Oocyte-specific inactivation of Omcg1 leads to DNA damage and c-Abl/TAp63-dependent oocyte death associated with dramatic remodeling of ovarian somatic cells

Aberrant loss of oocytes following cancer treatments or genetic mutations leads to premature ovarian insufficiency (POI) associated with endocrine-related disorders in 1% of women. Therefore, understanding the mechanisms governing oocyte death is crucial for the preservation of female fertility. Here, we report the striking reproductive features of a novel mouse model of POI obtained through oocyte-specific inactivation (ocKO) of Omcg1/Zfp830 encoding a nuclear zinc finger protein involved in pre-mRNA processing. Genetic ablation of OMCG1 in early growing oocytes leads to reduced transcription, accumulation of DNA double-strand breaks and subsequent c-Abl/TAp63-dependent oocyte death, thus uncovering the key role of OMCG1 for oocyte genomic integrity. All adult Omcg1(ocKO) females displayed complete elimination of early growing oocytes and sterility. Unexpectedly, mutant females exhibited a normal onset of puberty and sexual receptivity. Detailed studies of Omcg1(ocKO) ovaries revealed that the ovarian somatic compartment underwent a dramatic structural and functional remodeling. This allowed the cooperation between oocyte-depleted follicles and interstitial tissue to produce estradiol. Moreover, despite early folliculogenesis arrest, mutant mice exhibited sexual cyclicity as shown by cyclical changes in estrogen secretion, vaginal epithelium cytology and genital tract weight. Collectively, our findings demonstrate the key role of Omcg1 for oocyte survival and highlight the contribution of p63 pathway in damaged oocyte elimination in adulthood. Moreover, our findings challenge the prevailing view that sexual cyclicity is tightly dependent upon the pace of folliculogenesis and luteal differentiation.

Genome engineering via homologous recombination in mouse embryonic stem (ES) cells: an amazingly versatile tool for the study of mammalian biology

The ability to introduce genetic modifications in the germ line of complex organisms has been a long-standing goal of those who study developmental biology. In this regard, the mouse, a favorite model for the study of the mammals, is unique: indeed not only is it possible since the late seventies, to add genes to the mouse genome like in several other complex organisms but also to perform gene replacement and modification. This has been made possible via two technological breakthroughs: 1) the isolation and culture of embryonic stem cells (ES), which have the unique ability to colonize all the tissues of an host embryo including its germ line; 2) the development of methods allowing homologous recombination between an incoming DNA and its cognate chromosomal sequence (gene "targeting"). As a result, it has become possible to create mice bearing null mutations in any cloned gene (knock-out mice). Such a possibility has revolutionized the genetic approach of almost all aspects of the biology of the mouse. In recent years, the scope of gene targeting has been widened even more, due to the refinement of the knock-out technology: other types of genetic modifications may now be created, including subtle mutations (point mutations, micro deletions or insertions, etc.) and chromosomal rearrangements such as large deletions, duplications and translocations. Finally, methods have been devised which permit the creation of conditional mutations, allowing the study of gene function throughout the life of an animal, when gene inactivation entails embryonic lethality. In this paper, we present an overview of the methods and scenarios used for the programmed modification of mouse genome, and we underline their enormous interest for the study of mammalian biology.

A 2-Mb YAC/BAC-based physical map of the ovum mutant (Om) locus region on mouse chromosome 11

The embryonic lethal phenotype observed when DDK females are crossed with males from other strains results from a deleterious interaction between the egg cytoplasm and the paternal pronucleus soon after fertilization. We have previously mapped the Om locus responsible for this phenotype, called the DDK syndrome, to an approximately 2-cM region of chromosome 11. Here, we report the generation of a physical map of 28 yeast and bacterial artificial chromosome clones encompassing the entire genetic interval containing the Om locus. This contig, spanning approximately 2 Mb, was used to map precisely genes and genetic markers of the region. We determined the maximum physical interval for Om to be 1400 kb. In addition, 11 members of the Scya gene family were found to be organized into two clusters at the borders of the Om region. Two other genes (Rad51l3 and Schlafen 2) and one EST (D11Wsu78e) were also mapped in the Om region. This integrated map provides support for the identification of additional candidate genes for the DDK syndrome.

