Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
The impact of dissociation on transposon-mediated disease control strategies John Marshall, Department of Biomathematics, UCLA IR Transposase gene Resistance gene IR IR Transposase gene IR “It is misguided to continue to use scarce resources from the malaria-research budget for activities which could not control malaria but only produce NPS (Nature-paper synthetase).” C. F. Curtis, Ecological aspects for application of genetically modified mosquitoes (2003). How is linkage lost between a TE and resistance gene? 1. Rare recombination events gamete with dissociated transposable element construct intact transposable element construct IR Tr R IR IR Tr IR rare crossover event R homologous chromosome 2. gamete with dissociated resistance gene Internal deletion of DNA sequences within a TE 2. Abortive gap repair (Part 1) intact transposable element construct gap introduced into host chromosome IR Tr R IR host chromosomal DNA transposition IR Tr R IR or deletion 2. Abortive gap repair (Part 2) homologous chromosome, sister chromatid, or ectopic chromosomal site successful gap repair IR Tr R IR IR Tr R IR IR Tr R IR or IR Tr R IR gap in host chromosome gap repair mechanism abortive gap repair IR Tr IR dissociated transposable element construct Measurement of dissociation rate for P elements in D. melanogaster MALE FEMALE X chromosome white white P element IR Tr wild-type IR white Y chromosome Phenotype: Interpretation: Wild-type female Non-excision White-eyed male Non-insertion White-eyed female Excision Wild-type male Insertion Dissociation rate = 0.05 TE-1gen-1 W. R. Engels, Mobile DNA (1989). Internal deletion rates vary between TEs and host species: Fast: Slow: •P and hobo elements in Drosophila melanogaster •Herves element in Anopholes gambiae remains almost exclusively intact throughout evolutionary history •Ds elements in maize Question: What rate of dissociation can be tolerated by a TE-mediated disease control strategy? Question: What if the resistance gene compromises the transposition rate? Kanomycin resistance gene TE sequences Exogenous DNA used to increase transposon size D. J. Lampe et al., Genetics (1988). Question: What if the resistance gene has an impact on the host fitness? Fitness cost: Fitness benefit: •Mounting an immune response is generally associated with an evolutionary cost in insects •Transgenic mosquitoes have been created that have no noticeable fitness cost when fed on Plasmodium-free blood •These mosquitoes are in fact more fit when fed on Plasmodiuminfected blood M. T. Marrelli et al., PNAS (2007), L. A. Moreira et al., Genetics (2004), A. R. Kraaijeveld & H. C. Godfray, Nature (1997). Mathematical model: x( m,n ) (t ) = proportion of disease vectors having: m copies of the intact construct and n copies of the dissociated construct at time t Changes in host genotype: Total birth rate = Total death rate Birth rate: •All TE copies segregate independently Death rate: •Function of copy number •Impact of resistance gene Transposition: •Repression with increasing copy number •Handicap of resistance gene Deletion Dissociation: •Proportional to transposition rate Mathematical model: TER TER TE TE TER TER (3, 1) TER TE dis so cia tio n th r i b TER transposition TE excision TER excision ath e d TE TE TE TER (m, n) = (2, 2) transposition (2, 1) TER transposition TE excision TER (3, 2) excision TE transposition TER TER (2, 3) dis s oc iat ion TER TE TE TER TE TE (1, 2) TE (1, 3) Mathematical model: TER (2, 0) excision transposition TER th bir TER TER TE excision ath e d excision (m, n) = (1, 0) (1, 1) dis s oc iat ion TE (0, 1) (0, 0) Repression of transposition Transposition rate Handicap of resistance gene Transposition rate ui u TER TE copy number i ui u1 2 c ( i 1) Sources: • Self repression • Host repression TE uTER (1 )uTE Default parameter values: • u1 = 0.1 TE-1gen-1 • c=3 Fitness impact of resistance gene Host fitness Host death rate Fitness impact ui d d TER TE TE copy number i i 1 di p Sources: • Insertional mutagenesis • Ectopic recombination • Act of transposition ( m,n ) m 1 d ( m n ) p mn Default parameter values: • μ1 = 0.001 TE-1gen-1 • p = 1.5 What this all means mathematically Birth dx( m ,n ) (t ) dt Death (t ) p(i , j ),( k ,l ),( m ,n ) x(i , j ) (t ) x( k ,l ) (t ) ( m,n ) x( m ,n ) (t ) i , j , k ,l (m 1)(1 )um n 1 x( m 1,n ) (t ) (n 1)um n 1 x( m ,n 1) (t ) (m 1)vx( m 1,n ) (t ) (n 1)vx( m ,n 1) (t ) Dissociation (m 1) w( m 1,n 1) x( m 1,n 1) (t ) m(1 )um n num n mv nv mw( m ,n ) x( m ,n ) (t ) Transposition Deletion Loss of resistance gene is most rapid during the early stages of TE spread Dissociation rate = 0.01 TE-1gen-1: Dissociation often occurs during the act of transposition; hence the proportion of dissociated TEs increases rapidly early on. Following equilibrium, transposition still occurs to counteract selection and excision; hence the proportion of dissociated TEs continues to decrease slowly. Loss of resistance gene depends strongly on dissociation rate Dissociation rate = 0.001 TE-1gen-1: Dissociation rate = 0.05 TE-1gen-1: Prevalence of disease-resistant vectors strongly depends on dissociation rate For dissociation rates < 0.013 TE-1gen-1, the disease resistance gene reaches a maximum prevalence of < 80% For P elements in D. melanogaster, the resistance gene would reach a maximum prevalence of ~20% Transpositional handicap reduces the prevalence of disease-resistance For transpositional handicaps > 0.25, dissociation rate must be extremely small to achieve a high maximum prevalence For a Himar1 element increased in size by ~75%, the transpositional handicap is ~0.18 Fitness benefit greatly increases the prevalence of disease-resistance For a fitness cost > 0.005 TE-1, dissociation rate must be extremely small to achieve a high maximum prevalence For a fitness benefit as small as 0.001 TE-1, the dissociation rate required for disease control is greatly relaxed Model conclusions Dissociation rate: • Critical parameter in determining the fate of the gene drive strategy • Recommend dissociation rate < 0.01 TE-1gen-1 • If the dissociation rate is too large, then the introduction of resistance genes is reversible within a human time frame Selective advantage of resistance gene: • Not sufficient to drive resistance gene into population on its own • When combined with gene drive strategy, greatly improves chances of success • A fitness benefit ~ 0.001 TE-1 will make disease control realistic for moderate dissociation rates Future research directions • Molecular biology: Measure dissociation rates once transposition has been observed in host species, TEs of interest • Ecology: Continue to seek accurate measurements of fitness consequences of disease-resistance genes • Epidemiology: Imbed the spread of a resistance gene within a model of the epidemiology of malaria or Dengue fever Acknowledgements • Molecular biology: Prof. David O’Brochta • Vector biology: Prof. Charles Taylor • Mathematical modeling: Assoc. Prof. Tom Chou