L1 retrotransposition occurs mainly in embryogenesis and creates somatic mosaicism.

Long Interspersed Element 1 (L1) is a retrotransposon that comprises approximately 17% of the human genome. Despite its abundance in mammalian genomes, relatively little is understood about L1 retrotransposition in vivo. To study the timing and tissue specificity of retrotransposition, we created transgenic mouse and rat models containing human or mouse L1 elements controlled by their endogenous promoters. Here, we demonstrate abundant L1 RNA in both germ cells and embryos. However, the integration events usually occur in embryogenesis rather than in germ cells and are not heritable. We further demonstrate L1 RNA in preimplantation embryos lacking the L1 transgene and L1 somatic retrotransposition events in blastocysts and adults lacking the transgene. Together, these data indicate that L1 RNA transcribed in male or female germ cells can be carried over through fertilization and integrate during embryogenesis, an interesting example of heritability of RNA independent of its encoding DNA. Thus, L1 creates somatic mosaicism during mammalian development, suggesting a role for L1 in carcinogenesis and other disease.

Pilot studies of pressure-immobilization bandages for rattlesnake envenomations.

STUDY OBJECTIVE: Pressure-immobilization bandages sequester venom in extremities and are recommended for snakebites without local toxicity. Pilot studies were performed to determine the time of onset of toxicity and efficacy of pressure-immobilizations bandages in a porcine model of rattlesnake envenomation. METHODS: After IACUC approval, anesthetized pigs were injected subcutaneously in a distal hind leg with 200 mg of Crotalus atrox venom. After 1 min, pigs received either a pressure-immobilization bandage (N = 3) or no treatment (N = 3). At 24 h, surviving pigs received antivenin and then the pressure-immobilization bandages were removed. Surviving subjects were followed for 1 week. Chi-square analysis and paired t-test were used. RESULTS: Pigs with pressure-immobilization bandages survived for 24 h, whereas untreated pigs died at 13.68 +/- 3.42 h (p = 0.014). Surviving pigs walked on the extremity at 7 days. Potassium rose from 4.033 +/- 0.252 at baseline to 17.767 +/- 5.218 mEq/L (p < 0.0001) at time of death in untreated pigs but was normal at 24 h in treated subjects. Widespread tissue necrosis was seen in the untreated group but only local necrosis in the treatment group. CONCLUSIONS: Pressure-immobilization bandages prevented death from severe C. atrox envenomations with a 24 h delay to treatment. Surviving pigs had recovery of limb use at 1 week.

L1 integration in a transgenic mouse model.

To study integration of the human LINE-1 retrotransposon (L1) in vivo, we developed a transgenic mouse model of L1 retrotransposition that displays de novo somatic L1 insertions at a high frequency, occasionally several insertions per mouse. We mapped 3' integration sites of 51 insertions by Thermal Asymmetric Interlaced PCR (TAIL-PCR). Analysis of integration locations revealed a broad genomic distribution with a modest preference for intergenic regions. We characterized the complete structures of 33 de novo retrotransposition events. Our results highlight the large number of highly truncated L1s, as over 52% (27/51) of total integrants were <1/3 the length of a full-length element. New integrants carry all structural characteristics typical of genomic L1s, including a number with inversions, deletions, and 5'-end microhomologies to the target DNA sequence. Notably, at least 13% (7/51) of all insertions contain a short stretch of extra nucleotides at their 5' end, which we postulate result from template-jumping by the L1-encoded reverse transcriptase. We propose a unified model of L1 integration that explains all of the characteristic features of L1 retrotransposition, such as 5' truncations, inversions, extra nucleotide additions, and 5' boundary and inversion point microhomologies.