Table 1.
Effects of Environmental Factors on Repeat Instability. Several environmental factors have been found to modulate repeat instability across a variety of repeat expansion diseases and model systems.
| References | Repeat Expansion Disease | Repeat Sequence | Model System | Environmental Factor | Exposure | Effect on Repeat Instability |
|---|---|---|---|---|---|---|
| [45] | Fragile X Syndrome | CGG•CCG | Fragile X premutation mice carrying ~130 CGG•CCG repeats in the endogenous Fmr1 gene | KBrO3 | 0.5 g/L KBrO3 in drinking water; Mice raised on this drinking water for at least two generations |
Parental exposure to KBrO3 increased expansion frequency from 37% to 70% and from 62% to 83% in maternal and paternal transmissions, respectively. Exposure to KBrO3 had no significant effect on maternal or paternal contraction rates. Paternal exposure resulted in larger repeat length changes than maternal exposure. |
| [46] | - | GAA | Human lymphoblast cell extracts from unaffected individual with (GAA)15 repeat tract in FXN gene | K2CrO4 | Treated with 0.5 mM K2CrO4 for 2 h | MSH2-MSH3 complex only formed at GAA repeat tract in FXN gene after exposure to oxidative agents |
| KBrO3 | Treated with 10 mM KBrO3 for 2 h | |||||
| [47] | Huntington’s Disease | CAG | Human HD fibroblasts | H2O2 | 0.2 or 0.5 mM H2O2 | Induced repeat expansion, resulting in medium-length and disease-length alleles |
| Brain, liver, and tail of young (7–15-week-old) and old (15–52-week-old) R6/1 transgenic mice | Aging | - | Level and accumulation of age-dependent oxidative damage correlated with degree of repeat expansion. Liver and brain exhibited high levels of oxidative damage and expansion continued to progress with age. | |||
| [48] | Huntington’s Disease | CAG | HD murine embryonic stem cells (mESCs) with 127 CAG repeats; Derived from R6/1 mice | H2O2 a | 150 µM H2O2 | Significantly increased repeat tract length, averaging a threefold increase when compared to untreated HD mESCs |
| KBrO3 a | 7.5 mM KBrO3 | Modest increase in repeat expansions and exhibited a significant increase in repeat expansions after the twelfth treatment | ||||
| Methyl methanesulfonate (MMS) a | 3 mM MMS | None | ||||
| Differentiating HD mESCs | H2O2 | 50 µM or 150 µM H2O2 | Significant and dose dependent increases in repeat expansion; Effect of H2O2 on differentiating cells was slightly lower than its effect on pluripotent cells; Differentiation itself induced repeat expansion | |||
| [49] | Huntington’s Disease | CAG | Murine embryonic fibroblasts isolated from hHD(−/+)/Msh2(+/+) and hHD(−/+)/Msh2(−/−) mice at embryonic day 13 |
H2O2 | Repeated exposure to low doses of H2O2 | Resulted in repeat expansion when Msh2 was present |
| [50] | Huntington’s Disease | CAG | Human HD fibroblasts derived from individuals with HD; Mutant allele ranged from 42–60 repeats | 5-azacytidine | 10 µmol/L 5-azacytidine intermittently administered over 35 days | Stabilized the CAG repeat tract in HD cells while untreated HD cells exhibited small expansions |
| [51] * | Huntington’s Disease | CAG | R6/1 HD mice | Anthocyanin antioxidants from bilberry and blackcurrant | Anthocyanin antioxidants added to drinking water daily from 4–22 weeks of age | Reduced repeat instability index in ears and cortex; Effects were modest and did not significantly affect behavior |
| [52] | - | CAG | HEK293 cells with a GFP(CAG)89 reportere | Cold | Cells incubated at 30 °C for 24 h | All four environmental stressors significantly increased the occurrence of repeat instability. Sequencing of independent GFP+ colonies isolated from cold, hypoxic, and H2O2 conditions showed substantially altered repeat tract lengths in the form of contractions or indels. |
