Sex chromosomes in vertebrates: XX/XY against ZZ/ZW.

It was noted only recently that the mammalian X/Y sex chromosome system originated late in our therian ancestors. The question is, what makes the X/Y sex chromosome system so special that it has replaced the original Z/W sex chromosome system? Two classes of genes are overrepresented on the X chromosome: sex and reproduction genes and brain-related genes. The X chromosome has acquired bursts of young male-biased genes engaged in sex and reproduction which exemplifies the dynamics of sex chromosome evolution. Brain genes are old genes and display the most conserved transcriptome. The new therian X chromosome was formed by fusion of chromosome building blocks already bearing the highest density of brain genes in the ancestral vertebrate karyotype. These building blocks constitute an X chromosome haplotype that undergoes strong selection and benefits both sexes by oscillating between males and females. We believe that this strategy is superior to male-driven Z/W sex chromosome evolution.

Chromosomal speciation of humans and chimpanzees revisited: studies of DNA divergence within inverted regions.

The human and chimpanzee karyotypes are distinguishable in terms of nine pericentric inversions. According to the recombination suppression model of speciation, these inversions could have promoted the process of parapatric speciation between hominoid populations ancestral to chimpanzees and humans. Were recombination suppression to have occurred in inversion heterozygotes, gene flow would have been reduced, resulting in the accumulation of genetic incompatibilities leading to reproductive isolation and eventual speciation. In an attempt to detect the molecular signature of such events, the sequence divergence of non-coding DNA was compared between humans and chimpanzees. Precise knowledge of the locations of the inversion breakpoints permitted accurate discrimination between inverted and non-inverted regions. Contrary to the predictions of the recombination suppression model, sequence divergence was found to be lower in inverted chromosomal regions as compared to non-inverted regions, albeit with borderline statistical significance. Thus, no signature of recombination suppression resulting from inversion heterozygosity appears to be detectable by analysis of extant human and chimpanzee non-coding DNA. The precise delineation of the inversion breakpoints may nevertheless still prove helpful in identifying potential speciation-relevant genes within the inverted regions.

Isochores and replication time zones: a perfect match.

The mammalian genome is not a random sequence but shows a specific, evolutionarily conserved structure that becomes manifest in its isochore pattern. Isochores, i.e. stretches of DNA with a distinct sequence composition and thus a specific GC content, cause the chromosomal banding pattern. This fundamental level of genome organization is related to several functional features like the replication timing of a DNA sequence. GC richness of genomic regions generally corresponds to an early replication time during S phase. Recently, we demonstrated this interdependency on a molecular level for an abrupt transition from a GC-poor isochore to a GC-rich one in the NF1 gene region; this isochore boundary also separates late from early replicating chromatin. Now, we analyzed another genomic region containing four isochores separated by three sharp isochore transitions. Again, the GC-rich isochores were found to be replicating early, the GC-poor isochores late in S phase; one of the replication time zones was discovered to consist of one single replicon. At the boundaries between isochores, that all show no special sequence elements, the replication machinery stopped for several hours. Thus, our results emphasize the importance of isochores as functional genomic units, and of isochore transitions as genomic landmarks with a key function for chromosome organization and basic biological properties.

Recruitment of old genes to new functions: evidences obtained by comparing the orthologues of human XLMR genes in mouse and chicken.

Gene mapping data indicate that the human X chromosome is enriched in genes that affect both, higher cognitive efficiency and reproductive success. This raises the question whether these functions are ancient, or whether conserved X-linked genes were recruited to new functions. We have studied three X-linked mental retardation (XLMR) genes by RNA in situ hybridization in mouse and in chicken, in which these genes are autosomal: Rho guanine nucleotide exchange factor 6 (ARHGEF6), oligophrenin (OPHN1), and p21 activated kinase 3 (PAK3). In the mouse these genes are specifically expressed in telencephalic regions. Their orthologues in the chicken gave patterns of similar specificity in ancient parts of the brain, i.e. cerebellum and mesencephalon, but were not expressed in the telencephalon. Also in the testes, specific expression was only found in mouse, not in chicken. These data are interpreted such that certain genes on the X chromosome gained novel functions during evolution.

Ellobius lutescens: sex determination and sex chromosome.

