Transcript Dynamics of Transposable Elements in Genetically Modified

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
th
a
de
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
r
i
b
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
c (i 1)
ui  u1 2
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
mn 

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   )u m  n 1 x( m 1,n ) (t )  (n  1)u m  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   )u m  n  num  n  m v  nv  m w( 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