Spermatogenesis as a model system for developmental analysis of regulatory mechanisms associated with tissue-specific gene expression

Sequence-specific promoter elements regulate temporal-specific changes in chromatin required for testis-specific activation of the Pgk2 gene

The phosphoglycerate kinase-2 (Pgk2) gene is regulated in a tissue-, cell type-, and developmental stage-specific manner during spermatogenesis and is required for normal sperm motility and fertility in mammals. Activation of Pgk2 transcription is regulated by testis-specific demethylation of DNA and binding of testis-specific transcription factors to enhancer and core promoter elements. Here, we show that chromatin remodeling including reconfiguration of nucleosomes and changes in histone modifications is also associated with transcriptional activation of the Pgk2 gene during spermatogenesis. Developmental studies indicate that the order of events involved in transcriptional activation of the Pgk2 gene includes demethylation of DNA in T₁- and T₂-prospermatogonia, binding of a factor to the CAAT box in type A and B spermatogonia, followed by recruitment of chromatin remodeling factors, displacement of a nucleosome from the Pgk2 promoter region, binding of factors to the Pgk2 core promoter and enhancer regions, and, finally, initiation of transcription in primary spermatocytes. Transgene studies show that Pgk2 core promoter elements are required to direct demethylation of DNA and reconfiguration of nucleosomes, whereas both enhancer and core promoter elements are required to direct changes in histone modifications and initiation of transcription. These results provide novel insight into the developmental order of molecular events required to activate tissue-specific transcription of the Pgk2 gene, the distinct elements in the 5'-regulatory region of the Pgk2 gene that regulate each of these events, and the relationship among these events in that each step in this process appears to be a necessary prerequisite for the subsequent step.

Microarray-based analysis of cell-cycle gene expression during spermatogenesis in the mouse

Mammalian spermatogenesis is a continuum of cellular differentiation in a lineage that features three principal stages: 1) a mitotically active stage in spermatogonia, 2) a meiotic stage in spermatocytes, and 3) a postreplicative stage in spermatids. We used a microarray-based approach to identify changes in expression of cell-cycle genes that distinguish 1) mitotic type A spermatogonia from meiotic pachytene spermatocytes and 2) pachytene spermatocytes from postreplicative round spermatids. We detected expression of 550 genes related to cell-cycle function in one or more of these cell types. Although a majority of these genes were expressed during all three stages of spermatogenesis, we observed dramatic changes in levels of individual transcripts between mitotic spermatogonia and meiotic spermatocytes and between meiotic spermatocytes and postreplicative spermatids. Our results suggest that distinct cell-cycle gene regulatory networks or subnetworks are associated with each phase of the cell cycle in each spermatogenic cell type. In addition, we observed expression of different members of certain cell-cycle gene families in each of the three spermatogenic cell types investigated. Finally, we report expression of 221 cell-cycle genes that have not previously been annotated as part of the cell cycle network expressed during spermatogenesis, including eight novel genes that appear to be testis-specific.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

Molecular cloning of a novel nuclear factor, TDRP1, in spermatogenic cells of testis and its relationship with spermatogenesis

We reported the identification of a novel gene termed TDRP (encoding testis development-related protein) that might be involved in spermatogenesis. The human TDRP gene had two distinct transcripts, TDRP1 and TDRP2, which encoded proteins of 183 aa and 198 aa respectively. Tdrp mRNA was predominantly expressed in testis tissue. We generated rabbit polyclonal antibodies specific against human TDRP1. Immunohistochemistry analysis showed TDRP1 was expressed in spermatogenic cells, especially with high expression in spermatocytes. We provided evidence that TDRP1 distributed in both cytoplasm and nuclei of spermatogenic cells. Expression patterns of Tdrp1 mRNA and its protein were investigated in the rat testis tissues of different developmental stages. Both Tdrp1 mRNA and its protein were barely detected in the testis of neonatal rats, increased remarkably at 3weeks postpartum, and peaked at 2months postpartum. We also investigated TDRP1 expressions in testis tissues of azoospermic men with defective spermatogenesis. Western blot analysis showed that TDRP1 expressions were significantly lower in the testis tissues of azoospermic men compared with normal controls. These current data demonstrated that as a nuclear factor, TDRP1 might play an important role in spermatogenesis.