The mole vole Ellobius lutescens is an interesting animal, not only concerning its sex determination mechanism without the Y-chromosomal Sry gene, that triggers sex determination in nearly all other mammalian species, but also regarding the karyotype with an odd number of chromosomes, being identical in male and female animals. The odd chromosome represents the X chromosome, and therefore, even males do not have a Y chromosome. We present an overview of a search for candidate genes of male sex determination in the mole vole Ellobius lutescens. A singular X raises questions about the need for X chromosome inactivation in female cells. We present preliminary data that support a hypothesis that the E. lutescens Xist gene may be degenerated and thus non-functional.

Localization of human X chromosomal mental retardation (MRX) genes in chicken and comparison with the chicken genome sequence data.

In an ongoing study human X chromosomal mental retardation genes (MRX) were mapped in the chicken genome. Up to now the homologs of 13 genes were localized by FISH techniques. Four genes from HSAXp (TM4SF2, RSK2/RPS6KA3, NLGN4, ARX) map to GGA1q13-->q31, and seven genes from HSAXq (OPHN1, AGTR2, ARHGEF6, PAK3, FACL4/ACS4, FMR2, ATRX) to GGA4p. The gene-rich region of HSAXq28 proved to be much less conserved. GDI1 localized to GGA1pter and SLC6A8 to a mid-sized microchromosome. The order of the genes was determined from the newly available genome sequence data from chicken, which reveals exact colinearity between the genes in HSAXp and GGA1q13-->q31, but completely scrambled gene order between the genes with common synteny from HSAXq and GGA4p. This result supports the hypothesis that the human X chromosome is a real ancient autosomal linkage group.

Breakpoint analysis of the pericentric inversion between chimpanzee chromosome 10 and the homologous chromosome 12 in humans.

During this study, we analysed the pericentric inversion that distinguishes human chromosome 12 (HSA12) from the homologous chimpanzee chromosome (PTR10). Two large chimpanzee-specific duplications of 86 and 23 kb were observed in the breakpoint regions, which most probably occurred associated with the inversion. The inversion break in PTR10p caused the disruption of the SLCO1B3 gene in exon 11. However, the 86-kb duplication includes the functional SLCO1B3 locus, which is thus retained in the chimpanzee, although inverted to PTR10q. The second duplication spans 23 kb and does not contain expressed sequences. Eleven genes map to a region of about 1 Mb around the breakpoints. Six of these eleven genes are not among the differentially expressed genes as determined previously by comparing the human and chimpanzee transcriptome of fibroblast cell lines, blood leukocytes, liver and brain samples. These findings imply that the inversion did not cause major expression differences of these genes. Comparative FISH analysis with BACs spanning the inversion breakpoints in PTR on metaphase chromosomes of gorilla (GGO) confirmed that the pericentric inversion of the chromosome 12 homologs in GGO and PTR have distinct breakpoints and that humans retain the ancestral arrangement. These findings coincide with the trend observed in hominoid karyotype evolution that humans have a karyotype close to an ancestral one, while African great apes present with more derived chromosome arrangements.

The genomic organization of the murine Mlc1 (Wkl1, KIAA0027) gene.

The human MLC1( WKL1, KIAA0027) gene encodes a putative transmembrane protein expressed exclusively in brain. Recessive mutations within this gene cause megalencephalic leukoencephalopathy with subcortical cysts (MLC, MIM 604004, 605908). Furthermore, a missense mutation in this gene is suggestively linked with hereditary catatonic schizophrenia in a large pedigree. The murine gene Mlc1is composed of 12 exons spanning approximately 20 kb, and all exon-intron boundaries conform to the GT/AG consensus. The single copy transcript after splicing is approximately 2.8 kb in length, it contains 496 bp of 5' untranslated region (5'-UTR) and 1143 bp of 3'-UTR, and encodes a protein of 382 amino acids. Potential binding sites for transcription factors including CCAAT-boxes are present in the 5'-flanking region. Fluorescent in situ hybridization localizes the gene to mouse chromosome 15E-F, a region syntenic to human chromosome 22q13. The characterization of the genomic structure of the murine gene will facilitate studies of gene function and physiological properties of the encoded protein in transgenic mouse models.

A comparative ZOO-FISH analysis in bats elucidates the phylogenetic relationships between Megachiroptera and five microchiropteran families.

