Regulated expression of the small nuclear ribonucleoprotein particle protein SmN in embryonic stem cell differentiation

Disruption of genes regulated during hematopoietic differentiation of mouse embryonic stem cells

A retroviral gene trap vector (U3Tkneo) that selects for integrations in or near expressed 5' exons has been used to identify genes that are repressed during hematopoietic differentiation of mouse totipotent embryonic stem cells. The vector contains coding sequences for an HSV-thymidine kinase/neomycin phosphotransferase fusion protein in the U3 region of a Moloney murine leukemia virus LTR and allows selection for (G418) and against (Ganciclovir; GC) U3 gene expression. A total of 208 neomycin-resistant clones were isolated following infection with U3tkneo and screened for integrations into regulated genes by using a two-step, semisolid culture system that supports hematopoietic differentiation. Two clones contained U3Tkneo integrations in genes that were repressed selectively in hematopoietic cells. Analysis of upstream proviral flanking sequences indicated that both integrations occurred into unknown genes. One up-stream sequence identified a cellular transcript that was expressed differentially in the kidneys and liver of adult mice. When this fusion gene was passaged to the germ line, homozygous offspring with nearly null mutations were obtained. However, mutant mice were normal, suggesting that potential loss of function phenotypes are subtle and may be restricted to the kidneys and the liver.

Expression and methylation of imprinted genes during in vitro differentiation of mouse parthenogenetic and androgenetic embryonic stem cell lines

Messenger RNA and methylation levels of four imprinted genes, H19, Igf2r, Igf-2 and Snrpn were examined by northern and Southern blotting in mouse parthenogenetic, androgenetic and normal or wild-type embryonic stem cell lines during their differentiation in vitro as embryoid bodies. In most instances, mRNA levels in parthenogenetic and androgenetic embryoid bodies differed from wild type as expected from previously determined patterns of monoallelic expression in midgestation embryos and at later stages of development. These findings implicate aberrant mRNA levels of these genes in the abnormal development of parthenogenetic and androgenetic embryos and chimeras. Whereas complete silence of one of the parental alleles has previously been observed in vivo, we detected some mRNA in the corresponding embryonic stem cell line. This ‘leakage’ phenomenon could be explained by partial erasure, bypass or override of imprints, or could represent the actual activity status at very early stages of development. The mRNA levels of H19, Igf2r and Igf-2 and the degree of methylation at specific associated sequences were correlated according to previous studies in embryos, and thereby are consistent with suggestions that the methylation might play a role in controlling transcription of these genes. Paternal-specific methylation of the H19 promoter region is absent in sperm, yet we observed its presence in undifferentiated androgenetic embryonic stem cells, or before the potential expression phase of this gene in embryoid bodies. As such methylation is likely to invoke a repressive effect, this finding raises the possibility that it is part of the imprinting mechanism of H19, taking the form of a secondary imprint or postfertilization epigenetic modification necessary for repression of the paternal allele.

Genetic imprinting in the mouse: implications for gene regulation

Genetic imprinting specifies a germline marking that subsequently results in the repression of one or other parental allele at some point in development. Genetic manipulations to generate maternal and paternal duplications of specific chromosome regions have been used to screen almost the entire mouse genome for evidence of imprinting. As a result, 15 imprinting effects involving 10 regions on 6 different chromosomes have been detected that range from early embryonic lethalities to various growth and developmental defects seen only after birth. Genes with important roles in development therefore appear to be involved. Diverse studies have identified four imprinted genes, all of which show monoallelic expression in some, but not necessarily all, tissues. A correlation with methylation is indicated but the pattern of methylation is not consistent for each of the genes; methylation is therefore unlikely to be the imprinting signal. Methods being used to identify further imprinted genes are summarized and some of the difficulties posed are indicated.

Enhanced transcription of the gene encoding the SmN autoantigen in patients with systemic lupus erythematosus does not result in enhanced levels of the SmN protein

The SmN, protein is closely related to the constitutively expressed SmB and SmB' autoantigens and can also act as a target for human autoimmune sera. In contrast to the single gene encoding SmB and SmB' which is expressed in all tissues, the distinct gene encoding SmN is expressed at high levels only in brain and heart tissue. We show that the SmN gene is transcribed at significantly elevated levels in peripheral blood mononuclear cells (PBMCs) from SLE patients compared to normal controls. In contrast no significant elevation in transcription of the genes encoding SmB/B' or the U1-associated 70kD RNP autoantigen is observed in these patients. The elevation in SmN gene transcription in patient PBMCs does not result however, in enhanced levels of the SmN protein in the PBMCs of these patients. The significance of transcriptional and post-transcriptional processes in regulating the expression in SLE patients of SmN and other autoantigens is discussed.