BALB/c alleles at modifier loci increase the severity of the maternal effect of the "DDK syndrome"

The Om locus was first described in the DDK inbred mouse strain: DDK mice carry a mutation at Om resulting in a parental effect lethality of F(1) embryos. When DDK females are mated with males of other (non-DDK) inbred strains, e.g., BALB/c, they exhibit a low fertility, whereas the reciprocal cross, non-DDK females x DDK males, is fertile (as is the DDK intrastrain cross). The low fertility is due to the death of (DDK x non-DDK)F(1) embryos at the late-morula to blastocyst stage, which is referred to as the "DDK syndrome." The death of these F(1) embryos is caused by an incompatibility between a DDK maternal factor and the non-DDK paternal pronucleus. Previous genetic studies showed that F(1) mice have an intermediate phenotype compared to parental strains: crosses between F(1) females and non-DDK males are semisterile, as are crosses between DDK females and F(1) males. In the present studies, we have examined the properties of mice heterozygous for BALB/c and DDK Om alleles on an essentially BALB/c genetic background. Surprisingly, we found that the females are quasi-sterile when mated with BALB/c males and, thus, present a phenotype similar to DDK females. These results indicate that BALB/c alleles at modifier loci increase the severity of the DDK syndrome.

lacZ sequences prevent regulated expression of housekeeping genes

In order to dissect the MHC class I H-2K gene regulatory sequences, we p reviously generated transgenic mice containing various H-2K/lacZ fusion genes. However contrary to transgenes where H-2K sequences were fused to other coding sequences, none of the lacZ fusion transgenes was widely ex pressed like H-2K gene. We now show that this silencing also occurs when lacZ is inserted into a larger H-2K genomic construct including promoter and other regulatory elements. Because the 5'H-2K region contains a CpG island, we suspected that the presence of lacZ coding sequences was inte rfering with the mechanism by which the H-2K promoter region is normally unmethylated and transcriptionally active. Indeed, we show that in high ( >10) copy number transgenic mice, insertion of lacZ sequences in the v icinity of the H-2K promoter results in partial or complete methylation of the H-2K CpG island. However, in low (1-3) copy number transgenic mic e no methylation was observed but the transgene was still silent, sugges ting that the silencing effect of lacZ does not only rely on abnormal CpG methylation. Intriguingly, when the H -2/lacZ construct was introduced via embryonic stem (ES) cells, regulate d transgene expression was observed in several chimaeric embryos derived from independent ES clones, but never in adult chimeras. Combined with t he fact that, despite much effort, it has been very difficult to generat e 'blue' mice, our results highlight the transcription-silencing effect of lacZ sequences when they are associated with regulatory sequences of ubiquitously expressed genes.

Role of thalamic axons in the expression of H-2Z1, a mouse somatosensory cortex specific marker

In the H-2Z1 mouse line, postnatal expression of the lacZ containing transgene in the cerebral cortex is restricted to layer IV neurons of the somatosensory area. We have used H-2Z1 embryos in previous heterotopic transplantation experiments to investigate the chronology of determination of areal identity. From the onset of neurogenesis, the cortex was regionalized in domains fated to express or not the somatosensory area-specific transgene. Determination occured 1 day later. In the present study, we show that, in vivo, H-2Z1 expression coincides with invasion of the cortical plate by thalamic afferents. We therefore investigated the role of thalamic innervation in the onset of H-2Z1 expression. For this purpose, we examined the pattern of H-2Z1 expression in perinatal cortical explant, in reeler mutant and MaoA deficient mice, or in animals which had received neonatal lesions affecting the somatosensory cortex or the thalamocortical projection. We found that, around birth, a switch occurs in the control of H-2Z1 expression: whereas H-2Z1 expression developed autonomously in embryonic parietal cortex in the absence of thalamic fibers, a transient requirement for a thalamic axon derived signal was observed postnatally. This property has interesting implications for the plasticity of cortical areas in development and evolution.