Fluorescence in-situ hybridization with human whole chromosome painting probes (WCPs) was applied to compare the karyotypes of members of five bat families. Twenty-five evolutionarily conserved units (ECUs) were identified by ZOO-FISH analysis. In 10 of these 25 ECUs, thorough GTG-band comparison revealed an identical banding pattern in all families studied. Differences in the remaining ECUs were used as characters to judge the phylogenetic relationships within Chiroptera. Close relationships were found between Rhinolophidae and Hipposideridae. Also closely related are the representatives of the yangochiropteran families Phyllostomidae (genus studied: Glossophaga, Volleth et al. 1999), Molossidae and Vespertilionidae. All microchiropteran species studied here share four common features not found in the megachiropteran species Eonycteris spelaea. Two of these are considered as derived characters with a high probability of parallel evolution. On the other hand, Eonycteris shares one common, probably derived feature with the rhinolophoid families Rhinolophidae and Hipposideridae and an additional one only with Hipposideridae. At the moment, the relationships between Yangochiroptera, Rhinolophoidea and Megachiroptera must be left in an unsolved trichotomy. Comparison of neighboring segment combinations found in Chiroptera with those found in other mammalian taxa revealed six synapomorphic features for Chiroptera. Therefore, for karyological reasons, monophyly of Chiroptera is strongly supported.

The sex determination in Ellobius lutescens remains bizarre.

Mammalian sex determination and gonad differentiation are the result of a complex interaction of fine-tuned spatial and temporal gene expression with threshold levels of individual genes. The male pathway is initiated by SRY. Some exceptional mammals determine male sex without the SRY gene and even without a Y chromosome. Ellobius lutescens in this report is one example of this "weird" species. We provide key data on the genomic level that there are no coarse differences in the genomes of male and female animals by comparative genomic hybridization. On the gene level we studied the gene Nr5a1 for the orphan nuclear receptor, steroidogenic factor SF-1, a central constituent for gonad differentiation and adrenal gland development. The Ellobius lutescens Nr5a1 gene was mapped to the proximal short arm of chromosome 2 by fluorescence in situ hybridization. In addition, we provide evidence by linkage analysis in two E. lutescens pedigrees that Nr5a1 is not the key male sex-determining gene in Ellobius lutescens.

Evolution of the human X--a smart and sexy chromosome that controls speciation and development.

In humans, as in other mammals, sex is determined by an XX female/XY male chromosome system. Most attention has focused on the small, degenerate Y chromosome, which bears the male-dominant gene SRY. The X, in contrast, has been considered a well-behaved and immaculately conserved element that has hardly changed since the pre-mammal days when it was just another autosome pair. However, the X, uniquely in the genome, is present in two copies in females and only one in males. This has had dire consequences genetically on the evolution of its activity--and now it appears, on its gene content and/or the function of its genes. Here we will discuss the origin of the human X, and the evolution of dosage compensation and gene content, in the light of recent demonstrations that particular functions in sex and reproduction and cognition have accumulated on it.

A high density of X-linked genes for general cognitive ability: a run-away process shaping human evolution?

The incidence of mental disability is 30% higher in males than in females. We have examined entries in the OMIM database that are associated with mental disability and for several other common defects. Our findings indicate that compared with the autosomes, the X chromosome contains a significantly higher number of genes that, when mutated, cause mental impairment. We propose that these genes are involved in the development of cognitive abilities and thus exert a large X-chromosome effect on general intelligence in humans. We discuss these conclusions with regard to the conservation of the vertebrate X-chromosomal linkage group and to human evolution.

Molecular characterization and gene content of breakpoint boundaries in patients with neurofibromatosis type 1 with 17q11.2 microdeletions.

Homologous recombination between poorly characterized regions flanking the NF1 locus causes the constitutional loss of approximately 1.5 Mb from 17q11.2 covering > or =11 genes in 5%-20% of patients with neurofibromatosis type 1 (NF1). To elucidate the extent of microheterogeneity at the deletion boundaries, we used single-copy DNA fragments from the extreme ends of the deleted segment to perform FISH on metaphase chromosomes from eight patients with NF1 who had large deletions. In six patients, these probes were deleted, suggesting that breakage and fusions occurred within the adjacent highly homologous sequences. Reexamination of the deleted region revealed two novel functional genes FLJ12735 (AK022797) and KIAA0653-related (WI-12393 and AJ314647), the latter of which is located closest to the distal boundary and is partially duplicated. We defined the complete reading frames for these genes and two expressed-sequence tag (EST) clusters that were reported elsewhere and are associated with the markers SHGC-2390 and WI-9521. Hybrid cell lines carrying only the deleted chromosome 17 were generated from two patients and used to identify the fusion sequences by junction-specific PCRs. The proximal breakpoints were found between positions 125279 and 125479 in one patient and within 4 kb of position 143000 on BAC R-271K11 (AC005562) in three patients, and the distal breakpoints were found at the precise homologous position on R-640N20 (AC023278). The interstitial 17q11.2 microdeletion arises from unequal crossover between two highly homologous WI-12393-derived 60-kb duplicons separated by approximately 1.5 Mb. Since patients with the NF1 large-deletion syndrome have a significantly increased risk of neurofibroma development and mental retardation, hemizygosity for genes from the deleted region around the neurofibromin locus (CYTOR4, FLJ12735, FLJ22729, HSA272195 (centaurin-alpha2), NF1, OMGP, EVI2A, EVI2B, WI-9521, HSA272196, HCA66, KIAA0160, and WI-12393) may contribute to the severe phenotype of these patients.

