Globin Gene Cluster Research Articles

Beta-Globin Gene Haplotypes in Manipur, North-east India

A sample of 204 individuals belonging to five different population units of Manipur, viz. the Meitei, Gangte, Thadou, other Kuki, and Kabui has been analyzed for beta globin gene cluster, considering seven loci to bring out a first hand information on haplotype frequencies of beta globin in the state. A common trend of allelic frequencies observed in the present data are the high frequencies of HindIIe, Avallβ and Hinflβ and relatively low frequencies of HindlllGγ, HindIIIAγ, HindII5'ψ β and HindII3'ψβ. The frequency of Framework 1(presence of both AvaIIβ and HinfIβ sites) ranges from nil to 17.65% in the present data of Manipur. The frequency of Framework 2 ranges from 33% to 66% and that of Framework 3 from 33% to 55%. One noteworthy observation is the association of HbβEE exclusively with Framework 2, which suggests a similar mutational origin of HbE in South-east Asia and North-east India. Only five beta globin haplotypes are found in the present data. Haplotypes +——+ and +——+-, the most common in Japanese, Cambodians, Koreans, South Chinese, Samoans and Thais, are the most frequent haplotypes in Manipur too.

Compound heterozygosity for KLF1 mutations associated with remarkable increase of fetal hemoglobin and red cell protoporphyrin

The persistence of high fetal hemoglobin level in adults may ameliorate the clinical phenotype of beta-thalassemia and sickle cell anemia. Several genetic variants responsible for hereditary persistence of fetal hemoglobin, linked and not linked to the beta globin gene cluster, have been identified in patients and in normal individuals. Monoallelic loss of KLF1, a gene with a key role in erythropoiesis, has been recently reported to be responsible for persistence of high levels of fetal hemoglobin. In a Sardinian family, high levels of HbF (22.1-30.9%) were present only in compound heterozygotes for the S270X nonsense and K332Q missense mutations, while the isolated S270X nonsense (haploinsufficiency) or K332Q missense mutation were associated with normal HbF levels (<1.5%). Functionally, the K332Q Klf1 mutation impairs binding to the BCl11A gene and activation of the γ- and β-globin promoters. Moreover, we report for the first time the association of KLF1 mutations with very high levels of zinc protoporphyrin.

Developmental regulation of hemoglobin synthesis in the green anole lizard Anolis carolinensis

Tetrapod vertebrates possess multiple α- and β-like globin genes that are ontogenetically regulated, such that functionally distinct hemoglobin (Hb) isoforms are synthesized during different stages of development. The α- and β-like globin genes of amphibians, birds and mammals are differentially expressed during embryonic development and postnatal life, but little is known about the developmental regulation of globin gene expression in non-avian reptiles. Here we report an investigation into the developmental regulation of Hb synthesis in the green anole lizard Anolis carolinensis. We tested two hypotheses derived from comparative genomic studies of the globin gene clusters in tetrapod vertebrates. First, we tested whether the product of the Anolis α(D)-globin gene is incorporated into embryonic Hb, thereby performing the role that would normally be performed by the embyronic α(E)-globin gene (which has been deleted from the green anole genome). Second, we tested whether two 'lizard-specific' β-globin paralogs have independently evolved a division of labor between an early-expressed embryonic gene and a later-expressed adult gene. Results of a proteomic analysis revealed that α- and β-like globin genes of the anole are differentially expressed during embryonic development. However, the same repertoire of α- and β-chain Hb isoforms was expressed during all stages of development and postnatal life, and the ontogenetic shifts in isoform composition were relatively subtle. In contrast to the pattern that has been documented in other tetrapod vertebrates, it appears that the developmental regulation of Hb synthesis in the green anole lizard does not involve discrete, stage-specific switches in gene activation and gene silencing.

What influences Hb fetal production in adulthood?

