Cell Cycle and Radiosensitivity

Tc

The time that elapses from one mitosis to the next mitosis

Cell cycle time or mitotic cycle time

Tc parameters

Tm: Mitotis; total time in mitosis

Ts: Synthesis; total time cell in synthesis

Tg1: Gap 1; total time in gap 1

Tg2: Gap 2; total time in gap 2

These parameters relate to phase timing and these times vary depending on the cell cycle phase and type of cell

Parameter that contributes the most to variations in Tc between different types of cells

TG1 length!!

Varies heavily

Hamster: 1 hour

HeLa cells: 11 hours

Total cell cycle time for hamster

11

Total cell cycle time for HeLa cells

11 hours

General cell survival curves are based on

Asynchronous populations

Asynchronous populations

At any given time, the cells in our experiment are distributed mostly equally through each phase of cell cycles

Any number could be in any position

Synchronous populations

When we want to know how radiation effects cells at a precise moment in the cell cycle

Gets all cells in the same phase

Ways to synchronize cells in vitro

Mitotic Harvest

and

Hydroxyurea

Mitotic Harvest (In vitro cell synching)

Done to get cells synchronized

Cells nearing mitosis have a tendency to loosen their attachment of their containment vessel (less sticky)

Physically shake the experiment container in any given population, and all cells that fall off shoulder should be near mitosis

Take all mitotic endings and move them

Use phase timing!

When do cells loosen their attachment?

When they are near mitosis

Hydroxyurea (In vitro cell synching)

Used to get cells synchronized

Hydroxyurea is EXTREMELY TOXIC to cells, but ONLY if they are in the DNA SYNTHESIS PHASE

In any other phase, it is not toxic, only toxic in S phase

If administering hydroxyurea, all cells in S phase will die and the remaining cells will progress through the cycle and get bottlenecked at G1 phase

after blocking with hydroxyurea

hydroxyurea is removed, and all cells are in sync and ready to enter the S phase

done by an antidote chemical

The most radiosenstive phase of the cell cycle to radiation is

MITOSIS (M PHASE)

G2 is second most radiosensitive

The most radioresistant phase of the cell cycle to radiation is

LATE SYNTHESIS (LATE S PHASE)

2nd most radioresistant is early S phase

On these graphs of radiosensitivity for cell cycles

Dose is on the x axis

Single-cell survival is on the y axis

Big shoulder with less steep curve: radioresistant

No shoulder with steep curve: more radiosensitive

G1 lies in middle

Cell survival curves that hold radiation dose consistent

X Axis: Time (hours after shake off from mitotic harvest)

Y Axis: Colony surviving fractions

Shown this way when there is a CONSISTENT DOSE

Consistent dose cell curve for hamsters

Cells most radioresistant in late synthesis phase!

Cell most radiosensitive in M phase, worse when irradiated during this time.

Consistent dose cell curve for HeLa Cells

Cells most radiosensitive at M (mitosis)

Cells most radioresistant at late S and early G1 phase

Since G1 varies between different types of cells (phase timing)

It is more difficult to see differences in G1 cells that are an hour compared to one that is 11 hours

As demonstrated by the increased radioresistance of early G1 in the HeLa cells

Difficult to see this in the hamster cells

General patterns of radiosensitivity and radioresistance at different phases of the cell cycle

1) mitosis seems to be the most radiosensitive time for cells

2) Late S phase is the most radioresistant time

3) Early G1 and G2 appear to be radioresistant

4) Late G1 and G2 appear to be radiosensitive

Red curve represents mitotic cells irradiated under hypoxic conditions

Old models

S phase radioresistance

-hypothesized that since DNA is synthesized here and there is an identical chromosome available, and that homologous recombination is possible and less error prone, that it makes it more resistant

Mitosis radiosensitivity

-only one chromosome and error-prone non-homologous end joining is used more here, that makes it more sensitive

Current model for explanation of phase timing and radiosensitivity

Checkpoint Genes

Checkpoint genes

Quality control stop before Mitosis (M) and Synthesis (S)

Prevents cell cycle progression at these key checkpoints

Initiates repair of chromosome damage before cell DNA synthesis or division occurs

Cells that lack check point genes may be more sensitive

-radiation induced cell killing

-carcinogenesis (cancer)

A longer G1 may give checkpoint genes

More time to work

Checkpoint genes in G1 may influence

radiosensitivity

Given a set of cell survival curves for various types of radiation, explain the importance of LET on radiosensitivity through the cell cycle.

As LET increases, the variation in radiosensitivity (difference between highest and lowest survival values) decrease

This means that cell cycle position does NOT have as much of an influence on radiosensitivity for HIGH LET

Cell cycle position

Does not matter as much for HIGH LET

Cell cycle matters for

LOW LET

Has a higher survival range compared to high LET

Range of radiosensitivity is much less for

HIGH LET

Radiosensitivity ranges

Gamma range 2-100%

16 mev neutron range 0.7-14%

Effect of dose fractionation on synchronization

Can use dose fractionation to selectively "sensitize" populations of cells for therapuetic purposes

Through exposing asynchronous cells to radiation in vovo (inside the body), it tends to

-kill more cells at or close to mitosis

-kill less cells in DNA synthesis

-this produces a natural synchronization of cells where most are in radioresistant phase

-if you wait awhile, then expose the organism to another dose, these cells may have cycled to sensitive phase (good for therapuetic reasons)