Mosaicism of tyrosinase-locus transcription and chromatin structure in dark vs. light melanocyte clones of homozygous chinchilla-mottled mice

Boundary sequences flanking the mouse tyrosinase locus ensure faithful pattern of gene expression

Control of gene expression is dictated by cell-type specific regulatory sequences that physically organize the structure of chromatin, including promoters, enhancers and insulators. While promoters and enhancers convey cell-type specific activating signals, insulators prevent the cross-talk of regulatory elements within adjacent loci and safeguard the specificity of action of promoters and enhancers towards their targets in a tissue specific manner. Using the mouse tyrosinase (Tyr) locus as an experimental model, a gene whose mutations are associated with albinism, we described the chromatin structure in cells at two distinct transcriptional states. Guided by chromatin structure, through the use of Chromosome Conformation Capture (3C), we identified sequences at the 5′ and 3′ boundaries of this mammalian gene that function as enhancers and insulators. By CRISPR/Cas9-mediated chromosomal deletion, we dissected the functions of these two regulatory elements in vivo in the mouse, at the endogenous chromosomal context, and proved their mechanistic role as genomic insulators, shielding the Tyr locus from the expression patterns of adjacent genes.

Ectopic expression of the Stabilin2 gene triggered by an intracisternal A particle (IAP) element in DBA/2J strain of mice

Stabilin2 (Stab2) encodes a large transmembrane protein which is predominantly expressed in the liver sinusoidal endothelial cells (LSECs) and functions as a scavenger receptor for various macromolecules including hyaluronans (HA). In DBA/2J mice, plasma HA concentration is ten times higher than in 129S6 or C57BL/6J mice, and this phenotype is genetically linked to the Stab2 locus. Stab2 mRNA in the LSECs was significantly lower in DBA/2J than in 129S6, leading to reduced STAB2 proteins in the DBA/2J LSECs. We found a retrovirus-derived transposable element, intracisternal A particle (IAP), in the promoter region of Stab2^DBA which likely interferes with normal expression in the LSECs. In contrast, in other tissues of DBA/2J mice, the IAP drives high ectopic Stab2^DBA transcription starting within the 5′ long terminal repeat of IAP in a reverse orientation and continuing through the downstream Stab2^DBA. Ectopic transcription requires the Stab2-IAP element but is dominantly suppressed by the presence of loci on 59.7–73.0 Mb of chromosome (Chr) 13 from C57BL/6J, while the same region in 129S6 requires additional loci for complete suppression. Chr13:59.9–73 Mb contains a large number of genes encoding Krüppel-associated box-domain zinc-finger proteins that target transposable elements-derived sequences and repress their expression. Despite the high amount of ectopic Stab2^DBA transcript in tissues other than liver, STAB2 protein was undetectable and unlikely to contribute to the plasma HA levels of DBA/2J mice. Nevertheless, the IAP insertion and its effects on the transcription of the downstream Stab2^DBA exemplify that stochastic evolutional events could significantly influence susceptibility to complex but common diseases.

Functional validation of mouse tyrosinase non-coding regulatory DNA elements by CRISPR-Cas9-mediated mutagenesis

Newly developed genome-editing tools, such as the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system, allow simple and rapid genetic modification in most model organisms and human cell lines. Here, we report the production and analysis of mice carrying the inactivation via deletion of a genomic insulator, a key non-coding regulatory DNA element found 5' upstream of the mouse tyrosinase (Tyr) gene. Targeting sequences flanking this boundary in mouse fertilized eggs resulted in the efficient deletion or inversion of large intervening DNA fragments delineated by the RNA guides. The resulting genome-edited mice showed a dramatic decrease in Tyr gene expression as inferred from the evident decrease of coat pigmentation, thus supporting the functionality of this boundary sequence in vivo, at the endogenous locus. Several potential off-targets bearing sequence similarity with each of the two RNA guides used were analyzed and found to be largely intact. This study reports how non-coding DNA elements, even if located in repeat-rich genomic sequences, can be efficiently and functionally evaluated in vivo and, furthermore, it illustrates how the regulatory elements described by the ENCODE and EPIGENOME projects, in the mouse and human genomes, can be systematically validated.

The use of mouse models to study epigenetics

Much of what we know about the role of epigenetics in the determination of phenotype has come from studies of inbred mice. Some unusual expression patterns arising from endogenous and transgenic murine alleles, such as the Agouti coat color alleles, have allowed the study of variegation, variable expressivity, transgenerational epigenetic inheritance, parent-of-origin effects, and position effects. These phenomena have taught us much about gene silencing and the probabilistic nature of epigenetic processes. Based on some of these alleles, large-scale mutagenesis screens have broadened our knowledge of epigenetic control by identifying and characterizing novel genes involved in these processes.

PTEN and melanomagenesis

The PI3K-PTEN-AKT signaling pathway is involved in various cellular activities, including proliferation, migration, cell growth, cell survival and differentiation during adult homeostasis as well as in tumorigenesis. It has been suggested that the constitutive activation of PI3K/AKT signaling with concurrent loss of function of the tumor suppressor molecule PTEN contributes to cancer formation. Members of the PI3K-PTEN-AKT pathway, including these proteins and mTOR, are altered in melanoma tumors and cell lines. A hallmark of activation of the pathway is the loss of function of PTEN. Indeed, loss of heterozygosity of PTEN has been observed in approximately 30% of human melanomas, implicating this signaling pathway in this cancer. PI3K signaling activation, via loss of PTEN function, can inhibit proapoptotic genes such as the FoxO family of transcription factors, while inducing cell growth- and cell survival-related elements such as p70S6K and AKT. Determining how the PI3K-PTEN-AKT signaling pathway, alone or in cooperation with other pathways, orchestrates the induction of target genes involved in a diverse range of activities is a major challenge in research into melanoma initiation and progression. Moreover, the acquisition of basic knowledge will help patient management with appropriate therapies that are already, or will shortly be, on the market.

