A Case for Evolutionary

Thinking

Understanding HIV

An Evolutionary Perspective can Inform

Researchers about the following

questions:

• Why have promising Aids treatments, like the drug

azidothymidine (AZT) proven ineffective in the long run?

• Why does HIV kill people?

• Why are some people resistant to becoming infected or

to progressing to disease once they are infected?

• Where did HIV come from?

Background of the HIV/AIDS

Epidemic

What is HIV?

Life Cycle of HIV

How Does HIV Cause AIDS

• HIV infects macrophages and helper T cells (CD4+ cells).

• Once these cells are depleted, the immune system

collapses.

• Patients are diagnosed with AIDS when their helper T

cell count falls below 200 cells per cubic millimeter.

An Evolutionary Perspective Informs

Researchers about the following

questions:

• Why have promising Aids treatments, like the drug

azidothymidine (AZT) proven ineffective in the long run?

• Why does HIV kill people?

• Why are some people resistant to becoming infected or

to progressing to disease once they are infected?

• Where did HIV come from?

Why Does AZT Work in the Short

Run but Fail in the Long Run?

AZT works by stoping reverse

transcriptase

• AZT uses an anolog of

Thymidine.

• The Thymidine in AZT

has an azide group (-N3)

in place of the normal

hydroxyl group (-OH) on

the 3’ carbon.

• Reverse transcriptase

can not add more

nucleotides to the 3’ end

of the growing strand.

The effectiveness of AZT

• In early test, AZT worked, though it caused side effects

• After a few years of use, patients stopped responding to treatment and their CD4 cell counts again began to decline.

• Why?– Patient’s own cellular physiology could change.

– The population of virions living inside the patient changes so that their reverse transcriptase no longer is effected by AZT.

HIV Populations Evolve

Brendan Larder

Hypothetical difference between AZT

sensitive and resistant reverse transcrtiptase

Create an experiment to test the

hypothesis

• Use a mutagenic chemical or ionizing radiation

to generate strains of HIV with altered nucleotide

sequences.

• Grow strains of HIV in the presence of AZT.

• Strains that have the ability to differentiate

between AZT thymidine and normal thymidine

will produce more often

• Strains unable to replicate in the presence of

AZT would decline in numbers

Analytical Results From Resistant

Patients

• Researchers sequenced the reverse transcriptase gene.

• Strains of HIV that were resistant produced changes in the gene that were not their previously.

• The mutations changed amino acids in the active site of the reverse transcriptase.

• In most cases it was the same amino acid substitutions from patient to patient.

How do mutations occur in HIV’s

reverse transcriptase• Reverse transcriptase is

error prone when it copies the RNA into DNA.

• HIV has the highest mutation rate of any organism.

• Changes in the genetic makeup of HIV over time have led to drug resistance.

How Populations of HIV Evolve

1. Transcription errors

produce mutations in the

reverse transcriptase

gene. Some mutations

bestow resistance to

AZT.

2. The mutant virions pass

their AZT resistant

reverse transcriptase

genes to their progeny.

How Populations of HIV Evolve

Cont.

• In an AZT rich environment the resistant mutant strains survive and reproduce better than others.

• The population evolves and becomes resistant.

Understanding how resistance evolves,

helps in the design of better treatments

• With any single drug just one or a few mutations can render the virus resistant.

• What is needed is a way to increase the number of mutations that must occur before the virion can become resistant.

• The simplest way to increase the number of mutations that must occur at once is to use two or more drugs.

• These Multidrug treatments are known as High Active Anti-Retroviral Therapy (HAART)

Drugs in use or under development

• Reverse transcriptase inhibitors

– AZT

– 3TC

• Protease inhibitors

– Indinavir

• Fusion inhibitors

• Integrase inhibitors

Resistant Strains

• In the presence of AZT, mutated new strains of HIV or most efficient at reproduction.

• Are these new resistant strains also more efficient at reproduction when host cells do not contain AZT?

• NO. If AZT therapy is stopped, the proportion of AZT-resistant virions in the virus population falls until it reaches the level before therapy.

Mutations and the Environment

• Mutations occur randomly - but

• it is the environment that selects the type of

genetic variation that will survive and reproduce.

• Or, traits conducive to surviving and reproducing

spread throughout the population while traits

that are conducive to dying - - - disappear.

Why is HIV Fatal?

• The rapid Evoluton of the HIV population

inside a host hastens the collapse of

host’s immune system.

• This is short-sighted, because it also

hasten the extinction of the HIV population

The immune System’s Reaction.

• The body produces an immune response with antibodies and killer T-cells.

• These bind to epitopes on the surface of the virus or infected cells.

• It takes antibodies and T- cells about a week to become active against a particular epitope.

• An HIV population can produce up to 100 million new virions each day.

HIV’s Ability to Mutate

• Researchers periodically harvested and checked the genetic make-up of gp 120 of virons in a patient.

• First seven years: went from nearly identical to the original, to differing by 8%.

• By year eight the strain differentiation slowed. Why?

Viral Load.

• The patient’s viral

load increased over

time.

• Error prone Reverse

Transcriptase still

churning out

genetically variable

types.

T-cell count

• Immune system crashes at the same time the HIV strains stops diverging.

• HIV population free from the selective agent that was forcing it to evolve.

• The strains most capable of rapid replication simply spread, and those less capable became rare.

• Evolution by natural selection does not

look to the future. It is just a natural

process that happens automatically.

• As a result, the HIV population in any

particular host ultimately evolves itself

right out of existence.

CCR5 and CXCR4

• Virons that attach to the co-receptor CCR5 are predominant early and then again late in the infection

• CXCR4 become predominant between 5 and 8 years and then rare again by year 11.

• Why?

CCR5 and CXCR4

• At the beginning of an infection CXCR4 T-cells divide less often than T-cells displaying CCR5

• As the infection progresses CXCR4 T-cells begin to divide more rapidly.

• Short sighted evolution?– Virions that use the CXCR4 T-cells do not get

transmitted to a new host

– Also, virions that kill CXCR4 T-cells cause the immune system to collapse and the host to die.

A Correlation between Lethality

and Transmission

• There are rare strains of HIV that may not

kill their host.

– Sydney Bloodbank Cohort

• Fail to synthesis Nef protein.

• Important in helping HIV virions gain entry into the

host.

– HIV-2

• Viral loads are lower

• Why are these less harmful strains rare?

Correlation between Lethality and

Transmission cont-

• Evolutionary point of view.

– All host die at some point.

– To persist beyond the life span of the host the successful strain must colonize new host.

– Thus there are two levels that virions are selected at.

• The ability to survive inside a particular host.

• The ability to transfer to a new host.

– The second level is more important than the first.

Why are some people resistant to

infection by HIV?

• Some people who are repeatedly exposed to HIV do not become infected?

• Studies show that some of these individuals the CCR5 has been mutated that inhibits HIV– CCR5-D32

Where did HIV Come From?

• Nucleotide sequences can be used to

determine phylogeny and to construct

cladograms.

• The closer two nucleic acid sequences are

the closer the two organism are related to

each other.

The Origin of HIV

• Two main types of HIV,

HIV-2 and HIV-1, were

transmitted from different

sources.

• HIV-2 originated in sooty

mangabeys.

• HIV-1 was originally

transmitted to humans

from chimpanzees