Characterization of Spindlin1 isoform2 in mouse testis

AIM

To investigate the expression of Spindlin 1 (Spin 1) isoform2 and assess its function in mouse testis.

METHODS

First, reverse-transcription polymerase chain reaction (RT-PCR) was used to determine whether Spin1 isoform2 is present in mouse testis. Then the expression patterns of the isoform between newborn and adult mice testes were compared by immunoblot analysis. Finally, the diversity of its localization in mice testes at different ages (days 0, 7, 14, 21, 28 and 60) was observed by immunohistochemistry. The localization of the protein in mouse sperm was also investigated by immunofluorescence.

RESULTS

The RT-PCR results show that Spin1 isoform2 is present in mouse testis. As shown by immunoblot analysis, the isoform was more highly expressed in adult testes compared with newborn testes. Interestingly, Spin1 isoform2 did not show up in the cytoplasm of primary spermatocytes until day 14. Also, the protein exists at the tail of the mouse sperm.

CONCLUSION

Spin1 isoform2 is a protein expressed highly in adult testis, which might be involved in spermatogenesis and could be necessary for normal sperm motility.

In vivo analysis of developmentally and evolutionarily dynamic protein-DNA interactions regulating transcription of the Pgk2 gene during mammalian spermatogenesis

Transcription of the testis-specific Pgk2 gene is selectively activated in primary spermatocytes to provide a source of phosphoglycerate kinase that is critical to normal motility and fertility of mammalian spermatozoa. We examined dynamic changes in protein-DNA interactions at the Pgk2 gene promoter during murine spermatogenesis in vivo by performing genomic footprinting and chromatin immunoprecipitation assays with enriched populations of murine spermatogenic cells at stages prior to, during, and following transcription of this gene. We found that genes encoding the testis-specific homeodomain factor PBX4 and its coactivator, PREP1, are expressed in patterns that mirror expression of the Pgk2 gene and that these factors become bound to the Pgk2 enhancer in cells in which this gene is actively expressed. We therefore suggest that these factors, along with CREM and SP3, direct stage- and cell type-specific transcription of the Pgk2 gene during spermatogenesis. We propose that binding of PBX4, plus its coactivator PREP1, is a rate-limiting step leading to the initiation of tissue-specific transcription of the Pgk2 gene. This study provides insight into the developmentally dynamic establishment of tissue-specific protein-DNA interactions in vivo. It also allows us to speculate about the events that led to tissue-specific regulation of the Pgk2 gene during mammalian evolution.

Biology of sperm chromatin structure and relationship to male fertility and embryonic survival

Embryonic mortality in mammals is typically thought to result from 'female factor' infertility. There is growing evidence, however, that the status of sperm chromatin (DNA) at the time of fertilisation can also influence embryonic survival. During the final stages of spermatogenesis (spermiogenesis) a number of unique biochemical, morphological and physiological processes take place that are associated with marked changes in the structure of sperm chromatin. In early stages of spermatogenesis, sperm DNA is associated with histone nucleoproteins and structured into classical nucleosome core particles similar to other somatic cells. As spermiogenesis proceeds, the histone nucleoproteins are replaced by transition proteins which are subsequently replaced by protamines. At the completion of spermiogenesis the chromatin of mature sperm has a toroidal structure that is tightly compacted and resistant to denaturation. The compaction is necessary to protect sperm chromatin during transit through the epididymis and female reproductive tract. Disruption to chromatin remodelling during spermiogenesis results in chromatin that is susceptible to denaturation. Inappropriate chromatin structure has been shown in a number of mammalian species to be related to male infertility, and specifically the failure of embryonic development. A range of techniques are available to assess chromatin status in sperm but arguably the most informative is the sperm chromatin structure assay (SCSA). The SCSA is a flow cytometric assay that uses the metachromatic properties of acridine orange to measure the susceptibility of sperm chromatin to acid-induced denaturation. A relationship has been demonstrated, primarily in men, between the SCSA outcome and the probability of continued embryonic development and the establishment of pregnancy after fertilisation. The contribution of sperm chromatin instability to reproductive wastage in both natural mating and assisted reproduction warrants further investigation as it may prove valuable as a means of decreasing the incidence of embryonic mortality. In this regard, it is possible that 'male factor' infertility may emerge as an even more important component in embryonic development.

