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ENU Mutagenesis Protocol


Modified from Lyle Zimmerman. Special thanks to Nick Hirsch, Mary Burns, and Rob Grainger for advice.

Note: ENU is a potent toxin, mutagen, carcinogen. Please take appropriate precautions to protect yourself. Properly dispose of ENU according to your Institutions requirements. We highly recommend you discuss this with your Environmental Health & Safety group to insure that you use ENU safely.

Sperm Mutagenesis

Prepare animals

Day 0:
prime 4 females (10 U hCG)
boost 4 males (100 U hCG)

Day 1:
boost 4 females
sacrifice 4 males when females begin to lay.

ENU work:

Necessary Reagents

MES (10 g)
Sodium thiosulfate (500 g)

Prepare hood, floor around hood, waste containers
double gloves, wristguards, respirators, etc.

Prepare decontamination bath: (20% sodium thiosulfate 2% sodium hydroxide). This is a 2x bath and can be used 1:1 with decontaminated solutions.
Use a full 500 g sodium thiosulfate (just use a whole bottle) and 50 g NaOH into 2.5 L of solution.
All ENU-contaminated tips, tubes etc. go into the bath for 24 hours

Prepare 100mM ENU stock:
Make 50 ml 1 M MES
9.76 g MES into a final volume of 50 ml
Add 5 pellets of NaOH (helps get it into solution)
Add NaOH pellets or 10 M NaOH to final pH (7.0)

Make 5 mM MES pH 7.0 - 1 ml 1 M MES into 199 ml H20

Inject 85.4 ml 5mm MES pH 7.0 (2-[N-Morpholino]ethanesulfonic acid, Sigma M-3671) into ENU (N-Nitroso-N-ethylurea Sigma N3385 1g isopac), rock rt until dissolved (up to 4 hrs, triturate carefully w/ 18 gauge needle if necessary; last bits may refuse to go into solution); use fresh and freeze aliquots for later. We strongly recommend OD'ing to determine dose.

Determining [ENU] by Spectrophotometry


1. Dissolve 1 gm Isopac ENU in 100ml ENU dilution buffer. This solution is approximately 10 mg/ml.

2. Dilute 20µl ENU solution to 1 ml with ENU dilution buffer (i.e. 1:50 dilution)

3. Determine OD398 using a disposable plastic cuvette. Note: This is an important step. We and others have found that the amount of ENU varies substantially from lot to lot.

4. 1 mg/ml solution of ENU gives OD398 =0.72.
Therefore, [ENU] mg/ml = (OD398)(50)/0.72
or = (OD398)(69.4)
We would expect ~ 11.7 mg/ml

Divide measured concentration by 11.7 to generate correction factor. Multiply correction factor by doses to ensure accuracy.

BSA- or serum-coat tubes, dishes, pipets that will contact sperm solution

Treat Sperm with ENU

Make 1xMBS+0.2%BSA+3mM MES (pH 7)

Dissect 8 trop testis (4 frogs) into 1xMBS+0.2%BSA, place keep at RT.
Use a 1.5 ml Eppendorf tube
Remove 1xMBS+0.2%BSA, and add 500 ml 1xMBS+0.2%BSA + 3mM MES (pH 7)
Macerate testis

Bring up to 1.8 ml w/1xMBS+0.2%BSA +3 mm MES (pH 7)

Remove 0.1 ml and place aside – control for sperm fertility

Setup 15 ml conical tubes.
Transfer sperm solution to 15 ml conical tubes (see below)

ENU 100mM stock
1xMBS+0.1%BSA 3mM MES pH 7
Total volume (ml)
ENU Dose(mM)
100 ml
900 ml
100 (÷ by CF) ml
500 ml
500-ENU stock volume ml

Incubate at RT for 10 min mixing occasionally

Wash 2x by adding 10ml L15, spinning 5' 1000 rpm clinical fuge
Resuspend pellet by gentle flicking of the tube and not pipetting.
Start squeezing frogs after second spin
resusp. sperm pellet in ~1ml 1xMBS+0.2%BSA.

Fertilize Eggs

Squeeze eggs into dry 1xMBS+0.2%BSA coated dish
Apply sperm and wait 10 min.
Flood with 1/9XMR

Assay for cleavage two hours later (fertilizations can be delayed)

Dejelly and sort into 1/20XMMR+gent

Day 2:

Sort viable embryos into 1/20XMMR+gent. Also sort and count samples of control and mutagenized embryos to assay dominant effects.

Inject Frogs

Male frogs typically weigh between 6-9 g (Weigh frogs in 1 qt Glad plastic containers – this also acts as recovery chamber)

Inject 100 mg/kg or 0.1 mg/g of ENU

1 g ENU is diluted in ~85.4 ml of Buffer, therefore inject [frog wt (g)]*[0.1mg/g]*[1ml/11.7mg ENU] or 0.00855*frog wt.
This should be corrected based on actual measured amount of ENU determined by spectrophotometer

Volume injected should vary between 250-400 ml round to the nearest 50 ml. So dilute your ENU based on the ENU correction factor to make the injections between 250-400 ml.
So assuming that the heaviest frog is 10 g then:

Dilute 1:[correction factor]*4.68
inject [frog wt (g)]*0.04 in ml