Human hemoglobin genes are located in α and β globin gene clusters in chromosomes 16 and 11, respectively. Different types of hemoglobin are synthesized according to the stage of development with fetal hemoglobin (α2γ2) (Hb F) being the main hemoglobin in the fetal period. After birth, there is a reduction (to about 1%) in Hb F levels and adult hemoglobin, Hb A (2α2β2), increases to more than 96% of total hemoglobin. However, some genetic conditions whether linked to the β-globin gene cluster or not are associated with high Hb F levels in adults. Among those linked to β-globin are hereditary persistence of fetal hemoglobin, delta-beta thalassemia (δβ-Thalassemia) and the XmnI polymorphism (-158 C = T). Other polymorphisms not related to β-globin gene cluster are known to influence the γ-globin gene expression in adulthood. The most relevant polymorphisms that increase concentrations of Hb F are the HMIP locus on chromosome 6, the BCL11A locus on chromosome 2, the Xp22.2 region of the X chromosome and the 8q region on chromosome 8. Findings from our research group studying genetic factors involved in γ-globin gene regulation in adults without anemia in the northwestern region of São Paulo State showed that high Hb F levels are influenced by the presence of hereditary persistence of fetal hemoglobin mutations and the XmnI polymorphism, suggesting that both genetic alterations characterize the molecular basis of the evaluated population.

Rapid targeting and isolation of the -like globin gene cluster fragment from AA, AS and SS genotypesusing BamHI restriction enzyme

The β-globin gene cluster contains the β-globin gene on which the substitution of the 17thnucleotide gives rise to sickle cell anemia and other β-globin gene variants with varying severity. Recent therapy measures focus on understanding the structure and mechanism of expression of the β-globin gene. In this study, we attempted to isolate a gene fragment containing the complete set of the human β-globin gene cluster from AA, AS and SS blood types, using the restriction enzyme BamHI which has a recognition site within the 19-kb 3’ cluster region downstream the β-globin gene cluster. DNA fragment size of 69.65 kb was generated by the enzyme from the genome of AS and SS blood types while the fragment generated from the AA blood type was 70.28 kb. Fragments generated using Bam HI was all comparable and close to the documented size of the β-globin gene cluster. In addition to the 69.65 kb fragment generated from the DNA from SS blood type, a novel 5.73 kb fragment was also visualized which was absent in the lanes containing restricted DNA from the AA blood type. Key words: BamHI, beta-globin, genotype, isolation.

ATRX: Put Me on Repeat

Mutations in the chromatin-remodeling protein ATRX cause alpha thalassaemia and mental retardation, but the severity of the disorder is independent of the specific mutation. In this issue of Cell, Law etal. (2010) demonstrate that ATRX alters gene expression by binding to G-rich tandem repeats, and the degree of transcriptional silencing caused by ATRX mutations correlates with the number of repeats.

Developmental Silencing of Human ζ-Globin Gene Expression Is Mediated by the Transcriptional Repressor RREB1

The mammalian embryonic zeta-globin genes, including that of humans, are expressed at the early embryonic stage and then switched off during erythroid development. This autonomous silencing of the zeta-globin gene transcription is probably regulated by the cooperative work of various protein-DNA and protein-protein complexes formed at the zeta-globin promoter and its upstream enhancer (HS-40). We present data here indicating that a protein-binding motif, ZF2, contributes to the repression of the HS-40-regulated human zeta-promoter activity in erythroid cell lines and in transgenic mice. Combined site-directed mutagenesis and EMSA suggest that repression of the human zeta-globin promoter is mediated through binding of the zinc finger factor RREB1 to ZF2. This model is further supported by the observation that human zeta-globin gene transcription is elevated in the human erythroid K562 cell line or the primary erythroid culture upon RNA interference (RNAi)(2) knockdown of RREB1 expression. These data together suggest that RREB1 is a putative repressor for the silencing of the mammalian zeta-globin genes during erythroid development. Because zeta-globin is a powerful inhibitor of HbS polymerization, our experiments have provided a foundation for therapeutic up-regulation of zeta-globin gene expression in patients with severe hemoglobinopathies.

Lineage-Specific Patterns of Functional Diversification in the - and -Globin Gene Families of Tetrapod Vertebrates