Parent genes of retrotransposition-generated gene duplicates in Drosophila melanogaster have distinct expression profiles

Genes arising by retrotransposition are always different from their parent genes from the outset. In addition, the cDNA must insert into a region that allows expression or it will become a processed pseudogene. We sought to determine whether this class of gene duplication differs from other gene duplications based on functional criteria. Using amino acid sequences from Drosophila melanogaster, we identified retroduplicated gene pairs at various levels of sequence identity. Analysis of gene ontology annotations showed some enrichment of retroduplications in the cellular physiological processes class. Retroduplications show a higher level of nucleotide substitution than other gene duplications, suggesting a higher rate of divergence. Remarkably, analysis of microarray data for gene expression during embryogenesis showed that parent genes are more highly expressed relative to their retroduplicated copies, tandem duplications, and all genes. Furthermore, an expressed sequence tag library representation shows a broader distribution for parent genes than for all other genes and, as found previously by others, retroduplicated gene transcripts are found most abundantly in testes. Therefore, in examining retroduplicated gene pairs, we have found that parent genes of retroduplications are also a distinctive class in terms of transcript expression levels and distribution.

K-SPMM: a database of murine spermatogenic promoters modules & motifs

Background

Understanding the regulatory processes that coordinate the cascade of gene expression leading to male gamete development has proven challenging. Research has been hindered in part by an incomplete picture of the regulatory elements that are both characteristic of and distinctive to the broad population of spermatogenically expressed genes.

Description

K-SPMM, a database of murine Spermatogenic Promoters Modules and Motifs, has been developed as a web-based resource for the comparative analysis of promoter regions and their constituent elements in developing male germ cells. The system contains data on 7,551 genes and 11,715 putative promoter regions in Sertoli cells, spermatogonia, spermatocytes and spermatids. K-SPMM provides a detailed portrait of promoter site components, ranging from broad distributions of transcription factor binding sites to graphical illustrations of dimeric modules with respect to individual transcription start sites. Binding sites are identified through their similarities to position weight matrices catalogued in either the JASPAR or the TRANSFAC transcription factor archives. A flexible search function allows sub-populations of promoters to be identified on the basis of their presence in any of the four cell-types, their association with a list of genes or their component transcription-factor families.

Conclusion

This system can now be used independently or in conjunction with other databases of gene expression as a powerful aid to research networks of co-regulation. We illustrate this with respect to the spermiogenically active protamine locus in which binding sites are predicted that align well with biologically foot-printed protein binding domains.

Availability

Identification of a novel male reproduction-related gene and its regulated expression patterns in the prawn, Macrobrachium rosenbergii

To identify male-specific genes that could be involved in male development, we screened a subtracted male reproductive tract library and isolated a novel gene named Mar-Mrr (M. rosenbergii male reproduction-related gene). The Mar-Mrr cDNA sequence consists of 683 nucleotides with a 333 nucleotide open reading frame, encoding putative 110 amino acids (11.7473 kDa) precursor protein and a signal peptide consisting of 24 amino acids. Significant developmentally dependent accumulation of the mRNA was observed in the male reproductive tract, specifically in epithelial cells of vas deferens and terminal ampullae.

Basic mechanisms for the control of germ cell gene expression

The patterns of gene expression in spermatocytes and oocytes are quite different from those in somatic cells. The messenger RNAs produced by these cells are not only required to support germ cell development but, in the case of oocytes, they are also used for maturation, fertilization, and early embryogenesis. Recent studies have begun to provide an explanation for how germ-cell-specific programs of gene expression are generated. Part of the answer comes from the observation that germ cells express core promoter-associated regulatory factors that are different from those expressed in somatic cells. These factors supplement or replace their somatic counterparts to direct expression during meiosis and gametogenesis. In addition, germ cell transcription involves the recognition and use of specialized core promoter sequences. Finally, transcription must occur on chromosomal DNA templates that are reorganized into new chromatin-packaging configurations using alternate histone subunits. This article will review recent advances in our understanding of the factors and mechanisms that control transcription in ovary and testis and will discuss models for germ cell gene expression.

