A. SUMMARY OF SOME BASIC CONCEPTS IN RADIATION GENETICS:

1.  Major problems and questions in Radiation Genetics

1. Is there a threshold dose?
2. What is the shape of the close-response curve?
3. How reliable is extrapolation of human risk from experimental animal data?
4. Can genetic damage be repaired?
   
   
2.  Problems in Radiation Genetics
1. no unique mutations
2. most mutations are recessive
3. rare event
4. within the same species, genes vary both in their spontaneous mutation rate and in their vulnerability to radiation
   
   
3.  Doubling-Dose for Mutations in Male Mice (Low LET radiations)
1. Acute (High Dose-Rate): 200-300 mGy (20-30 rads)
2. Protracted (Low Dose-Rate): 0.9-1 Gy (90-100 rads)
   
   
4.  Estimated Mutagenic Hazard in Humans

2-3x10^-7 genetic generations of 30 yrs each = 300 years



5.  B Cell: a cell with one or more chromosomes with staining gaps
   
   
6.  C Cell: A cell containing one or more structurally abnormal chromosomes. Chromosome aberrations are subdivided in -stable (Cs), i.e. survivable, and unstable (Cu), i.e. fatal to the daughter cells inheriting them
   
   
7.  Linear Quadratic (L-Q) Model of Response: the model which states that biological effects of radiation are partly proportional to the first power of the Dose (the linear term) and partly proportional to the square of the Dose (the quadratic term), with A and B representing the linear and quadratic probability coefficients, respectively. Thus the yield of chromosome aberrations (Y) after dose (D), when plotted on log-log coordinates is:

Y = AD+BD²



8.  Genetic Maximum Permissible Dose Limits
   
   
9.  Occupational: 50 mSv (5 rem) per year
1. General population: 1.7 mSv (170 mrem) per year

 B. RADIATION EFFECTS ON DNA:

 Effects on DNA

 Four Kinds of Radiation Induced DNA Damage.

1. Chain breaks: Single or double, are caused by ionization. Single strand break in DNA following irradiation can lead to rejoin or, in the presence of O₂, to a peroxidized end which cannot rejoin.

Typically Low L.E.T. Radiation



Double chain break in DNA. For separation of fragments.

 Typically High L.E.T. Radiation



2. Cross-linking between strands, between adjacent DNA molecules, and between DNA and proteins. Caused by UV, x-rays, chemicals.

 

Crosslinking between two irradiated molecules. For diagrammatic purposes, the relative position of the two strands is shifted in the third illustration.

3.  Base damage: caused by UV light, carcinogens.
   
   
4.  Dimerization: within one strand or between strands dimers may be formed:

During DNA replication, a dimer is not recognized, replication stops. This means loss of information.

5.  Classification of Chromosomal Anomalies and Effect at Anaphase



  

Some chromosome aberrations.



 

6. Repair: Chain breaks --> polynucleotide ligase; base damage: "cut and snip" enzyme complex.



• Endonuclease
• Exonuclease
  
  
7.  G-Value (Chemical Yield)
   
   
• G = # molecules changed/100 eV
• Examples for Dan (in vitro)

 C. SOME SUMMARY POINTS TO KNOWN:

1. Many single-stranded breaks are produced in DNA by radiation but are readily repaired using the opposite DNA strand as a template (role of endonuclease).
   
   
2. Breaks in both strands, if well separated, are also readily repaired since they are handled individually.
   
   
3. Breaks in both strands that are opposite, or separated by only a few base pairs, may lead to a double-strand break (which cannot be easily repaired).
   
   
4. Energy from x-ray is deposited unevenly in "spurs" and "blobs." This may lead to multiple damaged sites, that is, a combination of double-stranded and base damage.
   
   
5. Radiation-induced breakage and incorrect rejoining in pre-replication chromosomes (G1 phase) may lead to chromosome aberrations.
   
   
6. Radiation-induced breakage and incorrect rejoining in post-replication chromosome (S and G2 phase) may lead to chromotid aberrations.
   
   
7. Principal aberrations include dicentrics, rings, acentric fragments, and anaphase bridges.
   
   
8.  The incidence of most radiation-induced aberrations is a linear-quadratic function of dose.
   
   
9. Scoring aberrations in lymphocytes from peripheral blood may be used to estimate total-body doses in humans accidentally irradiated. The lowest single dose that can be detected readily is 250 mGy (25 rads).
   
   
10. Aberrations can still be detected in irradiated persons up to 40 years after exposure.
    
    
11. There is a good correlation between cells killed and cells with an asymmetrical exchange aberration (i.e., a dicentric or ring formation).

D. Genetic Effects

1. Overview
   
   
1.  No evidence of effects in humans
   
   
• however, this does not mean there are no effects
• high background of effects
• recessive mutations may take many generations to express themselves with a significant frequency
  
  
2.  Insect Data (Fruit Fly)
   
   
• 1927 H.J. Muller. Work with Drosophila
  
  
• irradiation prior to procreation + measured frequency of lethal mutation in progeny
• dose response: linear non-threshold

 

•  no dose rate or fractionation effects were observed.
  
  
• proposed: single-hit model
  
  
• NCRP adopted the experimental evidence in its recommendation to lower the MPD to 5 rem/yr (1932).
  
  
3.  Animal Data (Mega-mouse Study)
   
   
• Russell & Russell (1945) Mouse Colony
• 7 x 10⁶ mice @ 0.001 -10³ rads/mouse
• observed 7 specific genes for easily identifiable characteristics (ear shape, coat color, eye color, etc.)
  