Demethylation, reactivation, and destabilization of human fragile X full-mutation alleles in mouse embryocarcinoma cells.

The major causes of fragile X syndrome are mutational expansion of the CGG repeat in the FMR1 gene, hypermethylation, and transcriptional silencing. Most fragile X embryos develop somatic mosaicism of disease-causing "full" expansions of different lengths. Homogeneity of the mosaic patterns among multiple tissues in the same individual indicates that these previously unstable expansions acquire mitotic stability early in fetal life. Since mitotic stability is found strictly associated with hypermethylation in adult tissues, current theory has fixed the time of instability to developmental stages when fully expanded CGG repeats exist in an unmethylated state. We used murine embryocarcinoma (EC) cells (PC13) as a model system of pluripotent embryonic cells. Hypermethylated and unmethylated full expansions on human fragile X chromosomes were transferred from murine A9 hybrids into EC cells, by means of microcell fusion. As demonstrated in the present study for the first time, even full expansion alleles that were fully methylated and stable in the donors' fibroblasts and in A9 became demethylated, reactivated, and destabilized in undifferentiated EC hybrids. When destabilized expansions were reintroduced from EC cells into A9, instability was reversed to stability. Our results strongly support the idea that fully expanded alleles are initially unstable and unmethylated in the human embryo and gain stability upon genetic or epigenetic change of the embryonic cells.

Protein kinase C isoenzyme: selective expression pattern of protein kinase C-θ during mouse development.

Protein kinase C (PKC)-θ, a serine/threonine protein kinase and novel PKC subfamily member, has been recently identified as an essential component of the T cell synapse which activates the NF-kB signaling cascade leading to expression of the IL-2 gene during T cell activation. By RNA in situ hybridization to whole-body embryo sections it is shown that the murine PKCθ is specifically expressed in tissues with hematopoietic and lymphopoietic activity. Expression is also evident in skeletal muscle. A further highly specific expression was observed in the peripheral and central nervous system which is described in detail. Expression in the brain persists up to adult stages.

Molecular cloning and characterization of the human protein kinase D2. A novel member of the protein kinase D family of serine threonine kinases.

We have isolated the full-length cDNA of a novel human serine threonine protein kinase gene. The deduced protein sequence contains two cysteine-rich motifs at the N terminus, a pleckstrin homology domain, and a catalytic domain containing all the characteristic sequence motifs of serine protein kinases. It exhibits the strongest homology to the serine threonine protein kinases PKD/PKCmicro and PKCnu, particularly in the duplex zinc finger-like cysteine-rich motif, in the pleckstrin homology domain and in the protein kinase domain. In contrast, it shows only a low degree of sequence similarity to other members of the PKC family. Therefore, the new protein has been termed protein kinase D2 (PKD2). The mRNA of PKD2 is widely expressed in human and murine tissues. It encodes a protein with a molecular mass of 105 kDa in SDS-polyacrylamide gel electrophoresis, which is expressed in various human cell lines, including HL60 cells, which do not express PKCmicro. In vivo phorbol ester binding studies demonstrated a concentration-dependent binding of [(3)H]phorbol 12,13-dibutyrate to PKD2. The addition of phorbol 12,13-dibutyrate in the presence of dioleoylphosphatidylserine stimulated the autophosphorylation of PKD2 in a synergistic fashion. Phorbol esters also stimulated autophosphorylation of PKD2 in intact cells. PKD2 activated by phorbol esters efficiently phosphorylated the exogenous substrate histone H1. In addition, we could identify the C-terminal Ser(876) residue as an in vivo phosphorylation site within PKD2. Phosphorylation of Ser(876) of PKD2 correlated with the activation status of the kinase. Finally, gastrin was found to be a physiological activator of PKD2 in human AGS-B cells stably transfected with the CCK(B)/gastrin receptor. Thus, PKD2 is a novel phorbol ester- and growth factor-stimulated protein kinase.