UTP14c is a recently acquired retrogene associated with spermatogenesis and fertility in man

In the mouse, Utp14b is a retrogene transposed to an intron of Acsl3 (long-chain-fatty-acid coenzyme A ligase 3) on mouse chromosome 1. It represents a copy of Utp14a, a ubiquitously expressed, X-linked gene involved in 18S rRNA synthesis. The Utp14b is specifically expressed in male germ cells and, when mutated in the jsd (juvenile spermatogonial depletion) mouse, results in early spermatogenic arrest and male infertility. To understand the function and relevance of the orthologous human gene in testis pathology, we mapped transcripts and searched for mutations within the gene in infertile males. In humans, the strict ortholog of UTP14b has degenerated and is no longer functional. However, a second active retroposon, UTP14c, is found within a widely expressed, putative glycosyl transferase-containing gene, GT8, on human chromosome 13. Unlike mouse Utp14b, which is only expressed in the male germ line, human UTP14c is expressed in testis and ovary, which is consistent with having a gonad-specific function. To determine if UTP14c is functionally equivalent to mouse Utp14b and essential to spermatogenesis in humans, we screened DNA from 234 nonobstructive, azoospermic/severely oligospermic males and 208 proven-fertile controls for mutations within UTP14c. We identified a mutation in three unrelated patients that introduces an in-frame stop codon truncating the UTP14c protein near the carboxyl terminus. These data indicate that UTP14c may be functionally equivalent to mouse Utp14b and required for normal male fertility in humans. The novel evolution of retroposed UTP14 genes supports the hypothesis that retrogenes play an important role in evolution via regulation of male reproductive fitness.

Identification of a novel testis-specific gene and its potential roles in testis development/spermatogenesis

AIM

To identify and characterize a novel gene with potential roles in testis development and spermatogenesis.

METHODS

A cDNA microarray was constructed from a human testis large insert cDNA library and hybridized with probes of human or mouse adult and fetal testes. Differentially expressed genes were isolated and sequenced. RT-PCR was used to test the tissue distribution of the genes of interest and in situ hybridization was performed to localize the gene expression in the mouse testis. A range of bioinformatical programs including Gene Runner, SMART, NCBI Blast and Emboss CpGPlot were used to characterize the new gene's feature.

RESULTS

A novel testis-specific gene, NYD-SP5, was differentially expressed in fetal and adult testes. The deduced protein structure of NYD-SP5 was found to contain an IQ motif (a short calmodulin-binding motif containing conserved Ile and Gln residues), a Carbamate kinase-like domain, a Zn-dependent exopeptidase domain and a lactate dehydrogenase (LDH) C-terminal-like domain. RT-PCR analysis revealed that NYD-SP5 was predominantly expressed in the testis but not in other 15 tissues examined. In situ hybridization and RT-PCR examinations revealed that the expression of NYD-SP5 was confined in the male germ cell but not present in the somatic cell in the testes.

CONCLUSION

NYD-SP5 is a newly found testis-specific gene with potential roles in testis development and spermatogenesis through a calmodulin-activated enzyme.

Molecular Cloning, Identification and Characteristics of a Novel Isoform of Carbamyl Phosphate Synthetase I in Human Testis

A gene coding a novel isoform of carbamyl phosphate synthetase I (CPS1) was cloned from a human testicular library. As shown by cDNA microarray hybridization, this gene was expressed at a higher level in human adult testes than in fetal testes. The full length of its cDNA was 3831 bp, with a 3149 bp open reading frame, encoding a 1050-amino-acid protein. The cDNA sequence was deposited in the GenBank (AY317138). Sequence analysis showed that it was homologous to the human CPS1 gene. The putative protein contained functional domains composing the intact large subunit of carbamoyl phosphate synthetase, thus indicated it has the capability of arginine biosynthesis. A multiple tissue expression profile showed high expression of this gene in human testis, suggesting the novel alternative splicing form of CPS1 may be correlated with human spermatogenesis.

