The mut-L homologue 1 gene (MLH1) is a DNA repair gene and has a key role in tumour-suppression in mammals. Genomic imprinting results in parent-of-origin-specific gene expression and has been identified in marsupials and eutherians. MLH1 is imprinted in the brushtail possum [1] and koala [2] and appears to be a marsupial-specific imprinted gene. Allele-specific methylation in the tammar wallaby indicated that MLH1 was maternally methylated in pouch young tissue. In kidney, liver, spleen and brain, the maternal MLH1 allele was completely silenced. In late term yolk sac placenta, the maternal MLH1 allele was methylated but incompletely silenced.
Expression analysis of the chromosome 3 MLH1 identified a second MLH1 site on chromosome 6. Alignment of the MLH1 transcript sequence indicated this second site was an intronless retrocopy, formed through the MLH1 mRNA incorporating into the genome. Assessment of marsupial reference genomes identified an orthologous MLH1 retrocopy in two macropodids and an additional two non-orthologous MLH1 retrocopies in the bilby and marsupial mole. Partial MLH1 retrocopies were identified in 9 out of 23 other marsupial genomes examined. The tammar, bilby and marsupial mole MLH1 retrocopy loci had an intact open reading frame. Analysis of transcriptomes and cDNA indicated transcripts were expressed from both the tammar and bilby retrocopy loci. Aligning the predicted protein sequences from the parental and retrocopy MLH1 indicated the retrocopy protein may have a comparable function to the parental MLH1 protein.
A transcriptionally active MLH1 retrocopy has previously been identified in the common marmoset [3], indicating there are at least four functional MLH1 retrocopies across mammalian lineages. This second site of MLH1 expression may introduce a redundancy to MLH1 expression and in marsupials, compensate for the reduced gene expression which results from silencing of the maternal allele of the parental MLH1.