Endogenous retroviruses in mammals: an emerging picture of how ERVs modify expression of adjacent genes

Endogenous retrovirsuses (ERVs) have long been known to influence gene expression in plants in important ways, but what of their roles in mammals? Our relatively sparse knowledge in that area was recently increased with the finding that ERVs can influence the expression of mammalian resident genes by disrupting transcriptional termination. For many mammalian biologists, retrotransposition is considered unimportant except when it disrupts the reading frame of a gene, but this view continues to be challenged. It has been known for some time that integration into an intron can create novel transcripts and integration upstream of a gene can alter the expression of the transcript, in many cases producing phenotypic consequences and disease. The new findings on transcriptional termination extend the opportunities for retrotransposons to play a role in human disease.

Forward signaling by EphB1/EphB2 interacting with ephrin-B ligands at the optic chiasm is required to form the ipsilateral projection

EphB receptor tyrosine kinases direct axonal pathfinding through interactions with ephrin-B proteins following axon-cell contact. As EphB:ephrin-B binding leads to bidirectional signals, the contributions of signaling into the Eph-expressing cell (forward signaling) or the ephrin-expressing cell (reverse signaling) cannot be assigned using traditional protein null alleles. To determine if EphB1 is functioning solely as a receptor during axon pathfinding, a new knock-in mutant mouse was created, EphB1(T-lacZ), which expresses an intracellular-truncated EphB1-β-gal fusion protein from the endogenous locus. As in the EphB1(-/-) protein null animals, the EphB1(T-lacZ/T-lacZ) homozygotes fail to form the ipsilateral projecting subpopulation of retinal ganglion cell axons. This indicates that reverse signaling through the extracellular domain of EphB1 is not required for proper axon pathfinding of retinal axons at the optic chiasm. Further analysis of other EphB and ephrin-B mutant mice shows that EphB1 is the preferred receptor of ephrin-B2 and, to a lesser degree, ephrin-B1 in mediating axon guidance at the optic chiasm despite the coexpression of EphB2 in the same ipsilaterally projecting retinal axons.

Differential gene expression of TRPM1, the potential cause of congenital stationary night blindness and coat spotting patterns (LP) in the Appaloosa horse (Equus caballus)

The appaloosa coat spotting pattern in horses is caused by a single incomplete dominant gene (LP). Homozygosity for LP (LP/LP) is directly associated with congenital stationary night blindness (CSNB) in Appaloosa horses. LP maps to a 6-cM region on ECA1. We investigated the relative expression of two functional candidate genes located in this LP candidate region (TRPM1 and OCA2), as well as three other linked loci (TJP1, MTMR10, and OTUD7A) by quantitative real-time RT-PCR. No large differences were found for expression levels of TJP1, MTMR10, OTUD7A, and OCA2. However, TRPM1 (Transient Receptor Potential Cation Channel, Subfamily M, Member 1) expression in the retina of homozygous appaloosa horses was 0.05% the level found in non-appaloosa horses (R = 0.0005). This constitutes a >1800-fold change (FC) decrease in TRPM1 gene expression in the retina (FC = -1870.637, P = 0.001) of CSNB-affected (LP/LP) horses. TRPM1 was also downregulated in LP/LP pigmented skin (R = 0.005, FC = -193.963, P = 0.001) and in LP/LP unpigmented skin (R = 0.003, FC = -288.686, P = 0.001) and was downregulated to a lesser extent in LP/lp unpigmented skin (R = 0.027, FC = -36.583, P = 0.001). TRP proteins are thought to have a role in controlling intracellular Ca(2+) concentration. Decreased expression of TRPM1 in the eye and the skin may alter bipolar cell signaling as well as melanocyte function, thus causing both CSNB and LP in horses.

The tyrosinase enhancer is activated by Sox10 and Mitf in mouse melanocytes

The terminal differentiation of melanocytes is associated with the transcriptional activation of genes responsible for pigment production such as tyrosinase. Pigment cell-specific transcription factors, such as Mitf, as well as specific proximal and distal regulatory elements (DRE) are implicated in the tight control of tyrosinase expression during development and adulthood. Proper tyrosinase expression in melanocytes depends upon the presence of a DRE that is located at -15 kb and provides enhancer activity via a central element termed core-enhancer. In this report, we show that the transcription factors Sox10, Mitf and USF-1 are able to activate the core-enhancer in luciferase reporter assays. Comparative sequence analysis identified evolutionarily motifs resembling Sox10 binding sites that were required for full enhancer activity in melanoma cells and in tyrosinase::lacZ transgenic mice. Sox10 was able to bind the DRE in vitro and mutation of the conserved motifs abolished the enhancer transactivation mediated by Sox10. In addition, two highly conserved CAGCTG E-box motifs were identified that were also required for enhancer activity and for transactivation by Mitf. The results suggest that Sox10 directly, and Mitf, most likely indirectly, activate the tyrosinase enhancer, underlining the contribution of Sox10 to tyrosinase gene regulation in melanocytes.

A comparative study of S/MAR prediction tools

BACKGROUND

S/MARs are regions of the DNA that are attached to the nuclear matrix. These regions are known to affect substantially the expression of genes. The computer prediction of S/MARs is a highly significant task which could contribute to our understanding of chromatin organisation in eukaryotic cells, the number and distribution of boundary elements, and the understanding of gene regulation in eukaryotic cells. However, while a number of S/MAR predictors have been proposed, their accuracy has so far not come under scrutiny.

RESULTS

We have selected S/MARs with sufficient experimental evidence and used these to evaluate existing methods of S/MAR prediction. Our main results are: 1.) all existing methods have little predictive power, 2.) a simple rule based on AT-percentage is generally competitive with other methods, 3.) in practice, the different methods will usually identify different sub-sequences as S/MARs, 4.) more research on the H-Rule would be valuable.

CONCLUSION

A new insight is needed to design a method which will predict S/MARs well. Our data, including the control data, has been deposited as additional material and this may help later researchers test new predictors.