The tissue specific SmN protein does not influence the alternative splicing of endogenous N-Cam and C-SRC RNAs in transfected 3T3 cells

The SmN protein is closely related to the constitutively expressed RNA splicing protein SmB but is expressed only in brain and heart tissue. The inclusion of the VASE exon in the N-Cam mRNA and of the N1 exon in the c-src mRNA correlates with the expression pattern of SmN, being observed in brain and heart but not in other tissues and increasing in amount as SmN levels increase during brain development. However, the artificial expression of SmN in cells in which it is normally absent does not affect the pattern of N-Cam and c-src splicing whilst a cell line lacking detectable SmN is able to include the VASE exon. Hence SmN does not appear to be necessary or sufficient for these tissue-specific and developmentally regulated alternative splicing events.

Genomic imprinting and candidate genes in the Prader-Willi and Angelman syndromes

The Prader-Willi and Angelman syndromes are now well established as the paradigm of genomic imprinting in human disease. Over the past year, much has been learnt about the mechanisms by which these syndromes arise and molecular diagnostics for the majority of patients are now available. Mouse models for aspects of the syndromes have been established, and the first association between a gene, located in chromosome 15, at 15q11-q13, and a phenotype (albinism) has been proven. Large parts of the critical regions have been cloned and at least six genes identified. Three genes or DNA sequences may be imprinted: two of these demonstrate DNA-methylation imprints and one is functionally imprinted in mouse. While the molecular mechanism of imprinting is not yet understood, it is beginning to yield its secrets to DNA methylation, replication, and chromatin structure studies of the phenomenon.

Maternal imprinting of the mouse Snrpn gene and conserved linkage homology with the human Prader-Willi syndrome region

Prader-Willi syndrome (PWS) is associated with paternal gene deficiencies in human chromosome 15q11-13, suggesting that PWS is caused by a deficiency in one or more maternally imprinted genes. We have now mapped a gene, Snrpn, encoding a brain-enriched small nuclear ribonucleoprotein (snRNP)-associated polypeptide SmN, to mouse chromosome 7 in a region of homology with human chromosome 15q11-13 and demonstrated that Snrpn is a maternally imprinted gene in mouse. These studies, in combination with the accompanying human mapping studies showing that SNRPN maps in the Prader-Willi critical region, identify SNRPN as a candidate gene involved in PWS and suggest that PWS may be caused, in part, by defects in mRNA processing.

Small nuclear ribonucleoprotein polypeptide N (SNRPN), an expressed gene in the Prader-Willi syndrome critical region

Prader-Willi syndrome (PWS) is associated with paternally derived chromosomal deletions in region 15q11-13 or with maternal disomy for chromosome 15. Therefore, loss of the expressed paternal alleles of maternally imprinted genes must be responsible for the PWS phenotype. We have mapped the gene encoding the small nuclear RNA associated polypeptide SmN (SNRPN) to human chromosome 15q12 and a processed pseudogene SNRPNP1 to chromosome region 6pter-p21. Furthermore, SNRPN was mapped to the minimal deletion interval that is critical for PWS. The fact that the mouse Snrpn gene is maternally imprinted in brain suggests that loss of the paternally derived SNRPN allele may be involved in the PWS phenotype.

Expression of the tissue specific splicing protein SmN in neuronal cell lines and in regions of the brain with different splicing capacities

The SmN protein is closely related to the ubiquitously expressed SmB and B' RNA splicing proteins but is expressed in only a limited range of tissues and cell types. The expression of SmN in a range of neuronal and non-neuronal cell lines correlates with their ability to splice the calcitonin/CGRP transcript to produce the mRNA encoding CGRP rather than that encoding calcitonin. Moreover, the SmN mRNA shows a widespread distribution within the brain and spinal ganglia being present in neuronal cells in all regions which naturally produce CGRP as well as in those areas which do not naturally express the calcitonin/CGRP gene but which can correctly splice the CGRP mRNA in transgenic mice expressing the calcitonin/CGRP gene in all cell types. Interestingly however the mRNA encoding SmN is also found in a few areas of the brain which can only carry out calcitonin-specific splicing in transgenic mice, such as the Purkinje layer of the cerebellum and the inferior colliculus. The possible role of SmN in the regulation of splicing in neuronal cells is discussed in the light of these results.

The intronless mouse gene for the tissue specific splicing protein SmN is a processed pseudogene containing a stop codon after thirty-one amino acids

The SmN protein is a component of small ribonucleoprotein particles which is closely related to the ubiquitously expressed splicing proteins SmB and B' but is expressed in only a small number of cells and tissues. We have isolated a mouse SmN-related sequence which lacks introns and contains multiple changes from the SmN cDNA sequence including a stop codon after thirty-one amino acids which would prevent it encoding functional SmN protein. This indicates that this intronless gene is a processed pseudogene and that the functional gene has yet to be isolated. In agreement with this southern blotting of mouse DNA with SmN probes reveals bands, additional to those derived from the pseudogene, which are characteristic of an intron-containing SmN gene. The relationship of the pseudogene to the functional SmN gene and to an intronless SmN-related sequence in the rat genome is discussed.