Tick Gene Drives and CRISPR Where the Research Actually Stands

Genetic tools that work routinely in mosquitoes and fruit flies have only recently been applied to ticks, and the foundations remain thin. A 2021 review in Frontiers in Cellular and Infection Microbiology framed the state of play directly: "Despite the growing burden of tick-borne diseases, research on ticks has lagged behind other arthropod vectors, such as mosquitoes." (Front 2021) That same review identified the reason as biological, not organizational — "challenges in applying functional genomics and genetic tools to the idiosyncrasies unique to tick biology, particularly techniques for stable genetic transformations" (Front 2021) — and recorded the status of CRISPR in ticks as of 2021 in a single sentence: "CRISPR-Cas9 is transforming non-model organism research; however, successful germline editing has yet to be accomplished in ticks." (Front 2021)

The same year, the review noted that CRISPR had crossed into a related group: "Recently, CRISPR-Cas was used to edit a mite, Tetranychus urticae, genome, the first for a chelicerate species and providing proof-of-concept that CRISPR-Cas9 can be used to create gene knockouts in mites." (Front 2021) Ticks, like mites, are chelicerates. A proof-of-concept in one chelicerate did not translate automatically to ticks.

The 2022 breakthrough

In 2022, the Gulia-Nuss laboratory at the University of Nevada-Reno published the first CRISPR genome editing in a tick, in the journal iScience. The authors described what had been achieved:

"This is the first demonstration of embryo injections in any chelicerate species and the first guided CRISPR/Cas9 mutagenesis in ticks. We also demonstrate that ReMOT Control can induce mutations in a non-insect arthropod, and both embryo injections and ReMOT Control had comparable editing efficiency for the gene tested." — iScience, 2022. Cas9-mediated gene editin...

The black-legged tick (Ixodes scapularis) was the target. Two hurdles had blocked tick work. First, "within the chelicerates, successful embryo injection has not yet been accomplished" (iScience 2022). Second, "Tick embryos are extremely difficult to inject due to multiple factors: high intra-ovular pressure, a hard chorion, and a wax layer outside the embryo that must be removed before injection." (iScience 2022) The team's workaround was surgical — "To eliminate the deposition of wax on eggs, the Gené’s organ of gravid I. scapularis was ablated prior to oviposition. Ablation of the Gené’s organ prevented wax deposition on the eggs but did not negatively impact tick survival or oviposition." (iScience 2022)

Results were modest in the way early technique papers usually are. "On average, ∼7% of the injected embryos hatched. Treated but un-injected embryos had better survival (∼60%–70% hatching), suggesting that further refinement in the injection protocol might increase hatch rates." (iScience 2022) Editing efficiency was heterogeneous: "Chitinase sgRNA injections led to the higher editing efficiency (up to 80%), compared to ProbP (1.8%). However, deep sequencing of ProbP -injected eggs showed less than 1% editing efficiency." (iScience 2022) The ReMOT results were lower but repeatable: "A total of 11 mutants ReMOT (R); R1-R11 were confirmed. 24-μM saponin resulted in 4.2% overall editing efficiency compared to 1.7% with 36-μM saponin" (iScience 2022).

The paper also reported an alternative delivery technique — "an alternative method that avoids the requirement to inject embryos has been developed, termed Receptor-Mediated Ovary Transduction of Cargo (ReMOT Control) which delivers the Cas9 RNP complex directly to the developing arthropod germline from the hemocoel, allowing targeted and heritable mutations to be generated by adult injection instead of embryo microinjection" (iScience 2022) — adapted from insect work. The tick paper reported "The G 0 gene-editing efficiency in ticks with ReMOT Control (1.7%–4.1%) is comparable to mosquitoes such as Ae. aegypti (∼1.5% with chloroquine)" (iScience 2022). ReMOT, however, has a fundamental limit: "gene knockin is currently not possible with ReMOT; therefore, any knockin experiments would still require embryo injections." (iScience 2022)

What the 2022 paper did — and did not — demonstrate

In the discussion, the authors wrote: "While these tools will accelerate tick genetic research, improvements are needed in embryo injection protocol for better survival and larval hatching and KO efficiencies. One of the significant limitations of this study is potential somatic editing because we did not attempt to generate heritable mutant lines due to the long lifecycle of ticks." (iScience 2022)

A second thread is open on repair mechanics. "Our data hints toward a DSB repair mechanism different from NHEJ in ticks because of the long deletions and less common edits distal to the cut site; however, more work is needed to understand the DSB repair mechanisms in ticks." (iScience 2022) Double-strand-break repair pathway matters because drive designs depend on predictable repair.