A Short Core Promoter Drives Expression of the ALF Transcription Factor in Reproductive Tissues of Male and Female Mice1

The control of gene expression in reproductive tissues involves a number of unique germ cell-specific transcription factors. One such factor, ALF (TFIIA tau), encodes a protein similar to the large subunit of general transcription factor TFIIA. To understand how this factor is regulated, we characterized transgenic mice that contain the ALF promoter linked to either beta-galactosidase or green fluorescent protein (GFP) reporters. The results show that as little as 133 base pairs are sufficient to drive developmentally accurate and cell-specific expression. Transgene DNA was methylated and inactive in liver, but could be reactivated in vivo by system administration of 5-aza, 2'-deoxycytidine. Fluorescence-activated cell sorting allowed the identification of male germ cells that express the GFP transgene and provides a potential method to collect cells that might be under the control of a nonsomatic transcription system. Finally, we found that transcripts from the endogenous ALF gene and derived transgenes can also be detected in whole ovary and in germinal vesicle-stage oocytes of female mice. The ALF sequence falls into a class of germ cell promoters whose features include small size, high GC content, numerous CpG dinucleotides, and an apparent TATA-like element. Overall, the results define a unique core promoter that is active in both male and female reproductive tissues, and suggest mouse ALF may have a regulatory role in male and female gametogenic gene expression programs.

Glycoconjugates of the urodele amphibian testis shown by lectin cytochemical methods

Lectin histochemistry is a useful method that allows the in situ identification of the terminal sugar moieties of the carbohydrates that form the glycoconjugates. Moreover, when it is combined with chemical or enzymatic deglycosylation pretreatments, lectin histochemistry can be employed to determine if carbohydrates are linked to the protein core by means of an N- or O-glycosidic linkage or, indeed, to partially sequence the sugar chains. One of the most interesting model organs for the study of spermatogenesis is the amphibian urodele testis. However, this organ has not been very widely investigated with lectin histochemical research. In the last few years, we have carried out a research project to identify and locate glycoconjugates in the testis of the urodele Pleurodeles waltl, the Spanish newt, as a first approach to identify possible carbohydrates with key roles in spermatogenesis. Our findings reveal some glycan chains located in a fusome-like structure in early (diploid) germ cells, oligosaccharides with terminal GalNAc in the acrosome, the occurrence of glycan modifications in the acrosomal contents during spermiogenesis, and changes in glycan composition of follicle and interstitial cells during the spermatogenetic cycle. Furthermore, the similar labeling pattern of follicle and duct cells supports the hypothesis for a common origin of both cell types.

A spermatogenesis-related gene expression profile in human spermatozoa and its potential clinical applications

Spermatogenesis is an essential stage in the human reproductive process. In a previously study aiming to determine which genes might be involved in spermatogenesis, we compared the gene expression profiles of adult and fetal testes by hybridizing cDNA probes prepared from adult and fetal testes to membranes dotted with gene clones derived from a commercial human testis library. We identified 266 differentially expressed genes that showed higher expression levels in adult testes, indicating their potential roles in spermatogenesis. In the present study, we applied the same cDNA microarray technique to the analysis of gene expression in the spermatozoa of normal fertile men and found 149 genes that were expressed at higher levels in adult testis. A further study of five sperm motility-related genes selected from this profile by real-time PCR revealed that there was significant difference in the expression levels of two genes ( TPX-1, testis-specific protein 1 and LDHC, lactate dehydrogenase C, transcript variant 1) between normal ( n=29) and motility impaired ( n=24) semen samples, indicating that these genes are involved in sperm function. Our results demonstrated that spermatogenesis-related gene profiling could help to assess sperm quality in humans, and further study of these genes will help us to elucidate the mechanisms involved in spermatogenesis and diseases relating to human infertility.