The alpha- and beta-globin gene families of jawed vertebrates have diversified with respect to both gene function and the developmental timing of gene expression. Phylogenetic reconstructions of globin gene family evolution have provided suggestive evidence that the developmental regulation of hemoglobin synthesis has evolved independently in multiple vertebrate lineages. For example, the embryonic beta-like globin genes of birds and placental mammals are not 1:1 orthologs. Despite the similarity in developmental expression profiles, the genes are independently derived from lineage-specific duplications of a beta-globin pro-ortholog. This suggests the possibility that other vertebrate taxa may also possess distinct repertoires of globin genes that were produced by repeated rounds of lineage-specific gene duplication and divergence. Until recently, investigations into this possibility have been hindered by the dearth of genomic sequence data from nonmammalian vertebrates. Here, we report new insights into globin gene family evolution that were provided by a phylogenetic analysis of vertebrate globins combined with a comparative genomic analysis of three key sauropsid taxa: a squamate reptile (anole lizard, Anolis carolinensis), a passeriform bird (zebra finch, Taeniopygia guttata), and a galliform bird (chicken, Gallus gallus). The main objectives of this study were 1) to characterize evolutionary changes in the size and membership composition of the alpha- and beta-globin gene families of tetrapod vertebrates and 2) to test whether functional diversification of the globin gene clusters occurred independently in different tetrapod lineages. Results of our comparative genomic analysis revealed several intriguing patterns of gene turnover in the globin gene clusters of different taxa. Lineage-specific differences in gene content were especially pronounced in the beta-globin gene family, as phylogenetic reconstructions revealed that amphibians, lepidosaurs (as represented by anole lizard), archosaurs (as represented by zebra finch and chicken), and mammals each possess a distinct independently derived repertoire of beta-like globin genes. In contrast to the ancient functional diversification of the alpha-globin gene cluster in the stem lineage of tetrapods, the physiological division of labor between early- and late-expressed genes in the beta-globin gene cluster appears to have evolved independently in several tetrapod lineages.

Clinical, hematologic and molecular variability of sickle cell-β thalassemia in western India

BACKGROUND:Sickle cell-β thalassemia (HbS-β thalassemia) is a sickling disorder of varying severity, which results from compound heterozygosity for sickle cell trait and β thalassemia trait. The present study was undertaken to determine the genetic factors responsible for the clinical variability of HbS-β thalassemia patients from western India.MATERIALS AND METHODS:Twenty-one HbS-β thalassemia cases with variable clinical manifestations were investigated. The α and β globin gene clusters were studied by molecular analysis.RESULTS:Thirteen patients showed milder clinical presentation as against eight patients who had severe clinical manifestations. Four β thalassemia mutations were identified: IVS 1-5 (G→C), codon 15 (G→A), codon 30 (G→C) and codon 8/9 (+G). α thalassemia and XmnI polymorphism in homozygous condition (+/+) were found to be common among the milder cases. The βS chromosomes were linked to the typical Arab-Indian haplotype (#31). Framework (FW) linkage studies showed that four β thalassemia mutations were associated with different β globin gene frameworks. Linkage of codon 15 (G→A) mutation to FW2 is being observed for the first time.CONCLUSION:The phenotypic expression of HbS-β thalassemia is not uniformly mild and α thalassemia and XmnI polymorphism in homozygous condition (+/+) are additional genetic factors modulating the severity of the disease in the Indian subcontinent.

Multiplex ligation‐dependent probe amplification screening of isolated increased HbF levels revealed three cases of novel rearrangements/deletions in the β‐globin gene cluster

Investigations of naturally occurring mutations, such as the deletional thalassaemias and hereditary persistence of fetal haemoglobins (HPFHs), have brought many insights into human globin switching, but limited data have been reported so far. We selected 15 individuals with elevated fetal haemoglobin (HbF) levels (>5%) from a previous screening of 27 006 Korean individuals and analysed dosage changes of the globin gene cluster using multiplex ligation-dependent probe amplification (MLPA). Dosage changes detected by the MLPA probes were followed up with gap-polymerase chain reaction and sequence analysis. Three subjects were found to have deletions in the globin gene cluster, including a beta-thalassaemia due to deletion of HBB (beta-globin gene), an HPFH due to deletions of HBD (delta-globin gene) and HBB, and an HPFH due to a novel HBG2-HBG1 fusion gene consisting of exons 1 and 2 of HBG2 ((G)gamma-globin gene) and exon 3 of HBG1 ((A)gamma-globin gene). The case with the HBG2-HBG1 fusion suggested the existence of another mechanism for the reactivation of HBG2 and HBG1. The IVS2 of HBG2 and HBG1might play a role in HbF regulation, and combinations of specific polymorphisms could influence the reactivation of these genes in adults.