Specification of somatosensory area identity in cortical explants

The H-2Z1 transgene is restricted to a subset of layer IV neurons in the postnatal mouse cortex and delineates exactly the somatosensory area. Expression of the H-2Z1 transgene was used as an areal marker to determine when the parietal cortex becomes committed to a somatosensory identity. We have shown previously that grafts dissected from embryonic day 13.5 (E13.5) H-2Z1 cortex and transplanted into the cortex of nontransgenic newborns express H-2Z1 according to their site of origin. Expression was not modified on heterotopic transplantation (). In the present study, whole cortical explants were isolated at E12.5 from noncortical tissues. The explants developed a regionalized expression of H-2Z1, indicating that regionalization takes place and is maintained in vitro. We used this property and confronted embryonic H-2Z1 cortex with presumptive embryonic sources of regionalizing signals in an in vitro grafting procedure. A great majority of E11.5-E13.5 grafts maintained their presumptive expression of H-2Z1 when grafted heterotopically on nontransgenic E13.5-E15.5 explants. However, a significantly lower proportion of E11.5 parietal grafts expressed H-2Z1 in occipital compared with parietal cortex, indicating that somatosensory identity may be partially plastic at E11.5. Earlier stages could not be tested because the E10.5 grafts failed to develop in vitro. The data suggest that commitment to the expression of a somatosensory area-specific marker coincides with the onset of neurogenesis and occurs well before the birth of the non-GABAergic neurons that express H-2Z1 in vivo.

Nonpermissiveness for mouse embryonic stem (ES) cell derivation circumvented by a single backcross to 129/Sv strain: establishment of ES cell lines bearing the Omd conditional lethal mutation

The inbred mouse strain DDK carries a conditional early embryonic lethal mutation that is manifested when DDK females are crossed to males of other inbred strains but not in the corresponding reciprocal crosses. It has been shown that embryonic lethality could be assigned to a single genetic locus called Ovum mutant (Om), on Chromosome (Chr) 11 near Syca 1. In the course of our study of the molecular mechanisms underlying the embryonic lethality, we were interested in deriving an embryonic stem cell bearing the Om mutation in the homozygous state (Omd/Omd). However, it turned out that DDK is nonpermissive for ES cell establishment, with a standard protocol. Here we show that permissiveness could be obtained using Omd/Omd blastocysts with a 75% 129/Sv and 25% DDK genetic background. Several germline-competent Omd/Omd ES cell lines have been derived from blastocysts of this genotype. Such a scenario could be extended to the generation of ES cell lines bearing any mutation present in an otherwise nonpermissive mouse strain.

Beyond 'knock-out' mice: new perspectives for the programmed modification of the mammalian genome

The emergence of gene inactivation by homologous recombination methodology in embryonic stem cells has revolutionized the field of mouse genetics. Indeed, the availability of a rapidly growing number of mouse null mutants has represented an invaluable source of knowledge on mammalian development, cellular biology and physiology and has provided many models for human inherited diseases. In recent years, improvements of the original 'knock-out' strategy, as well as the exploitation of exogenous enzymatic systems that are active in the recombination process, have considerably extended the range of genetic manipulations that can be produced. For example, it is now possible to create a mouse bearing a targeted point mutation as the unique change in its entire genome therefore allowing very fine dissection of gene function in vivo. Chromosome alterations such as large deletions, inversions or translocations can also be designed and will facilitate the global functional analysis of the mouse genome. This will extend the possibilities of creating models of human pathologies that frequently originate from various chromosomal disorders. Finally, the advent of methods allowing conditional gene targeting will open the way for the analysis of the consequence of a particular mutation in a defined organ and at a specific time during the life of a mouse.