Regulation of ALF gene expression in somatic and male germ line tissues involves partial and site-specific patterns of methylation

ALF (TFIIAalpha/beta-like factor) is a germ cell-specific counterpart of the large (alpha/beta) subunit of general transcription factor TFIIA. Here we isolated homologous GC-rich promoters from the mouse and human ALF genes and used promoter deletion analysis to identify sequences active in COS-7 and 293 cells. Further, bisulfite sequence analysis of the mouse ALF promoter showed that all 21 CpG dinucleotides between -179 and +207 were partially methylated in five somatic tissues, brain, heart, liver, lung, and muscle, and in epididymal spermatozoa from adult mice. In contrast, DNA from prepubertal mouse testis and from purified spermatocytes were unmethylated except at C(+19)G and C(+170)G. We also found that ALF expression correlates with a strong promoter-proximal DNase I-hypersensitive site present in nuclei from testis but not from liver. Finally we show that in vitro methylation of the ALF promoter inhibits activity and that 5-aza-2'-deoxycytidine treatment reactivates the endogenous ALF gene in a panel of seven different mouse and human somatic cell lines. Overall the results show that silencing in somatic cells is methylation-dependent and reversible and that a unique CpG-specific methylation pattern at the ALF promoter precedes expression in pachytene spermatocytes. This pattern is transient as remethylation of the ALF promoter in haploid germ cell DNA has occurred by the time spermatozoa are present in the epididymis.

A 1.3-kb upstream 5' region of the rat phosphoglycerate mutase m gene confers testis and skeletal muscle-specific expression in transgenic mice

Spermatogenesis is a complex process that occurs in successive mitotic, meiotic and post-meiotic phases and involves a highly regulated selective gene-expression pattern. However, this process has not been well characterised at the gene expression level due to the absence of germinal cell lines. We previously demonstrated that the rat skeletal muscle-specific gene for the glycolytic enzyme phosphoglycerate mutase is also specifically expressed in meiotic and haploid male germ cells from testis (12). To analyse the promoter elements that regulate the transcription of the phosphoglycerate mutase m gene (pgam-m)during spermatogenesis, we developed transgenic mice for a construct containing 1.3 kb from the pgam-m promoter linked to the Escherichia coli LacZ gene. RNA analysis by retrotranscription and PCR amplification of transgene expression showed transcriptional activity in the testis with a pattern during testis development that was identical to the endogenous gene. The transgene was also active in skeletal muscle but not in the adult heart in all the transgenic lines analysed. Collectively, these studies demonstrate that the 1.3 kb pgam-m promoter contains sufficient sequences to specify temporally regulated testis-specific expression as well as skeletal-muscle expression.

Reprogramming the male gamete genome: a window to successful gene therapy

Hematopoiesis and spermatogenesis both initiate from a stem cell capable of renewal and differentiation. Each pathway reflects the expression of unique combinations of facultative, i.e. tissue-specific and constitutive, i.e. housekeeping, genes in each cell type. In spermatogenesis, as in hematopoiesis, commitment is mediated by the mechanism of potentiation whereby specific chromatin domains are selectively opened along each chromosome. Within each open chromatin domain, a unique battery of gene(s) is availed to tissue-specific and ubiquitous transacting factors that are necessary to initiate transcription. In the absence of an open domain, trans-factor access is denied, and the initiation of transcription cannot proceed. Cell-fate is thus ultimately defined by the unique series of open-potentiated cell-specific chromatin domains. Defining the mechanism that opens chromatin domains is fundamental in understanding how differentiation from stem cells is controlled and whether cell-fate can be modified. A recent examination of the mammalian spermatogenic pathway [Kramer, J.A., McCarrey, J.M, Djakiew, D., Krawetz, S.A., 1998. Differentiation: the selective potentiation of chromatin domains. Development 125, 4749-4755] supports the view that cell fate is mediated by global changes in chromatin conformation. This stride underscores the possibility of moderating differentiation through chromatin conformation. It is likely that gene therapeutics capable of selectively potentiating individual genic domains in populations of differentiating and/or replicating cells that modify cellular phenotype will be developed in the next millennium.