Karen Ruth Davies
Department of Development and Genetics; The Babraham Institute
Mnt is a dominant mutation that was induced in a Harwell radiation mutagenesis Programme and found to show imprinting inheritance. Following paternal transmission of the mutation, heterozygous mice display intra-uterine growth retardation arising around days 13-14 pcm, such that the mutants can be clearly recognized by size alone (about 60% of normal) both before and after birth. There is no postnatal catch up growth, so adults are also small. The mutant is also characterized by a domed head. Maternal transmission results in fetal overgrowth, and homozygous mice die between days 15-17 pcm.
RNA analysis showed a major reduction of Igf2 message on the paternal Mnt chromosome in mesoderm derived tissues (such as heart or kidney) and in placenta, but normal levels in endoderm derived tissue such as the liver. When the maternal chromosome carried the Mnt mutation, H19 expression levels were shown to be substantially reduced, and this was associated with methylation of the maternal H19 allele. Maternal transmission also results in overexpression of Igf2 in endoderm derived tissues through biallelic expression.
The mutation maps to the distal chromosome 7 imprinting domain and co-segregates with D7MIT46, which is in Igf2. Molecular characterization of the mutation has shown that both Igf2 and H19 genes and the region between the genes are intact. It has been determined that the mutation has probably resulted from an inversion or translocation. One breakpoint has been identified 22 Kb downstream of H19 and the novel adjoining sequence has been characterized and shown to contain an EST from mouse heart adjacent to a large block of repeats. The location of the second breakpoint is currently under investigation.
We propose that the elusive mesoderm enhancers and an imprinting control element that is required to keep the maternal H19 allele unmethylated have been affected by the mutation. Alternatively, the new sequences brought into the vicinity of H19 exert a dominant position effect that leads to methylation and silencing of the gene.