I-SceI-induced gene replacement at a natural locus in embryonic stem cells

Gene targeting is a very powerful tool for studying mammalian development and physiology and for creating models of human diseases. In many instances, however, it is desirable to study different modifications of a target gene, but this is limited by the generally low frequency of homologous recombination in mammalian cells. We have developed a novel gene-targeting strategy in mouse embryonic stem cells that is based on the induction of endogenous gap repair processes at a defined location within the genome by induction of a double-strand break (DSB) in the gene to be mutated. This strategy was used to knock in an NH2-ezrin mutant in the villin gene, which encodes an actin-binding protein expressed in the brush border of the intestine and the kidney. To induce the DSB, an I-SceI yeast meganuclease restriction site was first introduced by gene targeting to the villin gene, followed by transient expression of I-SceI. The repair of the ensuing DSB was achieved with high efficiency (6 x 10[-6]) by a repair shuttle vector sharing only a 2.8-kb region of homology with the villin gene and no negative selection marker. Compared to conventional gene-targeting experiments at the villin locus, this represents a 100-fold stimulation of gene-targeting frequency, notwithstanding a much lower length of homology. This strategy will be very helpful in facilitating the targeted introduction of several types of mutations within a gene of interest.

CD4 T cell tolerance to nuclear proteins induced by medullary thymic epithelium

Thymic epithelium is involved in negative selection, but its precise role in selecting the CD4 T cell repertoire remains elusive. By using two transgenic mice, we have investigated how medullary thymic epithelium (mTE) and bone marrow (BM)-derived cells contribute to tolerance of CD4 T cells to nuclear beta-galactosidase (beta-gal). CD4 T cells were not tolerant when beta-gal was expressed in thymic BM-derived cells. In contrast, CD4 T cells of mice expressing beta-gal in mTE were tolerized. Tolerance resulted from presentation of endogenous beta-gal by mTE cells but not from cross-priming. mTE cells presented nuclear beta-gal to a Th clone in vitro, while thymic dendritic cells did not. The data indicate that mTE but not thymic BM-derived cells can use a MHC class II endogenous presentation pathway to induce tolerance to nuclear proteins.

Medullary thymic epithelial cells induce tolerance to intracellular proteins

The role of the medullary thymic epithelial cells in tolerance induction to MHC class I restricted self peptides has been analyzed by studying the beta-galactosidase (beta-gal)-specific cytotoxic T cell response of a transgenic mouse expressing beta-gal in the thymus, skin, and central nervous system (Tg beta-gal mouse). Our results showed that: 1) beta-gal expression in the thymus was limited in a subpopulation of medullary epithelial cells, and bone marrow-derived thymic cells were beta-gal-1; 2) Tg beta-gal mice did not mount an anti-beta-gal CTL response even in the presence of exogenous IL-2, while Tg beta-gal-->B6 chimeras responded to beta-gal as strongly as NTg beta-gal mice; 3) Tg beta-gal mice did not generate CTL against the immunodominant Kb-restricted beta-gal 497-504 peptide; 4) tolerance was due to the thymic epithelial cells that expressed beta-gal because nude mice grafted with thymus from Tg beta-gal mice were also unable to respond to beta-gal; 5) the Tg beta-gal mouse-derived beta-gal+ medullary epithelial TEC.X10 line presented the Kb-restricted beta-gal 497-504 epitope. In conclusion, these results demonstrate that medullary thymic epithelial cells induce a complete tolerance towards class I-restricted self peptides presented on their own surface.