Association of globin gene quadruplication and heterozygous thalassemia in patients with thalassemia intermedia

Ten patients with thalassemia intermedia with variable severity and apparent simple heterozygosis for beta0 39 C>T nonsense mutation were submitted to clinical, hematologic and molecular studies. The presence of an unknown molecular defect (silent beta-thalassemia) unlinked to the beta cluster interacting with the heterozygous beta thalassemia, was previously postulated in these families. Analysis of the alpha globin gene cluster with PCR-based methods (MLPA, GAP-PCR, digestion with restriction enzymes) detected complex rearrangements in the alpha cluster. A duplication of the alpha globin gene locus, including the upstream regulatory region, was present in all the patients, associated in some of them with deletion or non-deletion alpha thalassemia. The variability of the clinical phenotype correlates with the degree of the globin chain imbalance. The presence of alpha globin cluster duplication should be considered in patients heterozygote for beta-thalassemia with thalassemia intermedia phenotype and in the carriers of suspected silent beta thalassemia.

P18.07: Managing severe fetal anemia due to a new form of εγγδβ thalassemia in a Nordic family: intrauterine transfusions prior to final diagnosis

This case highlights the need to consider diagnoses of rare conditions not normally associated with a particular population. A hydropic fetus presented at 29 + 1 weeks with extensive ascites, pericardial fluid, enlarged heart and hyperkinetic circulation. Maximum velocity in the middle cerebral artery was 76 cm/s. The etiology of the fetal anemia was unknown. Both parents were of Nordic origin. The family history revealed that the father was born with anemia, in need of neonatal transfusions. No conclusive explanation for his anemia was found. His physical and neurological development was normal, although the microcytic anemia persisted. The family history made us consider a hereditary anemia in the fetus. The first cordocentesis revealed a microcytic anemia with Hb 5.8 g/dl, Hct 0.19, MCV 91 fl. Six intrauterine transfusions, until 34 completed weeks, were performed. The female infant was delivered by Cesarean section at 36 weeks. Her initial Hb level (transfused) was 14.1 g/dl; at one month of age her Hb was 8.8 g/dl, MCV 78 fl. She was transfused at this stage and again at about 2 months; since then her Hb has been stable at about 9 g/dl. She is healthy and her physical and neurological development has been normal. In the father, a deletion of the 5′ end of the β globin gene cluster was characterised, the breakpoints sequenced and a new type of eγγδβ thalassemia identified. Analysis of the daughter’s DNA by MPLA and Southern blotting confirmed that she had inherited the deletion chromosome from her father. The clinical presentation of eγγδβ thalassemia may vary but neonatal transfusions are frequently required due to marked neonatal anemia. A thorough family history may provide the suspicion of the potential diagnosis. Intrauterine transfusions should always be considered while further investigation takes place. A correct diagnosis provides a probable prognosis and also allows early prenatal diagnosis and optimal surveillance in future pregnancies.

Genomic organization of zebra finch alpha and beta globin genes and their expression in primitive and definitive blood in comparison with globins in chicken

How alpha and beta globin genes are organized and expressed in amniotes is of interest to researchers in a wide variety of fields. Data regarding this from avian species have been scarce. Using genomic and proteomic approaches, we present here our analysis of alpha and beta globins of zebra finch, a passerine bird. We show that finch alpha globin gene cluster has three genes (alphas 1-3), each orthologous to its chicken counterpart. Finch beta globin gene cluster has three genes (betas 1-3), with an additional pseudogene at the 3' end. Finch beta3 is orthologous to chicken betaA, but the orthology of beta1 and beta2 to chicken counterparts is less clear. All six finch globins are confirmed to encode functional proteins. Gene expression in both globin gene clusters is regulated developmentally. Adult finch blood has a globin profile similar to that of adult chicken, with high levels of beta3 and alpha3 and moderate levels of alpha2. Finch embryonic primitive blood exhibits a globin profile very different from that of equivalent stage chick embryos, with all six globins expressed at high levels. Overall, our data provide a valuable resource for future studies in avian globin gene evolution and globin switching during erythropoietic development.

Severe intrauterine anemia: a new form of thalassemia presenting in utero in a Norwegian family

Severe intrauterine anemia of unknown cause presents a diagnostic challenge. We describe a Norwegian case, managed successfully by intrauterine transfusions, that further investigations demonstrated to be due to a rare type of thalassemia. A deletion of the 5' end of the beta globin gene cluster was characterized, the breakpoints sequenced and a new type of epsilongammagammadeltabeta thalassemia identified. This case highlights the need to consider diagnoses of rare conditions not normally associated with a particular population.