  
•  demonstrated:
  
  
• dose rate effect (indicates repair)
• males more sensitive than females
• linear non-threshold fit to data no "unusual" mutations
• mutation rate = 15x's fruit fly results
  
  
4.  Doubling Dose (Results of the RUSSELL Experiments)
   
   
• doubling dose: That dose of radiation which will produce twice the frequency of genetic mutations as would have been observed without the radiation.
  
  
• estimated doubling dose
  
  
• mouse: 200 mGy – 2 Gy (20-200 rads)
• human: 330 mGy (33 rads)
  
  
5.  Genetically Significant Dose (GSD)
   
   
•  GSD: The gonadal dose which, if received by every member of the population, would be expected to produce the total genetic effect on the population as the sum of the individual doses actually received.
  
  
• mathematical formalism
  
  
• GSD =

where:

D = average gonadal dose per examination

N= # of people irradiated

N = total # of people in the population

P = expected future # of children per person

 

• estimated GSD from diagnostic x-rays

 

2. Generalization

1. radiation mutations are usually harmful
2. any dose carries with it some genetic risk (i.e.: non threshold)
3. frequency, mutation a dose (i.e. linear hypothesis)
4. effects are dependent an
   
   
•  dose rate
• fractionation
• Therefore repair processes exist
  
  
5. males more sensitive than female
6. most mutations are recessive, therefore may take many generations to express
7. radiation induced frequency is very low:
   
   
• 10^-7 mutations/rad/gene
  
  
8. spontaneous incidence is very high
   
   
• 10^-5 mutations/gene
  
  
3.  Other Effects
    
1. Cataracts (Non-Stochastic Effect)
   
   
• First Effect seen (1949: Cyclotron physicist)
• Radiosensitivity of the lens of the eye is:
• Age dependent. As age ­ --> latent period ¯
• Range: 5-30 years. Average: 15 years
• Charged particles (e.g. protons) are more effective than photons
• Dose response relationship: threshold, nonlinear
• < 2 Gy (200 rads) = threshold
• 10 Gy (1000 rads) = 100% cataracts
  
  
2.  Life Span Shortening
   
   
•  Many experiments with animals have demonstrated
• chronically irradiated animals die young
• close response is linear -- non-threshold

•   Epidemiologic Study: Three Physician Groups
  
  
• RSNA: Radiological Society of North America
• ACP: American College of Physicians
• AAOO: American Academy of Ophthalmology

• Epidemiologic Study (comparison between American Radiologist & US Population)

• Note:
  
  
•  similar studies in other countries (e.g. Great Britain) have failed to confirm any positive effect.
• any effects observed in "early radiologist" have disappeared, presumably related to better health physics practices and better equipment.
  
  

Genetic Effects:

To date, there is no data that proves that radiation can cause inherited genetic damage in any irradiated human population. There is no significant increase in any indicator of inherited (genetic) damage in any irradiated human population.

Our knowledge of the genetic effects of radiation therefore comes entirely from laboratory studies with animals, plants, and cell cultures. Risk estimates for man rely heavily on animal data (especially data from the mouse). Best human data available = follow-up of children born to survivors of Hiroshima and Nagasaki. Major findings from these data are summarized from Bier V.

1. Pregnancy termination studies--Study of 75,000 births (38,000 of which had at least 1 exposed parent). No significant effect of radiation on still births, birth rate, congenital abnormalities, infant mortality, childhood mortality, leukemia, or sex ratio.
2. Study of all live-born children born in Hiroshima and Nagasaki between may 1946 and December 1958 No statistically significant effects noted to date.
3. Cytogenetic study of children born to exposed parents show no significant effect demonstrated.
4. BLOOD PROTEIN STUDIES: Studies of rare electrophoretic varients of 28 proteins of blood plasma and erythrocytes, of deficiency variants of 10 erythrocyte enzymes, of children of exposed parents: 667,404 tests - 3 probable mutations, and of Control population: 466,881 tests - 3 probable mutations show showed no significant differences.

 

 

 

BIER V ESTIMATE OF SPONTANEOUS BURDEN OF GENETIC DISEASE IN MAN.

TYPE	INCIDENCE/1,000 LIVE BIRTHS
DOMINANT	10
X-LINKED	0.4
RECESSIVE	2.5
CHROMOSOMAL ABNORMALITY	4.4
CONGENITAL ABNORMALITY	20-30
OTHER MULTIFACTORAL TRAITS*	120

*Includes heart disease, cancer, and other diseases of complex etiology with probable hereditary compound

PROBLEMS FOR RADIATION GENETICS STUDIES IN PEOPLE:

1. Detecting increases in genetic disease due to radiation in the face of this large spontaneous burden is difficult.

 

2. Controlling for other variables which affect the incidence of genetic diseases is also difficult.

FINDINGS FROM EXPERIMENTAL ANIMALS (General findings)

1. Most genetic mutations are deleterious

 

2. Radiation does not produce novel mutations, but instead increases the incidence of mutations occurring naturally.

 

3. No evidence for threshold has been found.

 

4. Fractionation/protraction of radiation dose generally protects.

 

5. Quality of radiation - important (RBE increases with LET)

 

6. In mice - response of males and females differ. This reflects difference in the cell proliferation patterns of the oocytes and spermatogonia, and differences in the relative sensitivities of the various cell types.

 

7. Genetic consequences are generally reduced when time interval is allowed between irradiation and conception.