A comparison of the properties of Sox-3 with Sry and two related genes, Sox-1 and Sox-2

The Sox gene family consists of a large number of embryonically expressed genes related via the possession of a 79-amino-acid DNA-binding domain known as the HMG box. Partial clones for the first three Sox genes (al-a3) were isolated by homology to the HMG box of the testis-determining gene Sry and are now termed Sox-1, Sox-2 and Sox-3, Sox-3 is highly conserved amongst mammalian species and is located on the X chromosome. This has led to the proposal that Sry evolved from Sox-3. We present the cloning and sequencing of Sox-1, Sox-2 and Sox-3 from the mouse and show that Sox-3 is most closely relate to Sry. We also confirm that mouse Sox-3 is located on the X chromosome between Hprt and Dmd. Analysis of the distribution of Sox-3 RNA shows that its main site of expression is in the developing central nervous system, suggesting a role for Sox-3 in neural development. Moreover, we demonstrate that Sox-3, as well as Sox-1 and Sox-2, are expressed in the urogenital ridge and that their protein products are able to bind the same DNA sequence motif as Sry in vitro, but with different affinities. These observations prompt discussion of an evolutionary link between the genes and support the model that Sry has evolved from Sox-3. However our findings imply that if this is true, then Sry has undergone concomitant changes resulting in loss of CNS expression and altered DNA-binding properties.

Medullary thymic epithelial cells induce tolerance to intracellular proteins

The role of the medullary thymic epithelial cells in tolerance induction to MHC class I restricted self peptides has been analyzed by studying the beta-galactosidase (beta-gal)-specific cytotoxic T cell response of a transgenic mouse expressing beta-gal in the thymus, skin, and central nervous system (Tg beta-gal mouse). Our results showed that: 1) beta-gal expression in the thymus was limited in a subpopulation of medullary epithelial cells, and bone marrow-derived thymic cells were beta-gal-1; 2) Tg beta-gal mice did not mount an anti-beta-gal CTL response even in the presence of exogenous IL-2, while Tg beta-gal-->B6 chimeras responded to beta-gal as strongly as NTg beta-gal mice; 3) Tg beta-gal mice did not generate CTL against the immunodominant Kb-restricted beta-gal 497-504 peptide; 4) tolerance was due to the thymic epithelial cells that expressed beta-gal because nude mice grafted with thymus from Tg beta-gal mice were also unable to respond to beta-gal; 5) the Tg beta-gal mouse-derived beta-gal+ medullary epithelial TEC.X10 line presented the Kb-restricted beta-gal 497-504 epitope. In conclusion, these results demonstrate that medullary thymic epithelial cells induce a complete tolerance towards class I-restricted self peptides presented on their own surface.

A high-resolution map around the locus Om on mouse Chromosome 11

The locus Om (ovum mutant) identified in the mouse strain DDK affects the viability of (DDK x non-DDK)F1 preimplantation embryos. We previously located this locus on Chromosome (Chr) 11 close to Scya2 (Baldacci et al. Mamm. Genome 2, 100-105, 1992). Here we report a high-resolution map of the region around Om based on a large number of backcross individuals. The same region has been analyzed on the EUCIB backcross, and the two maps have been compared. The results define the proximal and distal boundaries for the Om mutation as Scya2 and D11Mit36 respectively. The distance between these two markers is about 2 cM. These data should facilitate the positional cloning and molecular characterization of Om.

Molecular heterogeneity of progenitors and radial migration in the developing cerebral cortex revealed by transgene expression

We have analyzed the developmental pattern of beta-galactosidase (beta-gal) expression in the cerebral cortex of the beta 2nZ3'1 transgenic mouse line, which was generated using regulatory elements of the beta 2-microglobulin gene and shows ectopic expression in nervous tissue. From embryonic day 10 onward, beta-gal was expressed in the medial and dorsal cortices, including the hippocampal region, whereas lateral cortical areas were devoid of labeling. During the period of cortical neurogenesis (embryonic days 11-17), beta-gal was expressed by selective precursors in the proliferative ventricular zone of the neocortex and hippocampus, as well as by a number of migrating and postmigratory neurons arranged into narrow radial stripes above the labeled progenitors. Thus, the transgene labels a subset of cortical progenitors and their progeny. Postnatally, radial clusters of beta-gal-positive neurons were discernible until postpartum day 10. At this age, the clusters were 250 to 500 microns wide, composed of neurons spanning all the cortical layers and exhibiting several neuronal phenotypes. These data suggest molecular heterogeneity of cortical progenitors and of the cohorts of postmitotic neurons originating from them, which implies intrinsic molecular mosaicism in both cortical progenitors and developing neurons. Furthermore, the data show that neurons committed to the expression of the transgene migrate along very narrow, radial stripes.