Distinct distal regulatory elements control tyrosinase expression in melanocytes and the retinal pigment epithelium

Pigment cells of mammals are characterized by two different developmental origins: cells of the retinal pigment epithelium (RPE) originate from the optic cup of the developing forebrain, whereas melanocytes arise from the neural crest. The pigmentation gene tyrosinase is expressed in all pigment cells but differentially regulated in melanocytes and RPE. The tyrosinase promoter does not confer strong expression in pigment cells in vivo, while inclusion of a distal regulatory element at position -15 kb is necessary and sufficient to provide strong expression in melanocytes. Nevertheless, the regulatory elements responsible for correct spatial and temporal tyrosinase expression in the RPE remained unidentified so far. In this report, we show that a 186 kb BAC containing the tyrosinase gene provides transgene expression in both RPE and melanocytes indicating the presence of regulatory sequences required for expression in the RPE. A deletion analysis of the BAC was performed demonstrating that a RPE-regulatory element resides between -17 and -75 kb. Using multi-species comparative genomic analysis we identified three conserved sequences within this region. When tested in transgenic mice one of these sequences located at -47 kb targeted expression to the RPE. In addition, deletion of this regulatory element within a tyrosinase::lacZ BAC provided evidence that this sequence is not only sufficient but also required for correct spatial and temporal expression in the RPE. The identification of this novel element demonstrates that tyrosinase gene expression is controlled by separate distal regulatory sequences in melanocytes and RPE.

The Tyr (albino) locus of the laboratory mouse

The albino mouse was already known in ancient times and was apparently selectively bred in Egypt, China, and Japan. Thus, it is not surprising that the c or albino locus (now the Tyr locus) was among the first used to demonstrate Mendelian inheritance in mammals at the dawn of the past century. This locus is now known to encode tyrosinase, the rate-limiting enzyme in the production of melanin pigment, and the molecular basis of the albino ( Tyr(c)) mutation is known. Here we describe the congenic series of Tyr-locus alleles, from wild type to null ( albino). We compare eye and skin pigmentation phenotypes and the genetic lesions that cause each. We suggest that this panel of congenic mutants contains rich, untapped resources for the study of many questions of basic cell biological interest.

A strategy to study tyrosinase transgenes in mouse melanocytes

BACKGROUND

A number of transgenic mice carrying different deletions in the Locus Control Region (LCR) of the mouse tyrosinase (Tyr) gene have been developed and analysed in our laboratory. We require melanocytes from these mice, to further study, at the cellular level, the effect of these deletions on the expression of the Tyr transgene, without potential interference with or from the endogenous Tyr alleles. It has been previously reported that it is possible to obtain and immortalize melanocyte cell cultures from postnatal mouse skin.

RESULTS

Here, we describe the efforts towards obtaining melanocyte cultures from our Tyr transgenic mice. We have bred our Tyr transgenic mice into Tyr c-32DSD mutant background, lacking the endogenous Tyr locus. In these conditions, we failed to obtain immortalized melanocytes. We decided to include the inactivation of the Ink4a-Arf locus to promote melanocyte immortalisation. For this purpose, we report the segregation of the Ink4a-Arf null allele from the brown (Tyrp1b) mutation in mice. Finally, we found that Ink4a-Arf +/- and Ink4a-Arf -/- melanocytes had undistinguishable tyrosine hydroxylase activities, although the latter showed reduced cellular pigmentation content.

CONCLUSION

The simultaneous presence of precise genomic deletions that include the tyrosinase locus, such as the Tyr c-32DSD allele, the Tyr transgene itself and the inactivated Ink4a-Arf locus in Tyrp1B genetic background appear as the crucial combination to perform forthcoming experiments. We cannot exclude that Ink4a-Arf mutations could affect the melanin biosynthetic pathway. Therefore, subsequent experiments with melanocytes will have to be performed in a normalized genetic background regarding the Ink4a-Arf locus.

Molecular basis of the extreme dilution mottled mouse mutation: a combination of coding and noncoding genomic alterations

Tyrosinase is the rate-limiting enzyme in melanin biosynthesis. It is an N-glycosylated, copper-containing transmembrane protein, whose post-translational processing involves intracytoplasmic movement from the endoplasmic reticulum to the Golgi and, eventually, to the melanosome. The expression of the tyrosinase (Tyr) gene is controlled by several regulatory regions including a locus control region (LCR) located 15 kb upstream from the promoter region. The extreme dilution mottled mutant mice (Tyrc-em) arose spontaneously at the MRC Institute in Harwell (United Kingdom) from a chinchilla-mottled mutant (Tyrc-m) stock, whose molecular basis corresponds to a rearrangement of 5'-upstream regulatory sequences including the LCR of the Tyr gene. Tyrc-em mice display a variegated pigmentation pattern in coat and eyes, in agreement with the LCR translocation, but also show a generalized hypopigmented phenotype, not seen in Tyrc-m mice. Genomic analyses of Tyrc-em mice showed a C1220T nucleotide substitution within the Tyr encoding region, resulting in a T373I amino acid change, which abolishes an N-glycosylation sequon located in the second metal ion binding site of the enzyme. Tyrosinase from Tyrc-em displayed a reduced enzymatic activity in vivo and in vitro, compared with wild-type enzyme. Deglycosylation studies showed that the mutant protein has an abnormal glycosylation pattern and is partially retained in the endoplasmic reticulum. We conclude that the phenotype of the extreme dilution mottled mouse mutant is caused by a combination of coding and noncoding genomic alterations resulting in several abnormalities that include suboptimal gene expression, abnormal protein processing, and reduced enzymatic activity.

