتغيير فرشاة الأسنان

بلاستيك في أنسجة بشرية.. دراسة تكشف مفاجأة طبية

دراسة تحذر من الإفراط في ممارسة الرياضة.. كيف يضر دماءك؟

المزيد

الآخبار التكنلوجية

"ديب سيك" تحجب أحدث نماذجها عن شركات الرقائق الأميركية

"سامسونغ" تطلق سلسلة هواتف "غالاكسي إس 26"

حملات سرية ورسائل تشويه.. كيف يتم استغلال "تشات جي بي تي"؟

أثر نفسي غير متوقع للرياضة.. مرتبط بالغضب

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مواضيع ذات صلة

Molecular Analysis of Nucleic Acid Sequences

Genes and Genome Complexity

Structure of Nucleic Acids

Genomics: Genome-Wide Analysis of Gene Structure and Function

Chromosomal Organization of Genes and Noncoding DNA

RNA Interference Causes Gene Inactivation by Destroying the Corresponding mRNA

Linkage Studies Can Map Disease Genes with a Resolution of About 1 Centimorgan

DNA Polymorphisms Are Used as Markers for Linkage Mapping of Human Mutations

The Two- Hit Theory of Tumor Suppressor Gene Inactivation in Cancer

DNA Microarrays Can Be Used to Evaluate the Expression of Many Genes at One Time

Hybridization Techniques Permit Detection of Specific DNA Fragments and mRNAs

Cloned DNA Molecules Can Be Sequenced Rapidly by Methods Based on PCR