Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis
ReviewChromothripsis and epigenomics complete causality criteria for cannabis- and addiction-connected carcinogenicity, congenital toxicity and heritable genotoxicity
Graphical abstract
Section snippets
Introduction to seminal paper
In a remarkable and highly celebrated report, the Pellman lab recently showed that severe chromosomal fragmentation involving dozens of double stranded breaks and subsequent apparently random and disordered repair of some of the fragments, could rapidly occur during the DNA synthetic phase (G2 and S-phases) of the mitotic cell cycle, if chromosomes became isolated from the main nuclear mass [1]. In this technical tour de force, high resolution DNA sequencing of single cells and live cell
Dynamics of the cell cycle
The cell cycle has numerous check points which are designed to prevent such genetically catastrophic events from occurring. The mitotic spindle assembly checkpoint (SAC) in particular requires all chromosomes to be attached to the spindle, and sister replicates to be attached at their kinetochores with opposing polarity (bi-orientation) to bundles of microtubules of the mitotic spindle which will draw them to opposite poles of the cell [15]. Mostly errors in this complicated machinery [16], [17]
Mitotic spindle poisons
The Boston studies used nocodozole to induce cell cycle arrest [1] which acts by binding tubulin subunits and preventing their polymerization [15]. Vincristine, vinblastine and colchicine act similarly [15]. The chemotherapeutic agent taxol acts by binding to and stabilizing microtubules, inhibiting their dynamic instability [15].
Of significance and concern Δ-9 tetrahydrocannabinol (THC) [34], [35], [36], [37] and other cannabinoids [38] act similarly to taxol. Importantly it has been shown
Non-linear dose-response kinetics
One important observation to emerge from these studies is the non-linear dose response kinetics of cannabis in mutagenicity and genotoxicity studies (Fig. 5). Low dose THC and other cannabinoids have been found both in vitro (<5 μg/ml or <5 μmol/l) and in clinical studies (<1 joint/day) to be rarely associated with genotoxically mediated adverse outcomes [36], [37], [40], [41], [42], [44], [47], [48], [49], [58], [59], [60], [61]. Serum levels of 1 mmol/l have been reported after recreational use
Cannabis cancerogenesis
Importantly cannabis use has also been positively associated in epidemiological studies with several cancers including aerodigestive cancers (head and neck [56], larynx, lung [63], [64], [65]), leukaemia, brain [66], prostate [67], cervix, testes [68] and bladder cancer [69], [70], [71]. Parental cannabis exposure during pregnancy has also been associated with the emergence of rhabdomyosarcoma [70], neuroblastoma [72] and acute myeloid leukaemia [73], [74] in their young children (<5 years). The
Cannabis teratogenicity
Cannabis has also been associated with foetal abnormalities in many studies including low birth weight, foetal growth restriction, preterm birth spontaneous miscarriage [46], [51], [59], [60], [81], microotia/anotia, microphthalmia/anophthalmia, spina bifida, meningomyelocoele, anencephaly, cardiac defects including in particular cardiac septal defects, gastroschisis and many others [46], [82]. Phocomelia (short or truncated forelimbs) has also been shown in testing in a similar preclinical
Cannabis-related mitochondrial inhibition
THC has also been shown to inhibit mitochondria after both in vitro and in vivo exposure of lung cells, brain cells and sperm in part by increasing their expression of uncoupling protein 2 [61], [85], [87], [88], [89], [90], [91]. Cannabis pyrollysates (partially burnt products of the smoked plant) also increase oxidative stress on many tissues [52], [58], [78]. These findings are important for several reasons. Oxidative stress is one of the leading theories of the causes of ageing and
Cannabis-related gametotoxicity, zygote toxicity and reproductive impairment
Moreover cell division, and DNA and chromosomal replication are very energy intensive processes. This point is well illustrated by Fig. 6 which stains the mitotic spindle, chromosomes and the dense network of mitochondria surrounding the mitotic apparatus at the end of anaphase. Perhaps unsurprisingly mitotic errors including chromosomal mis-segregation have been shown to be more common in older cells [99]. Importantly it has also been shown that improved energy production from aged oocyte
Other microtubule functions
Microtubules are also essential to many other cell functions notably in stem cell niches and in neurons. It has been shown that the cell cycle, particularly in S and G2 phases, governs the human embryonic stem cell decision relating to the exit from pluripotency to cell differentiation (via a P53/ATM-ATR/CHEK2/CyclinB1/TGFβ/Nanog spindle checkpoint pathway) [110], and that microtubule structures (nanotubes) mediate the spreading of deterministic molecular signals (bone morphogenetic protein
Other addictions
Interestingly similar comments can be made about several other addictions. Dependency syndromes associated with alcohol, tobacco, opioids and benzodiazepines have been associated with tumourigenesis [117], [118], [119], [120], [121], [122], [123]. Dependency on alcohol, benzodiazepines, opioids, cocaine and amphetamine has been linked with adverse morphological and developmental outcomes in children exposed in utero. Most chemical addictions are associated with foetal growth restriction [47],
Epigenetic contributions to mutagenicity
It will also be noted that the discussion to this point has not considered the epigenetic revolution which is rapidly overtaking medicine. The origins of the Barker hypothesis of the foetal origins of adult disease has been attributed to the observation of the increased incidence of cardiovascular disease in children born to women exposed to the post-war famine in England [138], [139]. Since that time many environmental agents have been linked with epigenetic change including alcohol [140],
Conclusion
As mentioned above high dose cannabis and THC test positive in many genotoxicity assays, albeit often with a highly non-linear threshold like effects above low doses. As long ago as 2004 it was said that 3–41% of all neonates born in various North American communities had been exposed to cannabis [172]. Since cannabis is addictive [187], is becoming more potent [77], [83], [86], quickly builds up in adipose tissues [62], [82] and seems generally to becoming more widely available under fluid
Conflict interests
The authors have no competing financial interests to declare.
Funding
This study received no external funding.
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