The tyrosinase gene of the i(b) albino mutant of the medaka fish carries a transposable element insertion in the promoter region

The i locus of the medaka fish contains the tyrosinase gene whose product is the key enzyme required for melanin biosynthesis. The i(b) allele at this locus, also denoted as i( 5), causes oculocutaneous albinism in homozygous carriers. Its albino phenotype is very weak, characterized mainly by small and varying sized melanophores in juveniles. Cloning and sequencing analyses of the tyrosinase gene for the i (b) allele revealed the presence of a 4.7-kb extra DNA fragment in the 5' untranslated region, this being Tol2, a DNA-based transposable element of the hobo Activator Tam3 (hAT) family which had previously been identified as a cause of another mutant allele i(4). Its insertion point was 85 bp upstream of the main transcription initiation site and 50 bp downstream of the CATGTG motif that has been suggested to be essential for the promoter function of the tyrosinase gene. The transcription level of the tyrosinase gene was decreased in i(b)/i(b) fish, compared with wild-type fish. The insertion is thus a likely cause of the weak albino phenotype. The Tol2 element transposes in a cut-and-paste fashion, and its excision is mostly imprecise, leaving some nucleotides and/or removing excess nucleotides. The i (b) mutant strain can thus be expected to serve as a source from which various other mutations in the promoter region can be derived.

Identification and functional validation of a 5' upstream regulatory sequence in the human tyrosinase gene homologous to the locus control region of the mouse tyrosinase gene

Comparison analysis of the sequences of the mouse and human genomes has proven a powerful approach in identifying functional regulatory elements within the non-coding regions that are conserved through evolution between homologous mammalian loci. Here, we applied computational analysis to identify regions of homology in the 5' upstream sequences of the human tyrosinase gene, similar to the locus control region (LCR) of the mouse tyrosinase gene, located at -15 kb. We detected several stretches of homology within the first 30 kb 5' tyrosinase gene upstream sequences of both species that include the proximal promoter sequences, the genomic region surrounding the mouse LCR, and further upstream segments. We cloned and sequenced a 5' upstream regulatory sequence found between -8 and -10 kb of the human tyrosinase locus (termed h5'URS) homologous to the mouse LCR sequences, and confirmed the presence of putative binding sites at -9 kb, homologous to those described in the mouse tyrosinase LCR core. Finally, we functionally validated the presence of a tissue-specific enhancer in the h5'URS by transient transfection analysis in human and mouse cells, as compared with homologous DNA sequences from the mouse tyrosinase locus. Future experiments in cells and transgenic animals will help us to understand the in vivo relevance of this newly described h5'URS sequence as a potentially important regulatory element for the correct expression of the human tyrosinase gene.

Targeted transcriptional repression of Gfi1 by GFI1 and GFI1B in lymphoid cells

Growth factor independence-1 (GFI1) and GFI1B are closely related, yet differentially expressed transcriptional repressors with nearly identical DNA binding domains. GFI1 is upregulated in the earliest thymocyte precursors, while GFI1B expression is restricted to T lymphopoiesis stages coincident with activation. Transgenic expression of GFI1 potentiates T-cell activation, while forced GFI1B expression decreases activation. Both mice and humans with mutant Gfi1 display lymphoid abnormalities. Here we describe autoregulation of Gfi1 in primary mouse thymocytes and a human T-cell line. GFI1 binding to cis-element sequences conserved between rat, mouse and human Gfi1 mediates direct and potent transcriptional repression. In addition, dramatic regulation of Gfi1 can also be mediated by GFI1B. These data provide the first example of a gene directly targeted by GFI1 and GFI1B. Moreover, they support a role for auto- and trans-regulation of Gfi1 by GFI1 and GFI1B in maintaining the normal expression patterns of Gfi1, and suggest that GFI1B may indirectly affect T-cell activation through repression of Gfi1.

Functional dissection of the mouse tyrosinase locus control region identifies a new putative boundary activity

Locus control regions (LCRs) are complex high-order chromatin structures harbouring several regulatory elements, including enhancers and boundaries. We have analysed the mouse tyrosinase LCR functions, in vitro, in cell lines and, in vivo, in transgenic mice and flies. The LCR-core (2.1 kb), located at -15 kb and carrying a previously described tissue-specific DNase I hypersensitive site, operates as a transcriptional enhancer that efficiently transactivates heterologous promoters in a cell-specific orientation-independent manner. Furthermore, we have investigated the boundary activity of these sequences in transgenic animals and cells. In mice, the LCR fragment (3.7 kb) rescued a weakly expressed reference construct that displays position effects. In Drosophila, the LCR fragment and its core insulated the expression of a white minigene reporter construct from chromosomal position effects. In cells, sequences located 5' from the LCR-core displayed putative boundary activities. We have obtained genomic sequences surrounding the LCR fragment and found a LINE1 repeated element at 5'. In B16 melanoma and L929 fibroblast mouse cells, this element was found heavily methylated, supporting the existence of putative boundary elements that could prevent the spreading of condensed chromatin from the LINE1 sequences into the LCR fragment, experimentally shown to be in an open chromatin structure.

Identification and Characterization of a DNase Hypersensitive Region of the Human Tyrosinase Gene

Mutations of the tyrosinase gene produce oculocutaneous albinism type 1 (OCA1). Most affected individuals are compound heterozygotes with different maternal and paternal mutations, but a substantial number of presumed tyrosinase alleles in these individuals have no identifiable mutation in the coding or proximal promoter region of the gene. This suggests that mutations in other regions of the gene, such as regulatory regions that are removed from the direct proximity of the coding sequence, may account for these currently unidentifiable mutations. The mouse tyrosinase gene has a distal enhancer or locus control region (LCR) that provides position-independent stimulation of gene expression, and a homologous regulatory region (HR) of the human gene could be the site of some of these mutations. We report a region 9 kb upstream of the human tyrosinase transcriptional start site that may be involved in regulation of this gene. Analysis of this region shows DNase I hypersensitivity in a cell lineage-specific pattern, a pattern indicative of regulatory regions of a gene. This region also has significant enhancer function when reporter vectors containing it are transfected into either human or mouse melanocyte cell lines, and elimination of specific sequences with homology to the mouse core enhancer in this region extinguishes the enhancer function. We believe that this region of homology contains sequences critical in the regulation of the human tyrosinase gene and is a candidate for the location of OCA1 mutations.