The 2022 HHS Tick-Borne Disease Working Group subcommittee report summarized the state of the research from outside the lab. Its framing was cautiously optimistic, anchoring the "germline transformation" claim in a presentation and a preprint rather than a published outcome:

"A major impediment to genetic manipulation of ticks has been the need to understand the sequential patterns of early embryological events and development of an embryo injection technique to facilitate successful protocol development for CRISPR-Cas studies. A presentation to the Tick Ecology Subcommittee of TBDWG by Dr. Monika Gulia-Nuss described her success in achieving germline transformation of ticks. A Sharma et al., 2022 preprint provided by Dr. Gulia-Nuss describes that, for the first time with any tick, a successful embryo injection protocol has been developed for blacklegged ticks (I. scapularis) and CRISPR-Cas9 has successfully been used to achieve genome editing to generate a mutation. Dr. Gulia-Nuss stated that this tick embryo injection technology has been shared with at least four other laboratories where gene editing studies are in progress in multiple tick species. These findings are a significant step forward in tick research. Transgenic tick research is essentially in its infancy. The potential benefits of transgenic tick basic research are significant, and sustained research funding support for this research area is encouraged." — HHS, 2022. Changing Dynamics of Tick...

The blockquote above carries the status-of-the-science register: significant step forward, but still in its infancy — and the call for sustained research funding that reflects that honest scope.

Gene drives: the step beyond editing

Editing a single tick's genome is distinct from driving an engineered trait through a wild tick population. The latter requires a gene drive — a CRISPR-based system that biases inheritance so that a chosen allele spreads from generation to generation rather than being diluted out. A 2023 governance review defined the mechanism: "When this gene drive system is introduced into germ-line cells, it biases inheritance away from 50% (predicted by Mendelian inheritance) towards 100% (depending on the efficiency)." (IRGC 2023)

In mosquitoes and flies, the field has moved fast. A 2022 Nature Reviews Genetics review reported that "Recently developed CRISPR–Cas9-based gene-drive systems are highly efficient in laboratory settings, offering the potential to reduce the prevalence of vector-borne diseases, crop pests and non-native invasive species." (Nature 2022) The same review summarized its five-year window bluntly: "Progress in the gene-drive field has been remarkable over the past 5 years. In this brief period of intensive productivity, nearly all substantive technical barriers have been overcome for drive systems either modifying or suppressing mosquito populations." (Nature 2022) Two design strategies have emerged: one is the genetic equivalent of insecticide, the other does not kill the vector. The suppression strategy aims "to force deleterious traits into a population, leading those populations to crash or be much diminished" (Nature 2022); the modification strategy instead "leaves the insect in place in the environment but blocks disease transmission" (Nature 2022). For the strategic framework these options would map onto in ticks — including the reservoir-host route — see tick gene drive application approaches; this article stays on lab status rather than strategy.