| Heat | Cells incubated at 44 °C for 24 h | |||||
| Hypoxia | Cells incubated in 1% O2 in a hypoxic chamber for 48 h | |||||
| H2O2 | Cells treated with 0.5 mM H2O2 for 15 min | |||||
| [53] * | Myotonic Dystrophy type 1 | CTG-CAG | Lymphoblastoid cell lines from unaffected individuals and individuals with DM1 | Mitomycin C (MMC) | 5 ng/mL MMC continuously administered for 12 population doublings | Enhanced expansion bias of long-pathogenic repeats and promoted expansion of normal-length repeats |
| [54] * | Myotonic Dystrophy type 1 | CTG-CAG | Lymphoblast cell lines from individuals with DM1 | Mitomycin C b | 0.1 or 0.2 µg/mL MMC for 14–16 h | Reduction of repeat length by 100–350 repeats often occurred |
| Doxorubicin b | 0.1–5 µg/mL doxorubicin for 30 min | |||||
| Ethylmethanesulfonate (EMS) b | 500 or 700 µg/mL EMS for 14–16 h | |||||
| Mitoxantrone b | 50 or 100 nM mitoxantrone for 1 h | |||||
| [55] * | Myotonic Dystrophy type 1 | CTG-CAG | Primary fibroblasts from 22.5-week female fetus with DM1 and 76-year-old woman with DM1 | Aphidicolin c | 0.207 µM aphidicolin for a single population doubling | Enhanced the magnitude of repeat expansions |
| Emetine c | 1 µM emetine for 18 h | |||||
| Mimosine c | 200 µM mimosine for 18 h | None | ||||
| [56] | Myotonic Dystrophy | CTG-CAG | Human fibroblast cell lines derived from individuals with myotonic dystrophy with (CAG)80 or (CAG)150 repeat tracts | 5-aza-deoxycytidine | 0.5 µM 5-aza-deoxycytidine | Induced repeat instability in the DMPK gene with a bias toward expansion |
| [57] * | Myotonic Dystrophy type 1 | CAG-CTG | Dmt-D mouse kidney cell cultures | Caffeine d | 2 mM caffeine | High doses increased rate of expansion by ∼60% |
| Novobiocin d | 60 µM novobiocin | None | ||||
| Aspirin d | 5.6 µM aspirin | Highly significant decreases in the rate of expansion ranging from ∼25% to 75% | ||||
| Cytosine arabinoside (AraC) d | 500 nM AraC | |||||
| Ethidium bromide d | 250 nM ethidium bromide | |||||
| H2O2 d | 100 µM H2O2 | |||||
| Rhodamine 6G d | 50 nM rhodamine 6G | |||||
| 5-azacytidine d | 10 µm 5-azacytidine | |||||
| [58] * | Myotonic Dystrophy type 1 | CAG-CTG | D2763Kc2 cell line derived from kidney of 6-month-old Dmt-D knockin mouse | Manganese | 2 µM each day for 73 days | Significantly reduced rate of somatic repeat expansion; Reduced repeat size variability |
| Ascorbic acid | 200 µM each day for 73 days | Did not significantly alter repeat expansion rate; Reduced repeat size variability | ||||
| Trolox C | 500 µM in 0.1% ethanol each day for 73 days | Did not significantly alter repeat expansion rate | ||||
| Melatonin | 20 µM in 0.1% ethanol each day for 73 days | |||||
| Cadmium | 2 µM each day for 73 days | |||||
| H2O2 | 20 µM each day for 73 days | |||||
| Cobalt | 2 µM each day for 73 days | |||||
| Zinc | 2 µM each day for 73 days | |||||
| Ethanol | 0.1% each day for 73 days |
* These studies only observed treatment effects of environmental exposures on repeat instability. The possible mechanisms of environmentally induced repeat instability were not investigated. a Cells were exposed to their respective treatment once per passage for 12 passages. b Cell cultures were grown for up to 10–30 generations. In select experiments, a second dose of the drug was administered after recovery of the culture. c Cells underwent five rounds of their respective treatment. d Treated cultures were given fresh drug-supplemented medium every 2–3 days over the course of 3 months.