Tyrosinase gene correction using fluorescent oligonucleotides

Gene therapy and production of mutated cell lines or animal models should be improved significantly once efficient controlled gene targeting strategies are developed. We used short single-stranded oligodeoxynucleotides (ODN), in some cases coupled to the fluorescent dye fluorescein isothiocyanate (FITC), to correct an endogenic natural point mutation in melanocytes in culture. The addition of the FITC molecule to the 5' extremity of the ODN did not interfere with the efficiency of the reversion of the mutation and did not have any deleterious side-effects. The use of fluorescent ODN could lead to great improvement in the technique. In particular, it may facilitate sorting of the transfected cells in the treated population, and thereby significantly increase the percentage of corrected cells.

Gene transfer strategies in animal transgenesis

Position effects in animal transgenesis have prevented the reproducible success and limited the initial expectations of this technique in many biotechnological projects. Historically, several strategies have been devised to overcome such position effects, including the progressive addition of regulatory elements belonging to the same or to a heterologous expression domain. An expression domain is thought to contain all regulatory elements that are needed to specifically control the expression of a given gene in time and space. The lack of profound knowledge on the chromatin structure of expression domains of biotechnological interest, such as mammary gland-specific genes, explains why most standard expression vectors have failed to drive high-level, position-independent, and copy-number-dependent expression of transgenes in a reproducible manner. In contrast, the application of artificial chromosome-type constructs to animal transgenesis usually ensures optimal expression levels. YACs, BACs, and PACs have become crucial tools in animal transgenesis, allowing the inclusion of distant key regulatory sequences, previously unknown, that are characteristic for each expression domain. These elements contribute to insulating the artificial chromosome-type constructs from chromosomal position effects and are fundamental in order to guarantee the correct expression of transgenes.

Artificial chromosome transgenesis in pigmentary research

Pigmentary genes were among the first mammalian genes to be studied, mostly because of the obvious phenotypes associated with their mutations. In 1990, tyrosinase, encoding the limiting enzyme in the melanin synthesis pathway, was eventually assigned to the c (albino) locus by classical rescue experiments driven by functional constructs in transgenic mice. These pioneer reports triggered the study of the regulation of endogenous tyrosinase gene expression by combining different amounts of upstream regulatory and promoter regions and testing their function in vivo in transgenic animals. However, faithful and reproducible transgenic expression was not achieved until the entire tyrosinase expression domain was transferred to the germ-line of mice using artificial-chromosome-type transgenes. The use of these large tyrosinase transgenic constructs and the ease with which they could be manipulated in vitro enabled the discovery of previously unknown but fundamental regulatory regions, such as the tyrosinase locus control region (LCR), whose presence was required in order to guarantee position-independent and copy-number-dependent expression of tyrosinase transgenes, with an expression level, per copy, comparable to that of an endogenous wild-type allele. Subsequently, functional dissection of elements present within this LCR through the generation of new artificial-chromosome type tyrosinase transgenes has revealed the existence of different regulatory activities. The existence of some of these units had been suggested previously by standard-type transgenic analyses. In this review, we will discuss both independent approaches and conclude that optimal tyrosinase transgene expression requires the use of its complete expression domain.

Variegated expression and delayed retinal pigmentation during development in transgenic mice with a deletion in the locus control region of the tyrosinase gene

Deletion of the tyrosinase locus control region (LCR) in transgenic mice results in variegated expression in the skin. Here we investigate the pigmentation pattern of other tissues that express tyrosinase: iris, choroid, and retina in the same animals. A mosaic distribution of pigmentation appears in the iris and choroid. Interestingly, a markedly different mosaic pattern is found in the retina, where central areas contain little or no melanin while pigmentation rises to normal levels towards periphery. Further, there is a temporal delay in the initiation and accumulation of pigment in retinal pigmented epithelium (RPE) cells during development, and patterns of adult retinal melanisation in these mice appear arrested at a stage found in early embryogenesis in wild-type mice. These results demonstrate that the tyrosinase LCR is needed for the correct establishment and maintenance of this expression domain throughout development, but particularly during the later stages of retinal melanisation.

Increased transgene expression by the mouse tyrosinase enhancer is restricted to neural crest-derived pigment cells

In this study, we have addressed the impact of the mouse tyrosinase enhancer on regulated expression from the mouse tyrosinase promoter during embryonic development. Stable and transient transgenic experiments using the reporter gene lacZ reveal that (1) expression is detected in neural crest-derived melanoblasts from E11.5 onward, (2) the enhancer does not increase transgenic expression in optic cup-derived pigment cells of the retinal pigment epithelium (RPE), and (3) expression in the telencephalon is not any longer detected. The importance of the enhancer for expression in pigment cells of the eye was further investigated in adult mice using an attenuated diphtheria toxin A gene. This demonstrated that in presence of the enhancer the transgene expression is specifically targeted to neural crest-derived melanocytes of the choroid and not, or slightly, to the RPE. This suggests that tyrosinase is differentially regulated in the two pigment cell lineages, and that this promoter can be used to target expression preferentially to the neural crest-derived melanocyte lineage.

Distal upstream tyrosinase S/MAR-containing sequence has regulatory properties specific to subsets of melanocytes

A distal upstream regulatory element of the mouse tyrosinase gene has locus control region (LCR)-like activity and is required for position-independent expression of linked genes. It consists of a DNAse I hypersensitive site, which has enhancer activity in neural crest-derived melanocytes, embedded within a scaffold/matrix attachment region (S/MAR), both of which are necessary for LCR activity. To address the role of the S/MAR in position-independent expression, we assessed the ability of a fragment containing most of the S/MAR to insulate a transgene from position effects. The S/MAR sequence showed a striking cell type specificity in its function in all six multicopy transgenic lines, dampening position effects considerably in cutaneous melanocytes while allowing no expression in other neural crest-derived melanocytes, and causing elevated expression in ocular melanocytes derived from the neural tube. The specificity of transgene expression in the eye suggested the presence of both positive and negative regulatory elements in this enhancer/S/MAR region, which was confirmed by transient transfection analyses. This is the first known regulatory element to exhibit different activities in melanocytes of different developmental origins.