None of that work is yet in ticks. As of a 2023 inventory, "No gene drives have been released yet into the ecosystem; however, several laboratory cage trials have occurred. To date, synthetic gene drives have been developed in yeast (Saccharomyces cervisiae, Candida albicans), fruit flies (Drosophila melanogaster), the plant Arabidopsis thaliana, diamondback moths (Plutella xylostella), mosquitoes (Anopheles gambaie, Aedes aegypti, Anopheles stephensi) and mice (Mus musculus)." (IRGC 2023) Ticks are not in that list. The 2021 Frontiers review sketched what drives in ticks might one day look like — "a male dominant allele to produce a single sex to reduce tick populations, or a trait to increase refractoriness to pathogens, could be effective strategies for managing tick-borne diseases" (Front 2021) — but the same review placed drives squarely in the conditional: "Gene drives, which bias inheritance towards a natural or synthetic genetic element or specific allele and lead to a preferential increase of a specific phenotype throughout a population, are being developed for mosquito control." (Front 2021)

On transfer to other species, the Nature Reviews Genetics review was cautious: "Although gene drives are likely to be adaptable to other insects, there may be considerable challenges in devising such systems, including colonizing new or native model mosquito species in the laboratory, tuning methods for germline transgenesis to new insect species, or sustaining efficient gene conversion (for example, as has proven difficult in Ae. aegypti)." (Nature 2022) Ticks are not insects. They are chelicerates. For how the mosquito and tick research timelines line up side by side on the same clock, see mosquito vs. tick gene drive timeline comparison; the tick data in that comparison is the lab status this article documents.

A parallel path: editing the reservoir, not the vector

One proposed route to reducing tick-borne disease does not edit ticks at all. It edits white-footed mice, the main reservoir for the Lyme disease bacterium in the northeastern United States. The Mice Against Ticks project, described in a 2019 Philosophical Transactions of the Royal Society B paper, laid out the logic. On feasibility: "Combined with advances in the use of CRISPR for germline editing, attendees concluded that conferring heritable resistance to tick-borne diseases would be a challenging engineering problem that would take years to accomplish, but attainable using current CRISPR editing techniques." (RSocB 2019) On starting point: "Because heritable genome editing has not yet been achieved in white-footed mice, we will test a variety of delivery methods for CRISPR-based insertion, including embryo injection and i-GONAD." (RSocB 2019) The field-trial plan was confined: "the ecological effects of the intervention could be tested in field trials on small, mostly uninhabited private islands or one large private island" (RSocB 2019). Notably, the project did not propose a gene drive. It proposed introducing heritably resistant mice and allowing them to interbreed.

This is a different technical bet, at a different point in the disease cycle, than the tick-editing program. It also depends on germline editing that has not yet been demonstrated in the target species.

Tools currently in hand

For laboratories that want to study tick gene function today, the toolkit is narrow. A 2022 field summary put it plainly: "RNAi (RNAi) is currently the only technique available for functional genomics studies in ticks." (iScience 2022) The same paper gave the reason genetic approaches have been stalled: "Advances in tick genomics and genetics have mainly been stymied by challenges in applying available molecular tools to the unique biology of ticks to carry out reverse genetics that can directly validate gene functions and correlate with phenotypes of interest." (iScience 2022) Even the techniques adapted from insect work carry hard limits. Of the alternative CRISPR-delivery methods now available for ticks, "a limitation of both of these alternate strategies is that they cannot currently be used for gene knock-in (overexpression or replacement) as there is no mechanism for carrying template into the embryo for homology-directed repair, making them suitable only for gene knockout studies." (Front 2021)

The Nature Reviews Genetics field review made the same point about ReMOT from the drive-engineering side: "While this system has proven effective for ‘knockout’ gene editing, it remains to be determined whether it can deliver DNA repair templates to permit ‘knock-in’ modifications." (Nature 2022) Knock-in is what gene drives require.

The honest summary

CRISPR-Cas9 was first applied to ticks in 2022, in black-legged ticks, by one laboratory. Editing efficiency varied by gene; hatch rates were low. Heritable mutant lines were not produced in that paper. The available CRISPR-delivery methods in ticks are, as of publication, "suitable only for gene knockout studies" (Front 2021). Gene drives themselves have been released in no species anywhere — "No gene drives have been released yet into the ecosystem" (IRGC 2023) — and the species in which drives have been developed in the laboratory do not include any tick. The 2022 HHS summary ("essentially in its infancy," quoted earlier) is, on the available evidence, the most accurate single sentence about the state of the field.

Sources

    Not medical advice. See a healthcare provider for medical decisions. Medical Disclaimer