The Organization in Micro-Loops of an Extended Fragment of Chicken Chromosome 14, Including the Alpha Globin Gene Cluster in the Erythroid Cells

It has been shown that the activation of tissue-specific gene transcription during the course of cell differentiation is associated with a spatial reorganization of the genomic domains harboring those specific genes. This reorganization consists of the relocation to the nuclear matrix of the whole genomic domain containing one or more of the genes being transcribed. However, it remains unclear whether, during this process, extended areas of the genome also become attached to the nuclear matrix. We studied the genome´s pattern of interaction with the nuclear matrix in both erythroid and non-erythroid cells of chickens, using a 220Kb region of chromosome #14, which contains the alpha-globin gene cluster and some surrounding house-keeping genes. The results show that in erythroid cells, the fragment of the genome containing the alpha-globin gene domain became spatially arranged into micro-loops which could not be detected by mapping experiments.

Proteomic analysis of plasma protein in β‐thalassemia/HbE patients treated with curcumin

Thalassemia is a group of genetic disorders resulting from different mutations in the globin gene clusters and leading to an imbalance in globin synthesis. Unpaired globin chains are less stable and are susceptible to oxidation. Patients with beta‐thalassemia/HbE (β‐thal/HbE) are prone to increase oxidative stress as indicated by increased lipid peroxidation and protein oxidation. Curcumin, a strong antioxidant and iron chelator, has been shown to decrease oxidative stress in β‐thal/HbE patients. In this study, the effect of curcumin on plasma protein expression was investigated using proteomic analysis in untreated patients as compared to normal subjects and in patients before and after supplementing with 500 mg curcumin daily for 12 months. Plasma proteins involved in blood coagulation and hemostasis e.g. prothrombin, fibrinogen, hemopexin, and haptoglobin were significantly decreased in patients, particularly in splenectomized patients. The administration of curcumin led to significant increase of prothrombin, fibrinogen, and haptoglobin. Our results provide strong evidence for the existence of a chronic hypercoagulable state in thalassemia and the patients responded to curcumin supplement as shown by a rise in the level of procoagulant proteins. Therefore, the administration of curcumin could be beneficial for thalassemic patients as adjunct therapy to decrease hypercoagulable state.

HBA Mutations Detected By MLPA In Beta-Thalassemia Patients

Thalassemia syndromes are the most common monogenic disorders in humans and b-thalassemia major is one of the most important public health problems in Turkey. Many genetic factors play roles in modifying the disease severity such as mutations in the alpha globin gene (HBA), UGT1 gene (bilirubin metabolism) or HFE gene (iron metabolism). Deletions or duplications in HBA gene determine the accumulation of a chain in b-thalassemia major patients. The human alpha globin gene cluster is located on chromosome 16 and deletions or duplications are the most important mutations. Multiple ligation dependent probe amplification (MLPA) technique is a simple and rapid technique and used in many genetic disorders in which deletions and duplications are common. In this study we used a commercial MLPA kit for detecting alpha globin gene mutations. The kit contains 24 different probes in the HBA region. In this study we aimed to investigate the incidence of alpha globin gene mutations using MLPA technique in 19 b-thalassemia major patients which were diagnosed in our department.Alpha globin gene deletions were detected in 2 out of 19 (10.5%) b-thalassemia major patients. In one of the patients the deletion was located between the HBA2 gene and HBA1P gene. The deletion detected in the other patient was located between the HBA1 gene and HBA2 gene. This result suggests that alpha globin gene mutations in b-thalassemia major patients are not rare. MLPA technique is suitable to investigate the mutations in this complex gene cluster.