Changes in Chromatin Organization at the Neutrophil Elastase Locus Associated With Myeloid Cell Differentiation

Neutrophil elastase, proteinase-3, and azurocidin are primary components of neutrophil azurophilic granules and are encoded by closely linked genes (gene symbols ELA2, PRTN3, and AZU1, respectively) in a region of approximately 50 kb. These genes are coordinately expressed in a granulocyte-specific fashion, but the mechanisms defining this pattern of expression are unknown. To understand the role of chromatin organization in governing the expression of ELA2, PRTN3, and AZU1, we mapped this region of chromosome 19 and identified the adipsin (complement factor D) gene in proximity to the 3′ end of ELA2. We then examined the changes in chromatin structure at the locus which accompany myeloid cell differentiation and identified 17 DNase I hypersensitive sites (DHS 1 to 17) in U-937 cells, an early myelomonocytic cell line expressing high levels of neutrophil elastase. Chemically induced differentiation and concomitant downregulation of AZU1, PRTN3, and ELA2 transcription in U-937 cells is not accompanied by changes in the DHS-pattern. Mature neutrophils, however, do not carry any of these hypersensitive sites, indicating a large degree of chromatin remodeling at this locus accompanying terminal granulocytic differentiation. Sixteen of the 17 DHS identified in U-937 cells are also present in the HL-60 myelomonocytic cell line. Hematopoietic cell lines representing the early erythroid and lymphocyte lineages, and a nonhematopoietic cell line display a subset of the hypersensitive sites. The altered chromatin structure specific to cells that actively transcribe the AZU1-PRTN3-ELA2 genes suggests that chromatin reorganization is an important mechanism regulating the myeloid-specific transcription of this gene cluster.

Changes in Chromatin Organization at the Neutrophil Elastase Locus Associated With Myeloid Cell Differentiation

Neutrophil elastase, proteinase-3, and azurocidin are primary components of neutrophil azurophilic granules and are encoded by closely linked genes (gene symbols ELA2, PRTN3, and AZU1, respectively) in a region of approximately 50 kb. These genes are coordinately expressed in a granulocyte-specific fashion, but the mechanisms defining this pattern of expression are unknown. To understand the role of chromatin organization in governing the expression of ELA2, PRTN3, and AZU1, we mapped this region of chromosome 19 and identified the adipsin (complement factor D) gene in proximity to the 3′ end of ELA2. We then examined the changes in chromatin structure at the locus which accompany myeloid cell differentiation and identified 17 DNase I hypersensitive sites (DHS 1 to 17) in U-937 cells, an early myelomonocytic cell line expressing high levels of neutrophil elastase. Chemically induced differentiation and concomitant downregulation of AZU1, PRTN3, and ELA2 transcription in U-937 cells is not accompanied by changes in the DHS-pattern. Mature neutrophils, however, do not carry any of these hypersensitive sites, indicating a large degree of chromatin remodeling at this locus accompanying terminal granulocytic differentiation. Sixteen of the 17 DHS identified in U-937 cells are also present in the HL-60 myelomonocytic cell line. Hematopoietic cell lines representing the early erythroid and lymphocyte lineages, and a nonhematopoietic cell line display a subset of the hypersensitive sites. The altered chromatin structure specific to cells that actively transcribe the AZU1-PRTN3-ELA2 genes suggests that chromatin reorganization is an important mechanism regulating the myeloid-specific transcription of this gene cluster.

Mechanisms of mutation induction in germ cells of the mouse as assessed by the specific locus test

Mouse germ cell specific locus mutagenesis data and a molecular characterization of mutant alleles have been reviewed to arrive at an understanding of the mechanism of mutation induction in mammals. (a) The spermatogenic stage specificity for the sensitivity to mutation induction by 20 chemical mutagens is considered. (b) The effects of a saturable repair process and its recovery over time are examined for the mutagenic efficiency of ethylnitrosourea. (c) The mutagenic events following methylnitrosourea and chlorambucil are shown to be mainly deletions. In contrast the mutations recovered after ethylnitrosourea treatment are almost exclusively base pair substitutions. (d) It is emphasized that to date very few specific locus experiments have been designed to test for mutagenic events outside the interval stem cell spermatogonia-mature spermatozoa. A specific locus mutation has recently been shown to be due to loss of heterozygosity via mitotic recombination in an early zygote stage and suggests a broader range of possible mechanisms of mutation when these stages are considered. (e) With the cloning of all 7 marker loci mutation analysis at the molecular level will allow a more direct assessment of the mutation process in future studies.

Genetic and epigenetic control of the proliferation and differentiation of mouse epidermal melanocytes in culture

Serum-free culture of epidermal cell suspensions from neonatal skin of mice of strain C57BL/10JHir (B10) showed that alpha-melanocyte-stimulating hormone (alpha-MSH) was involved in regulating the differentiation of melanocytes by inducing tyrosinase activity, melanosome formation, and dendritogenesis. Dibutyryl adenosine 3':5'-cyclic monophosphate (DBcAMP) similarly induced the differentiation of melanocytes. On the other hand, DBcAMP induced the proliferation of epidermal melanocytes in culture in the presence of keratinocytes. Basic fibroblast growth factor (bFGF) was also shown to stimulate the sustained proliferation of undifferentiated melanoblasts in the presence of DBcAMP and keratinocytes. These results suggest that the proliferation and differentiation of mouse epidermal melanoblasts and melanocytes in culture are regulated by the three factors; namely, cAMP, bFGF, and keratinocyte-derived factors. Moreover, serum-free primary culture of mouse epidermal melanocytes derived from B10 congenic mice, which carry various coat color genes, showed that the coat color genes were involved in regulating the proliferation and differentiation of mouse epidermal melanocytes by controlling the proliferative rate, melanosome formation and maturation, and melanosome distribution.

