Sperm-mediated gene transfer: mechanisms and implications.
The finding that mature spermatozoa act as vectors of genetic material, not only for their own genome, but also for exogenous DNA molecules, has suggested a strategy for animal transgenesis alternative to DNA microinjection. Exploiting this ability, protocols for sperm-mediated gene transfer have been developed in a variety of animal species with extremely variable results. In the last years, we have devoted much effort to clarify the mechanism of sperm/DNA interaction, with the dual aim to improve the protocols for transgenesis and to expand our understanding of the molecular mechanism(s) underlying sperm-mediated gene transfer. The results from these studies challenge the traditional view that spermatozoa are metabolically inert cells, showing that the binding of exogenous DNA to sperm cells, and its nuclear internalization, are not casual events but steps of a regulated process mediated by specific factors. The binding of exogenous sequences triggers enzymatic functions that are otherwise repressed in mature spermatozoa under normal conditions. Among those, we have identified an endogenous retrotransposon-encoded reverse transcriptase activity able to reverse transcribe exogenous RNA molecules in cDNA copies that can be delivered to embryos at fertilisation. The reverse-transcribed molecules have peculiar features: they are propagated in tissues as low copy extrachromosomal structures, mosaic distributed, transcriptionally competent, able to inducing phenotypic variations in positive tissues, and sexually transferred from one generation to the next in a non-mendelian fashion. Taken as a whole, these results suggest that sperm-mediated gene transfer is a retrotransposon-mediated phenomenon: thus, the sperm endogenous retrotransposon machinery can be regarded as a novel source of genetic information.
Spadafora
Corrado
C
Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy. cspadaf@tin.it
D000818:Animals; D030801:Animals, Genetically Modified; D004247:DNA / Q000378:metabolism*; D018076:DNA, Complementary / Q000378:metabolism; D018014:Gene Transfer Techniques; D008297:Male; D012313:RNA / Q000378:metabolism; D012194:RNA-Directed DNA Polymerase / Q000378:metabolism; D018626:Retroelements; D013094:Spermatozoa / Q000502:physiology*
D018076:DNA, Complementary; D018626:Retroelements; D012313:RNA; D004247:DNA; D012194:RNA-Directed DNA Polymerase