RNA in the sperm and eggs of mice can transfer heritable traits, scientists report in this week's Nature. The findings add another wrinkle to the current understanding of what drives inheritance, and suggest that RNA found in human sperm may also influence genetic expression. "This could add a great possibility for variation into what is already known about classical genetic variation," study author Minoo Rassoulzadegan at the University of Nice in France told The Scientist. The focus of the current study is Kit, a tyrosine kinase receptor critical to development.
The researchers engineered a mutation, tm1Alf, which both suppressed Kit synthesis and generated lacZ reporter sequences. Homozygote tm1Alf mice die shortly after birth, while heterozygotes possess white feet and a white tail tip. When the researchers interbred heterozygotes, most offspring that did not possess the mutant tm1Alf allele still maintained the white patches of their parents.
Polymerase chain reaction (PCR) analyses indicated these mice lacked lacZ sequences, and Southern blot analyses confirmed they had wild-type genomes. Rassoulzadegan and her colleagues suggested this phenomenon was similar to paramutation, a heritable epigenetic change investigated most thoroughly in plants. Paramutated offspring also resulted when the researchers crossed heterozygotes with wild-type mice, regardless of the sex combination. Paramutated mice possessed half the levels of mature Kit mRNAs (similar to heterozygotes), and Northern analysis showed they also had aberrantly sized Kit RNA molecules.
These different RNA molecules may originate from abnormal arrest of transcription, abnormal initiation of transcription, or irregular post-transcriptional processing, Rassoulzadegan told The Scientist.
In normal mice, Kit transcription is restricted to spermatogonia and reduced or silent from meiosis onward. However, in paramutated and heterozygote mice, the researchers found significant amounts of Kit RNA in both spermatids and mature sperm. To test if RNA caused the paramutation, Rassoulzadegan and her colleagues injected RNA from heterozygotes into normal wild-type mouse embryos. Close to half of all resulting mice and their offspring showed the heterozygote white tail tip signature, suggesting the injected RNA drove the trait.
Since the abnormally sized RNAs linked with the paramutation apparently resulted from partial degradation of Kit RNAs, the investigators tried an additional experiment: degrading RNAs in normal wild-type embryos with two Kit-specific microRNAs (miRNAs). They found that either Kit-specific miRNA, or both, could result in high frequencies of the white tail phenotype that were heritable.
Future experiments should sequence and characterize the aberrant RNAs, Paul Soloway at Cornell University in Ithaca, N.Y., who did not participate in this study, told The Scientist. "Once you know what they are, you'd want a transgenic system to see if enforcing their expression is in and of itself sufficient for paramutation," he said. It's still not clear if RNA causes heritable traits naturally -- " the million dollar question," said Stephen Krawetz at Wayne State University in Detroit, also not a co-author. Natural RNA heredity "will be fairly hard to detect," Soloway added. Whole genome assays could look for changes due to RNA in instances where epigenetic effects are not immediately obvious, Krawetz suggested. RNA is also present in human sperm, and previous human studies involving the locus INS1 discovered fathers could pass diabetes resistance traits to offspring in an apparently epigenetic manner. The current findings suggest researchers might want to inspect sperm from these fathers and see whether they contain unusual INS1-related RNAs, Soloway suggested.
Links within this article
M. Rassoulzadegan et al. "RNA-mediated non-mendelian inheritance of an epigenetic change in the mouse." Nature, May 25, 2006. www.nature.com
B. Maher. "Getting repetitive in Keystone." The Scientist, Jan. 22, 2006. www.the-scientist.com/blog/display/23001/
J.M. Perkel. "MicroRNAs assume a developmental role." The Scientist, April 1, 2006. www.the-scientist.com/article/display/23301/
Paul Soloway www.people.cornell.edu/pages/pds28/
Stephen Krawetz compbio.med.wayne.edu/
S.T. Bennett el al. "Insulin VNTR allele-specific effect in type 1 diabetes depends on identity of untransmitted paternal allele." Nature Genetics, November 1997.
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