Depletion of L3MBTL1, a PcG Gene Deleted in Patients with 20q-Associated with Hematologic Malignancies, Accelerates Erythroid Differentiation of Hematopoietic Progenitor Cells

L3MBTL1 is a polycomb gene located in 20q12, within the common deleted region identified in patients with 20q deletion associated with polycythaemia vera (PV), myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML). L3MBTL1 is expressed within CD34+ haematopoietic progenitor cells from which myeloid malignancies arise and its Drosophila homologue encodes a tumour suppressor protein. L3MBTL1 represents then a candidate target gene in 20q deletion patients. To gain insight into the role of L3MBTL1 in hematopoiesis we knocked down the level of L3MBTL1 mRNA through lentiviral expression of short hairpin RNAs in CD34+ hematopoietic stem/progenitor cells isolated from human cord blood (CB) cells. We achieved an approximately 80% drop in endogenous L3MBTL1 mRNA level, as determined using quantitative RT-PCR. Transduced and sorted CD34+ GFP+ CB cells were plated in liquid cultures and induced to cytokine-driven differentiation. The effect of L3MBTL1-knock down was assessed by colony assays and by fluorescence-activated cell sorting (FACS), using lineage–specific cell surface markers. We demonstrate that the knock down (KD) of L3MBTL1 remarkably accelerates the differentiation of hematopoietic CD34+ cells into erythrocytes. The relative percentage of mature erythroid precursors cells, defined as CD71+ and Glycophorin A+ cells, consistently increased in the L3MBTL1-KD population, as the CD34+ cells reached this advanced stage of erythroid differentiation in fewer days than control cells. As confirmation, Giemsa staining after cytospin preparations of L3MBTL1-KD cells showed more mature morphology compared to the control cells, and benzidine staining revealed many more Hb containing-positive cells in the L3MBTL1-depleted cell population compared to the control. Monitoring of globin gene expression demonstrated that L3MBT-Knock down is involved in the regulation of only a subset of these genes, primarily the expression of the epsilon and zeta globin genes, the embryonic globin genes belonging to the beta and alpha globin gene clusters respectively. In addition to these effects a significant slowing of proliferation was seen, which likely reflects the increased differentiation of these cells. We have previously identified a role for L3MBT in binding histones H1 and H4 that contain monomethylated and dimethylated lysine residues H1k2b and H4k20 (Kalakonda et al. 2008) and compacting chromatin (Trojer et al. Cell 2007). Precisely how this feature of L3MBTL1 function in regulating the erythroid differentiation of hematopoietic cells is unknown. We however link lack of L3MBTL1 with the possible pathogenesis of PV associated with 20q deletion. We will present additional data that attempt to define the kinetics of alpha-like and beta-like globin gene expression during the erythroid maturation of L3MBTL1-KD cells in culture and to investigate a supposed synergism with the known molecular pathways of normal and malignant erythroid differentiation.

Hemoglobin research and the origins of molecular medicine

AbstractMuch of our understanding of human physiology, and of many aspects of pathology, has its antecedents in laboratory and clinical studies of hemoglobin. Over the last century, knowledge of the genetics, functions, and diseases of the hemoglobin proteins has been refined to the molecular level by analyses of their crystallographic structures and by cloning and sequencing of their genes and surrounding DNA. In the last few decades, research has opened up new paradigms for hemoglobin related to processes such as its role in the transport of nitric oxide and the complex developmental control of the α-like and β-like globin gene clusters. It is noteworthy that this recent work has had implications for understanding and treating the prevalent diseases of hemoglobin, especially the use of hydroxyurea to elevate fetal hemoglobin in sickle cell disease. It is likely that current research will also have significant clinical implications, as well as lessons for other aspects of molecular medicine, the origin of which can be largely traced to this research tradition.

In embryonic chicken erythrocytes actively transcribed alpha globin genes are not associated with the nuclear matrix

The spatial organization of a 250 Kb region of chicken chromosome 14, which includes the alpha globin gene cluster, was studied using in situ hybridization of a corresponding BAC probe with nuclear halos. It was found that in non-erythroid cells (DT40) and cultured erythroid cells of definite lineage (HD3) the genomic region under study was partially (DT40 cells) or fully (HD3 cells) associated with the nuclear matrix. In contrast, in embryonic red blood cells (10-day RBC) the same area was located in the crown of DNA loops surrounding the nuclear matrix, although both globin genes and surrounding house-keeping genes were actively transcribed in these cells. This spatial organization was associated with the virtual absence of RNA polymerase II in nuclear matrices prepared from 10-day RBC. In contrast, in HD3 cells a significant portion of RNA polymerase II was present in nuclear matrices. Taken together, these observations suggest that in embryonic erythroid cells transcription does not occur in association with the nuclear matrix.