function userFind(
    t: Transaction,
    options: {where: {username?: string, email?: string}},
    callbackThen: (data: any) => Promise<any>,
    callbackCatch?: (err: Error) => never
): Promise<any> {

    return User.findOne({
            ...options,
            transaction: t
        })
        .then(callbackThen)
        .catch(callbackCatch || (err => {
            throw new Error(); //Provoque un ROLLBACK
        }));

}

//username = 'toto'
//email = 'aaa@aol.com'

this.sequelize.transaction((t: Transaction) => {

    return userFind(t, {
        where: {username: username}
    }, (data: any) => {
        if(data) {
            throw new Error('...'); //Provoque un ROLLBACK
        }

        return userFind(t, {
            where: {email: email}
        }, (data: any) => {
            if(data) {
                throw new Error('...'); //Provoque un ROLLBACK
            }

            //etc...
        }
    });

});