Functional imprinting and epigenetic modification of the human SNRPN gene

Strain-specific differences in mouse oocytes and their contributions to epigenetic inheritance

Previous experiments revealed a strain-dependent effect of egg cytoplasm on the developmental potential of androgenetic (two paternal genomes) mouse embryos. Eggs obtained from C57BL/6 mice supported androgenone development to the blastocyst stage at a much higher frequency than eggs from DBA/2 mice. Transient exposure of paternal pronuclei to DBA/2 egg cytoplasm also compromised development, indicating that the DBA/2 egg cytoplasm negatively affected the ability of paternal pronuclei to support blastocyst formation. An essential first step toward understanding the molecular mechanism by which egg modifier factors influence gene expression is to determine the number of loci that are responsible for the strain difference. To do this, (B6D2)F1 hybrid females were backcrossed to DBA/2 males and the eggs from individual female progeny assayed for their ability to support androgenetic development. Approximately one fourth of the backcross females produced eggs that failed to support androgenone development, indicating that two independently segregating genetic loci are most likely responsible for the difference between DBA/2 and C57BL/6 egg phenotypes. Comparison of DBA/2 and C57BL/6 oocytes by two-dimensional protein gel electrophoresis revealed at least 17 proteins that exhibited significant, reproducible, quantitative differences in rates of synthesis. All of these proteins were synthesized in (B6D2)F1 oocytes. These data, combined with the previous observation that the C57BL/6 egg phenotype is dominant, are consistent with a model in which a C57BL/6 allele at either locus provides a protective function, either by antagonizing the actions of the DBA/2 alleles or by providing, through partial or complete redundancy, a function not provided by the DBA/2 alleles.

Deletions of a differentially methylated CpG island at the SNRPN gene define a putative imprinting control region

To determine the molecular basis of Prader-Willi syndrome (PWS) and Angelman syndrome (AS), we have isolated new transcripts from chromosome 15q11-q13. Two novel transcripts located within 300 kilobases telomeric to the small nuclear ribonucleoprotein-associated polypeptide N gene (SNRPN) were paternally expressed in cultured cells, along with SNRPN, defining a large imprinted transcriptional domain. In three PWS patients (two sibs), small deletions remove a differentially methylated CpG island containing a newly described 5' exon alpha of SNRPN, and cause loss of expression for the three imprinted transcripts and altered methylation over hundreds of kilobases. The smallest PWS deletion is familial and asymptomatic with maternal transmission. Our data imply the presence of a paternal imprinting control region near exon alpha.

Parental origin of transcription from the human GNAS1 gene

Variation in the phenotypic expression of Albright's hereditary osteodystrophy (AHO) determined by the parent of transmission, suggests that the human Gs alpha gene (GNAS1), in which mutations occur in AHO, may be under imprinted control. GNAS1 is also known to map to a chromosomal region (20q13.11) showing syntenic homology with the imprinted mouse region 2E1-2H3. To establish if GNAS1 is indeed imprinted, we have examined the parental origin of GNAS1 transcription in human fetal tissues. Of 75 fetuses genotyped, at gestational ages ranging from 6 to 13 weeks, 13 heterozygous for a FokI polymorphism in exon 5 of GNAS1 were identified whose mothers were homozygous for one or other allele. RNA from up to 10 different tissues from each fetus was analysed by RT-PCR. In all cases expression from both parental alleles was shown by FokI digestion of RT-PCR products and quantification of the resulting fragments. No tissue specific pattern of expression was discerned in these experiments. If genomic imprinting regulates the expression of the human GNAS1 gene, our data suggest that the effect must either be subtle and quantitative, or be confined to a small subset of specialised hormone responsive cells within the target tissues.

Genomic imprinting: control of gene expression by epigenetic inheritance

One parental copy of an imprinted gene is invariably expressed during development. The characteristic of the heritable epigenetic germline imprint remains elusive, but recent evidence stresses the necessity for additional post-zygotic epigenetic modifications to account for this temporal and tissue-specific mono-allelic expression. Preliminary insight into a diversity of post-zygotic control mechanisms is beginning to emerge.

Parental imprinting of autosomal mammalian genes

The molecular mechanisms introducing epigenetic modifications that lead to differential silencing of some autosomal alleles depending on their parental legacy are still largely unknown, but recent results from studies of endogenously imprinted genes and particular transgenes make DNA methylation a strong candidate. At the same time, these results have raised new questions about the details of the imprinting process.