Up-regulation of specific tyrosinase mRNAs in mouse melanomas with the c2j gene substituted for the wild-type tyrosinase allele: utilization in design of syngeneic immunotherapy models

The expression of cell-specialization genes is likely to be changing in tumor cells as their differentiation declines. Functional changes in these genes might yield unusual peptide epitopes with anti-tumor potential and could occur without modification in the DNA sequence of the gene. Melanomas undergo a characteristic decline in melanization that may reflect altered contributions of key melanocytic genes such as tyrosinase. Quantitative reverse transcriptase-PCR of the wild-type (C) tyrosinase gene in transgenic (C57BL/6 strain) mouse melanomas has revealed a shift toward alternative splicing of the pre-mRNA that generated increased levels of the Delta1b and Delta1d mRNA splice variants. The spontaneous c2j albino mutation of tyrosinase (in the C57BL/6 strain) changes the pre-mRNA splicing pattern. In c2j/c2j melanomas, alternative splicing was again increased. However, while some mRNAs (notably Delta1b) present in C/C were obligatorily absent, others (Delta3 and Delta1d) were elevated. In c2j/c2j melanomas, the percentage of total tyrosinase transcripts attributable to Delta3 reached approximately 2-fold the incidence in c2j/c2j or C/C skin melanocytes. The percentage attributable to Delta1d rose to approximately 2-fold the incidence in c2j/c2j skin, and to 10-fold that in C/C skin. These differences provide a basis for unique mouse models in which the melanoma arises in skin grafted from a C/C or c2j/c2j transgenic donor to a transgenic host of the same or opposite tyrosinase genotype. Immunotherapy designs then could be based on augmenting those antigenic peptides that are novel or overrepresented in a tumor relative to the syngeneic host.

The regulation of tyrosinase gene transcription

Tyrosinase is one of the key enzymes essential for melanogenesis. The control of its activity rests in part at the level of transcriptional regulation. The 5' promoter regions of the human, mouse, chicken, quail, snapping turtle, and frog tyrosinase sequences have been isolated and the mechanisms regulating the activity of these sequences are beginning to be elucidated. This review provides an update on the following aspects of tyrosinase gene regulation: basal promoter elements that determine the site of transcription initiation for RNA polymerase II; the cis-acting elements and DNA-binding factors that mediate melanocyte-specific expression of the tyrosinase gene; promoter elements involved in the temporal control of tyrosinase gene expression; additional elements that may be required to achieve wild-type levels of gene expression; and specific elements that may be required for modulation of tyrosinase gene expression in response to humoral factors or external stimuli that are known to influence the amounts of melanin synthesized by fully differentiated melanocytes. The wild type expression of tyrosinase is the result of the interaction of many different factors and it is becoming evident that certain elements and factors play more than one role in this process.

Inherited somatic mosaicism caused by an intracisternal A particle insertion in the mouse tyrosinase gene

A recessive, fully penetrant mutation (c(m1OR)) at the mouse albino locus that results in coat-color mottling has been characterized at the molecular level. Restriction mapping and DNA sequencing analyses provide evidence that mutants carry a 5.4-kb intracisternal A particle (IAP) element insertion upstream of the tyrosinase (Tyr) promoter. Northern blot analysis and reverse transcription-PCR results show that the tyrosinase gene is expressed at much lower levels in mutant than in wild-type mice. The mutant Tyr gene still retains the tissue-specific expression pattern, and the Tyr transcript is not initiated from the IAP long terminal repeat promoter. We propose that the IAP insertion isolates the promoter of the tyrosinase gene from upstream cis-acting regulatory elements, leading to a substantially decreased level of Tyr gene expression in mutants.

Tyrosinase is a new marker for cell populations in the mouse neural tube

Tyrosinase, the key enzyme in melanin synthesis, is expressed in pigment cells derived from both neural crest and neuroectoderm. The present study was performed to detect tyrosinase promoter activity and tyrosinase gene expression during murine brain development. Mouse tyrosinase 5' region (6.1 Kb) was used to direct lacZ expression in transgenic mice. During embryogenesis, the transgene reproduced tyrosinase expression in pigment cells but was also observed in embryonic neuroectoderm and migrating neural crest cells. Both tyrosinase and lacZ were detected in cell populations often organized in columnar arrangements and found throughout the entire neural tube, in the cranial region as well as in the spinal chord. In the developing brain, the highest density of positive cells was localized to ventricular and subventricular zones and to evaginations of the neural tube such as optic vesicle, pineal gland, and olfactory bulbs. These results demonstrate that tyrosinase promoter activity and tyrosinase expression are not restricted to differentiated pigment cells. We suggest that tyrosinase is a new marker for cell populations in the neural tube, and that expression is correlated to regions undergoing rapid cell proliferation.

Identification of nuclear factors that bind to the mouse tyrosinase gene regulatory region

Several nuclear factors that interact with sequences in the 5' flanking region of the mouse tyrosinase gene were identified using band shift and methylation interference assays. One of these factors bind to an AT-rich sequence, TATCAATTAG, located at -183 base pairs upstream of the transcription start site. To isolate cDNA clone encoding this DNA binding protein, we have screened a lambda gt11 cDNA expression library prepared from mouse melanocyte cell line with a labeled oligonucleotide probe containing its binding site. Complementary DNA clones encoding mouse high mobility group protein HMG-I and its isoform HMG-Y were obtained. HMG-I(Y) is a low molecular size, basic nuclear protein that binds specifically to AT-rich region of double-stranded DNA in vitro. In Northern blot analysis the level of HMG-I(Y) mRNA expression did not correlate with that of tyrosinase or TRP-1. Although the amount of HMG-I(Y) transcripts has no apparent influence on the mouse tyrosinase gene expression, it is possible that HMG-I(Y) binds to the 5' flanking sequence of the tyrosinase gene as an auxiliary factor, and facilitates the binding and activity of other transcription factors.

