modifiers of cell survival: repair

Ahmed GroupLecture 10

Modifiers of Cell Survival:Modifiers of Cell Survival:

RepairRepair


Modifiers of Cell Survival: Repair

• Sub-lethal damage repair

• Half-time of repair

• Potentially lethal damage repair

• Effect of dose, dose rate, and cell type

• Effect of dose fractionation

• Effect of LET

• Effect of oxygen/ hypoxia


Sub-lethal damage repair and half-time of Sub-lethal damage repair and half-time of repairrepair


Sub-lethal DamageSub-lethal Damage

Can be repaired under normal Can be repaired under normal circumstancescircumstances

Two split of doses of radiation can Two split of doses of radiation can overcome sub-lethal damage repair to overcome sub-lethal damage repair to cause lethal damage.cause lethal damage.


Sublethal Damage RepairSublethal Damage Repair

Increasing in cell survival that results from Increasing in cell survival that results from splitting RT into 2 fractions separated by a splitting RT into 2 fractions separated by a time intervaltime interval

Dose divided into 2 equal fractions, Dose divided into 2 equal fractions, separated by time, surviving fraction separated by time, surviving fraction increases until a plateau is reached at increases until a plateau is reached at about 2 hoursabout 2 hours


Cell cycle not in progress

Cell cycle in progress



SLDRSLDR

Previous experiment shows repair of SLD Previous experiment shows repair of SLD uncomplicated by movement of cells uncomplicated by movement of cells through the cell cyclethrough the cell cycle

Longer intervals between the 2 doses Longer intervals between the 2 doses results in the surviving fraction of cells results in the surviving fraction of cells decreasing againdecreasing again


Diagram explanationDiagram explanation

Cells in the sensitive part of the cell cycle Cells in the sensitive part of the cell cycle are killedare killed

In first 2 hours, SLDR occursIn first 2 hours, SLDR occurs

Resistant cells move to more sensitive part Resistant cells move to more sensitive part G2/M of cell cycle (REASSORTMENT)G2/M of cell cycle (REASSORTMENT)

When time interval exceeds cell cycle time, When time interval exceeds cell cycle time, 10-12 hours, repopulation occurs and cell 10-12 hours, repopulation occurs and cell survival increases survival increases


Repair SLD in-vivoRepair SLD in-vivoThe recovery factor is the ratio of SF resulting from 2-dose fractions to the The recovery factor is the ratio of SF resulting from 2-dose fractions to the SF from a single-dose fractionSF from a single-dose fraction

In neither case, there is dramatic dip in the curve at 6 hours resulting from movement of cells through the cell cycle, because the cell cycle is long

The mouse tumor data show more repair in small 1-day tumors than in large hypoxic 6-tumors


EXPERIMENTEXPERIMENT

Illustrates Illustrates 3 R’s3 R’s of radiobiology: of radiobiology: Repair Repair Reassortment Reassortment RepopulationRepopulation

Additional “R”:Additional “R”: Re-oxygenationRe-oxygenation


Sublethal Damage RepairSublethal Damage RepairHalf-time of repair

As the time interval between the two dose fractions is increased, there is a rapid increase in the fractions of cells surviving due to the prompt repair of sublethal damage.The half-time of sublethal damage repair in mammalian cells is about 1 hour, but it may be longer in late-responding normal tissues in vivo.


SLDRSLDR

When dose delivered in 2 fractions, increase When dose delivered in 2 fractions, increase in cell survival b/c shoulder of the curve in cell survival b/c shoulder of the curve must be expressed each timemust be expressed each time

Significant for x-rays but almost nonexistent Significant for x-rays but almost nonexistent for neutrons for neutrons

Repair of SLD reflects repair of DNA (DSB) Repair of SLD reflects repair of DNA (DSB) breaks before they can interact to form breaks before they can interact to form lethal chromosomal aberration (multi-track)lethal chromosomal aberration (multi-track)


Potentially lethal damage repair


Potentially Lethal DamagePotentially Lethal Damage

Component of radiation damage that can be Component of radiation damage that can be modified by post-irradiation environmental modified by post-irradiation environmental conditionsconditions

If left in same environment for 6-12 hours after If left in same environment for 6-12 hours after RT before being subcultured, survival increasesRT before being subcultured, survival increases

Resistant tumors like melanoma? Large amount Resistant tumors like melanoma? Large amount of PLDR is presentof PLDR is present


PLDRPLDR

In general, PLD is repaired and the In general, PLD is repaired and the fraction of cells surviving a given dose is fraction of cells surviving a given dose is enhanced if postirradiation conditions are enhanced if postirradiation conditions are suboptimal for growth so that cells do not suboptimal for growth so that cells do not have to attempt the complex process of have to attempt the complex process of mitosis while their chromosomes are mitosis while their chromosomes are damaged.damaged.If mitosis is delayed by suboptimal growth If mitosis is delayed by suboptimal growth conditions, DNA damage can be repairedconditions, DNA damage can be repaired


Potentially Lethal Damage RepairPotentially Lethal Damage Repair


Effects of dose, dose rate, and cell typeEffects of dose, dose rate, and cell type