Disruption of murine Hexa gene leads to enzymatic deficiency and to neuronal lysosomal storage, similar to that observed in Tay-Sachs disease

Tay-Sachs disease is an autosomal recessive lysosomal storage disease caused by beta-hexosaminidase A deficiency and leads to death in early childhood. The disease results from mutations in the HEXA gene, which codes for the alpha chain of beta-hexosaminidase. The castastrophic neurodegenerative progression of the disease is thought to be a consequence of massive neuronal accumulation of GM2 ganglioside and related glycolipids in the brain and nervous system of the patients. Fuller understanding of the pathogenesis and the development of therapeutic procedures have both suffered from the lack of an animal model. We have used gene targeting in embryonic stem (ES) cells to disrupt the mouse Hexa gene. Mice homozygous for the disrupted allele mimic several biochemical and histological features of human Tay-Sachs disease. Hexa-/- mice displayed a total deficiency of beta-hexosaminidase A activity, and membranous cytoplasmic inclusions typical of GM2 gangliosidoses were found in the cytoplasm of their neurons. However, while the number of storage neurons increased with age, it remained low compared with that found in human, and no apparent motor or behavioral disorders could be observed. This suggests that the presence of beta-hexosaminidase A is not an absolute requirement of ganglioside degradation in mice. These mice should help us to understand several aspects of the disease as well as the physiological functions of hexosaminidase in mice. They should also provide a valuable animal model in which to test new forms of therapy, and in particular gene delivery into the central nervous system.

Developmental expression of H-2K major histocompatibility complex class I transgenes requires the presence of proximal introns

The pattern of expression of the H-2K major histocompatibility complex (MHC) class I gene is complex. During embryonic development H-2K mRNA, detectable from midgestation, is poorly expressed. In the adult, H-2K expression is nearly ubiquitous but transcriptional regulation occurs leading to different mRNA levels in the different organs of the mouse. In vitro studies have shown that most of the regulatory elements controlling H-2K gene transcription are located in the 5' region of the gene. However, using fusion transgenes in which reporter genes were under the control of 2 kb of H-2K 5' regulatory region, we have previously shown that this region was not sufficient to ensure correct developmental transgene expression. By contrast, a native 9.25 kb H-2K transgene was expressed appropriately both in the adult and in the embryo. In order to localise more precisely the cis-acting regulatory sequences involved in H-2K developmental expression, we have now constructed new transgenic lines containing H-2Kb transgenes that were deleted from specific parts of the H-2Kb gene. We show that deletion of either the H-2K 3' flanking region or the 5 (out of 7) distal introns results in an expression of the transgenes which is similar to that of the endogenous H-2K gene, both in the adult and during embryonic development. By contrast, deletion of all the introns or of the two proximal ones abrogates H-2K transgene expression. Our data reveal the complexity of H-2K regulation and highlight the crucial role of proximal introns in H-2K expression in the living organism.