Neomorphic agouti mutations in obese yellow mice

Several dominant mutations of the mouse agouti coat colour gene have pleiotropic effects that include obesity and a yellow coat. The Ay allele is caused by a large deletion that affects the expression of several contiguous genes. We show that three other obesity-associated agouti mutations, Aiy, Asy and Avy, are due to different molecular alterations that result in ubiquitous expression of a chimaeric RNA that encodes a normal agouti protein. The Aiy and Avy alleles are caused by insertion of an intracisternal A particle element 1 kb or 100 kb, respectively, upstream of agouti coding sequences. These results provide a model for other genes that show allele-specific imprinting, and demonstrate that molecular mechanisms typically responsible for activation of proto-oncogenes can also lead to other disease phenotypes.

A distal tyrosinase upstream element stimulates gene expression in neural-crest-derived melanocytes of transgenic mice: position-independent and mosaic expression

We have assessed the importance of a melanocyte-specific DNase I hypersensitive site and matrix attachment region situated 15 kb upstream of the mouse tyrosinase gene by analysis in transgenic mice. Transgenes containing all, part, or none of this region linked to the tyrosinase promoter and human tyrosinase cDNA were introduced into genetically albino mice, and pigmentation and transgene message levels were analyzed in the resulting transgenic lines. The effect of the upstream region was to enhance significantly gene expression in melanocytes, and to provide position-independent expression of the transgene. Two exceptions to complete position independence were seen; these lines displayed a mosaic expression pattern in which the transgene was expressed fully in some melanocyte clones but less so in others, resulting in transverse stripes of colours ranging from near white to dark grey. Unexpectedly, pigmentation in the eye of all transgenic lines containing the upstream region was nonuniform, in that the neural-crest-derived melanocytes of the choroid and anterior iris contained significantly more pigment than those derived from the optic cup (retinal pigment epithelium and posterior iris). Transgenes containing a small part or none of the upstream region were expressed poorly and in a position-dependent manner; of those lines that were visibly pigmented, expression was equal in the neural crest and optic-cup-derived cells of the eye.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular basis of dark-eyed albinism in the mouse

Dark-eyed albino (C44H) is a recessive allele at the mouse albino (c) locus, which encodes tyrosinase (monophenol,L-dopa:oxygen oxidoreductase, EC 1.14.18.1), the key enzyme in melanin synthesis. Similar to type IB oculocutaneous albinism in humans, overall production of pigment is greatly reduced in dark-eyed albino mice and obvious only in the eyes. We have studied the molecular basis of the c44H mutation and show that expression of the tyrosinase gene is not affected. After sequencing tyrosinase cDNA isolated from c44H/c44H homozygotes, we uncovered a single base alteration from wild type leading to a serine-to-isoleucine exchange. The importance of this mutation was demonstrated by generating transgenic mice containing a mutated tyrosinase minigene. This showed that the single base change was sufficient to severely depress pigment production in transgenic mice. We therefore conclude that the point mutation is responsible and sufficient to generate the dark-eyed albino phenotype.

Genetics and molecular biology of mouse pigmentation

The formation of mouse coat color is a relatively complex developmental process that is affected by a large number of mutations, both naturally occurring and induced. The cloning of the genes in which these mutations occur and the elucidation of the mechanisms by which these mutations disrupt the normal pigmentation pattern is leading to an understanding of the way interactions between gene products lead to a final phenotype.

A yeast artificial chromosome covering the tyrosinase gene confers copy number-dependent expression in transgenic mice

Expression of transgenes in mice often fails to follow the normal temporal and spatial pattern and to reach the same level as the endogenous copies. Only in exceptional cases has position-independent and copy number-dependent expression been reproduced. The size constraint of standard constructs may prevent the inclusion of important remote regulatory elements. Yeast artificial chromosomes (YACs) provide a means of cloning large DNA fragments and the transfer of YAC DNA into somatic cells has been reported. We have previously produced transgenic mice carrying a 35 kilobase YAC construct. Here we report the transfer of a 250 kilobase YAC covering the mouse tyrosinase gene into mice by pronuclear injection of gel-purified YAC DNA. The YAC was inserted into the mouse genome without major rearrangements and expression of the YAC-borne tyrosinase gene resulted in complete rescue of the albino phenotype of the recipient mice. Expression from the transgene reached levels comparable to that of the endogenous gene and showed copy number dependence and position independence.

Spontaneous malignant transformation of melanocytes explanted from Wf/Wf mice with a Kit kinase-domain mutation

The W/Kit mouse locus, affecting proliferation and survival of pigment cells, blood cells, and germ cells, is known to encode a tyrosine kinase growth factor receptor and is considered a protooncogene; yet it has not heretofore been causally implicated in any malignancies of those cells. The Wf/Wf mutant mouse coat comprises viable and inviable melanoblast clones, seen ultimately as pigmented and white transverse stripes--the latter more prominent. Judging from the pattern, all clones initially expand, and the inviable ones then undergo programmed cell death prenatally. To observe skin melanocytes of the viable clones during extended proliferation, the cells were explanted from individual young mice. An unusually large number of primary explants failed to survive--a result consistent with a growth handicap. In 3 of the 10 surviving cell lines, many cells spontaneously underwent a series of striking changes with the classic features of transformation. The two transformed lines that have been tested by grafting to immunosuppressed hosts formed undifferentiated invasive tumors compatible with malignant amelanotic melanoma. None of our 52 other melanocyte lines of the coisogenic wild-type strain and 13 other natural genotypes have become transformed under the same culture conditions. Molecular analysis of the Wf gene revealed a single change from wild-type: a point mutation affecting the catalytic region in the kinase domain of the Kit protein. The apparent growth disadvantage due to the mutation may allow selection for melanocytes mobilizing more efficient pathways, thus leading to neoplasia. Production of both viable and inviable melanoblast clones is unlikely to be due only to the kinase mutation; possibly the degree, duration, and consistency of expression of this locus may be controlled by cis elements outside the coding region.