Effect of dose fractionationEffect of dose fractionation


DOSE-RATE EFFECTDOSE-RATE EFFECT

Repair secondary to long radiation exposure. Repair secondary to long radiation exposure. As dose rate decreases, survival increases As dose rate decreases, survival increases b/c of more SLDR. Curve becomes shallower b/c of more SLDR. Curve becomes shallower and shoulder disappearsand shoulder disappears

More SLDR occurs during the protracted More SLDR occurs during the protracted exposureexposure


Multiple small fractions approximate to a continuous exposure to a low dose rate

Because continuous low-dose rate irradiation may be considered to be an infinite number of infinitely small fractions, the survival curve under these conditions also would be expected to have no shoulder and to be shallower than for single acute exposures

Idealized fractionated Idealized fractionated experimentexperiment


Examples of the dose-rate Examples of the dose-rate effect in-vitro and in-vivoeffect in-vitro and in-vivo

Survival curves for HeLa cells exposed to -rays at high and low dose rates

As the dose rate is reduced, the survival curve becomes shallower tends to disappear (I.e., survival curve becomes an exponential function of the dose)

The dose rate effect caused by SLD is most dramatic between 1-100 rad/min. Above or below this dose-rate range, the survival curve changes little.


Dose-response curves for Chinese hamster cells (CHL-F line) exposed to cobalt 60-g-rays at various dose rates.

The decrease in cell killing becomes even more dramatic as the dose rate is reduced further


The survival curves fan out at LDR because of the presence of a range of repair times for SLD and further is associated with a range of inherent radiation sensitivities

Dose survival curves at high dose rate (HDR) and low dose rate (LDR)


Examples of the dose-rate Examples of the dose-rate effect in-vitro and in-vivoeffect in-vitro and in-vivo

Response of mouse jejunal crypt cells irradiated with -rays.

There isa dramatic dose-rate effect owing to the repair of sub-lethal radiation damage from acute exposure of 274 rad/min to a protracted exposure at 0.92 rad/min.

Cell division dominates at LDR because the exposure time is longer than the cell cycle time.


INVERSE DOSE-RATE EFFECTINVERSE DOSE-RATE EFFECT

In some cell lines, an inverse dose-rate In some cell lines, an inverse dose-rate effect is evident effect is evident

Reducing the dose rate increases the Reducing the dose rate increases the proportion of cells killedproportion of cells killed

Accumulation of cells in G2, sensitive Accumulation of cells in G2, sensitive phase of the cell cyclephase of the cell cycle


Example of the inverse dose-rate effect in-Example of the inverse dose-rate effect in-vitrovitro

Decreasing the dose-rate from 154 rad to 37 rad/h increases the efficiency of cell killing, which is as effective as an acute exposure.


Mechanism underlying inverse Mechanism underlying inverse dose-rate effect in-vitrodose-rate effect in-vitro


Dose-Rate Effect SummaryDose-Rate Effect Summary

As dose rate decreases, SLDR As dose rate decreases, SLDR occurs b/c of protracted occurs b/c of protracted exposureexposure

In some cell lines, lowering the In some cell lines, lowering the dose rate further allows cells to dose rate further allows cells to accumulate in G2, a sensitive accumulate in G2, a sensitive part of the cell cycle and part of the cell cycle and survival decreasessurvival decreases

A further reduction will allow A further reduction will allow cells to pass through G2 and cells to pass through G2 and proliferation occurs if exposure proliferation occurs if exposure time long compared to length of time long compared to length of cell cyclecell cycle


Very low dose rates continuous Very low dose rates continuous exposuresexposures

2 rad/day continuous testis exposure does not 2 rad/day continuous testis exposure does not affect reproductionaffect reproductionSmall intestine maintain cell division and steady-Small intestine maintain cell division and steady-state population at dose rate as high as 4 Gy/daystate population at dose rate as high as 4 Gy/dayThe blood forming tissues are intermediate The blood forming tissues are intermediate between these two extremesbetween these two extremesThese depend on three important aspects (i) The These depend on three important aspects (i) The cellular sensitivity of stem cells; (ii) The duration of cellular sensitivity of stem cells; (ii) The duration of cell cycle; and (iii) The ability of some tissues to cell cycle; and (iii) The ability of some tissues to adapt to the new trauma of continuous irradiationadapt to the new trauma of continuous irradiation


Effect of LET


LET and repair


Repair and radiation qualityRepair and radiation quality

The shoulder is present for X-rays suggesting the presence of SLD repair, whereas, in response to neutrons there is no shoulder suggesting loss of SLD repair.


Effects of oxygen/ hypoxia


From Sinclair W.K., Radiat Res. 33:620-643, 1968. The broken line is a calculated curve expected to apply to mitotic cells under hypoxia.

Cell-survival curves for Chinese hamster cells at various stages of the cell cycle


Survival curves for CHO cells exposed to X-rays in the presence of various oxygen concentrations. Oxygen is introduced graduallyinto the biologic system.

The introduction ofa very small quantityof oxygen, 100 ppm,is readily noticeablein a change in theslope of the survivalcurves. A concentration of2,200 ppm, which isabout 0.25% oxygen,moves the survivalcurve halfway toward the fullyaerated condition.


Effect of oxygen/hypoxia: in-vivo


SummarySummary

modifiers of cell survival: repair

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