Compartmentalization of lambda subtype expression in the B cell repertoire of mice with a disrupted or normal C kappa gene segment

The establishment of the B cell repertoire depends on two major parameters. The first is determined by mechanistic processes that give rise to a great diversity of B cell receptors from a combination of multiple gene segments. The second is dominated by selective processes that recruit B cell clones via their immunoglobulin receptors. To assess the impact of these parameters on the composition of B cell repertoire, we constructed a mouse model displaying a B cell repertoire limited in its diversity. To this end, we disrupted the C kappa segment by gene targeting. B cells from such mutant mice do not express the kappa light chain. Their light chain repertoire is therefore limited by the expression of only four main lambda light chains: lambda 1, lambda 2(V2), lambda 2(Vx) and lambda 3. In this study we described the proportions of each lambda subtype in various lymphoid compartments. Our results show that the lambda 1 subtype is dominant in the spleen and the bone marrow. Moreover, lambda 1 prevalence is independent of the wild or mutant C kappa genotype. These results suggest that the mechanistic processes are mainly responsible for the bias in lambda subtype expression. On the other hand, the lambda 2(V2) and/or lambda 3 subtypes are expressed at higher levels in the peritoneal cavity. Their prevalence is again observed regardless of the C kappa genotype and seems to be due to B1 cells. These results suggest that different mechanistic processes could control lambda subtype expression in B1 and B2 cell lineages.

Early determination of a mouse somatosensory cortex marker

The mammalian neocortex is subdivided into functionally distinct areas differing in cytoarchitecture and connectivity. Areal specification is thought to occur late in development and to be controlled by extrinsic cues, particularly thalamic afferents. We have produced a transgenic mouse line in which beta-galactosidase expression in the neocortex is largely restricted to layer-IV neurons of the somatosensory area. Transgene expression in these mice may be considered as an intrinsic marker of a somatosensory cortex identity. We investigated whether the fate of pieces of embryonic cortex from transgenic embryos is modified after transplantation to ectopic locations. Parietal or occipital cortex obtained on embryonic days 14-16 maintained their characteristics with respect to transgene expression after heterotopic transplantation to the cerebellum or neocortex of newborn hosts. This shows that the specification of neocortical areas involves a previously unsuspected early step of areal determination.

Disruption of Krox-20 results in alteration of rhombomeres 3 and 5 in the developing hindbrain

The zinc finger gene Krox-20 is transcribed in two alternate segments (rhombomeres) of the developing hindbrain. To investigate its function, we have used homologous recombination to generate mice carrying an in-frame insertion of the E. coli lacZ gene within Krox-20. Analysis of the beta-galactosidase pattern in heterozygous embryos confirmed the known profile with expression restricted to rhombomeres (r) 3 and 5. Mice homozygous for the mutation die during the first two weeks after birth. Anatomical analysis of the hindbrain and of the cranial nerves during embryogenesis, combined with the determination of the expression patterns of rhombomere-specific genes, demonstrated that Krox-20 inactivation results in a marked reduction or elimination of r3 and r5. We conclude that Krox-20, although not required for the initial delimitation of r3 and r5, plays an important role in the process of segmentation governing hindbrain development.

Unexpected position-dependent expression of H-2 and beta 2-microglobulin/lacZ transgenes

In order to study sequences involved in the developmentally regulated and tissue-specific expression of the class I Major Histocompatibility Complex (MHC) genes, we have constructed several H-2/lacZ transgenic lines in which the 5' regulatory sequences of the H-2Kb gene are linked to the Escherichia coli beta-galactosidase (lacZ) gene. In five H-2/lacZ lines, the pattern of lacZ expression, detected histochemically varied greatly from line to line. None of the H-2/lacZ transgenes were transcribed in cells normally expressing a high level of endogenous H-2 molecules, although these H-2 regulatory sequences have been shown to be sufficient to drive tissue-specific expression of other reporter genes. Interestingly, when constructs containing 5' beta 2-microglobulin (beta 2m) regulatory sequences linked to lacZ were used to derive transgenic lines, similar results were obtained. A survey of lacZ labeling in H-2/lacZ and beta 2m/lacZ transgenic mice strongly suggests that these transgenes are very sensitive to position effect, lacZ expression being controlled by endogenous chromosomal regulatory elements specific for each insertion site. Here we describe the complex pattern of lacZ expression in the different transgenic lines during development; we discuss the unusual properties of these transgenes and underline their potential use for developmental studies and characterization of genomic sequences involved in spatiotemporal gene expression.