Spectral karyotyping of mouse cell line WMP2.

We have evaluated the mouse cell line WMP2 using both GTG-banding analysis and spectral karyotyping to verify the reliability of using this established cell line derived from WMP/WMP mice. The WMP cell lines contain easily identifiable metacentric fusion chromosomes and are used extensively for gene mapping. Because of karyotypical changes in the WMP1 cell line, WMP2 was examined. Our results demonstrate that WMP2 is stable during culture, and the karyotype is simple and easy to use. Based on the findings discussed in this paper, we recommend the use of the WMP2 cell line for future prospective gene mapping in the mouse.

Identification of a novel putative Ran-binding protein and its close homologue.

In the process of cloning genes at the breakpoint of t(5;14) (q34;q11), a recurring translocation in acute lymphoblastic leukemia, we isolated and characterized a novel gene at 5q34, and a close human homologue (66% amino acid identity) located at 8p11-12. The presence of an importin-beta N-terminal domain at their N-terminus, their size of approximately 110 kD, their nuclear localization and the identity of the homologue to a gene of a recently submitted RanGTP binding protein (RanBP16), suggest that its protein is a novel member of the importin-beta superfamily of nuclear transport receptors, therefore called RanBP17. Northern blot analysis of human tissues revealed a ubiquitous expression pattern of the RanBP16 gene and a very restricted expression pattern of the RanBP17 gene, showing high expression in testis and pancreas. Both genes are evolutionary conserved and show a high (99 and 94%) amino acid conservation with their murine counterparts and a striking similarity (40%) to a protein product of Caenorhabditis elegans (C35A5.8).

A comparison of the expression pattern of five genes of the family of small leucine-rich proteoglycans during mouse development.

For five members of the family of the small leucine-rich proteoglycans (SLRPs), the expression pattern during fetal development was analyzed. RNA in situ hybridization on whole body sections of mouse embryos was performed for biglycan (Bgn), decorin (Dcn), fibromodulin (Fmod), chondroadherin (Chad), and lumican (Lum). Special attention was given to the question of whether these patterns coincide only with sites of collagen secretion in connective tissue during tissue modeling or if expression can be observed at specific sites of organ differentiation also. In general, Fmod, Lum, and Bgn are expressed at sites of cartilage and bone formation and interstitial tissue deposition; Chad is expressed only at sites of cartilage; and Dcn is expressed only at sites of interstitial tissue deposition. However, there are some distinct developmental stages where no collagen secretion is known to occur. For example, this applies for the expression of Fmod in the forming somites of stage 9.5 postconception (p.c.), for Dcn and Lum in later stage embryos in the pituitary gland and dorsal root ganglia, and for Bgn and Dcn during differentiation in the kidney. These studies provide further evidence for a role of these molecules during connective tissue organization but also for an involvement at specific sites of organ differentiation.

The 5' part of the mouse immunoglobulin kappa locus as a continuously cloned structure.

Five contigs of the 5' part of the immunoglobulin kappa locus (F. Röschenthaler et al., Eur. J. Immunol. 1999. 29: 2065 - 2071) have been linked by cosmid clones prepared from bacterial artificial chromosomes (BACs) and by PCR. One of the previously defined contigs which contains three pseudogenes (Z7) was shown by fluorescence in situ hybridization to be located near the kappa locus on chromosome 6, but not within the locus; the three Vkappa genes are therefore now classified as orphons. A Vkappa9 / 10 gene, which was sequenced previously, was now localized within the locus, and two additional Vkappa genes were identified, a potentially functional Vkappa24 gene and a pseudogene of the Vkappa9 / 10 family. This brings the number of localized and sequenced Vkappa genes in the locus to 140; 75 of them are functional, 21 potentially functional and 44 pseudogenes. The 5' part of the kappa locus is now one contig of 1.88 megabase (Mb); it comprises 82 Vkappa genes. Other contigs of the locus are 65 kb, 105 kb and 1.04 Mb in size and contain 2, 5 and 51 Vkappa genes, respectively. The contigs are separated by three gaps of 10 - 40 kb each. Detailed restriction maps and other structural details of the kappa locus are deposted in the Internet at http://www.med. uni-muenchen.de / biochemie / zachau / kappa.htm.