Fluent Validations. Inherit validation classes

I used Fluent Validator. But sometimes I need create a hierarchy of rules. For example:

[Validator(typeof(UserValidation))]
public class UserViewModel
{
    public string FirstName;
    public string LastName;
}

public class UserValidation : AbstractValidator<UserViewModel>
{
    public UserValidation()
    {
        this.RuleFor(x => x.FirstName).NotNull();
        this.RuleFor(x => x.FirstName).NotEmpty();

        this.RuleFor(x => x.LastName).NotNull();
        this.RuleFor(x => x.LastName).NotEmpty();
    }
}

public class RootViewModel : UserViewModel
{
    public string MiddleName;       
}

I want to inherit validation rules from UserValidation to RootValidation. But this code didn't work:

public class RootViewModelValidation:UserValidation<RootViewModel>
{
    public RootViewModelValidation()
    {
        this.RuleFor(x => x.MiddleName).NotNull();
        this.RuleFor(x => x.MiddleName).NotEmpty();
    }
}

How can I inherit validation class using FluentValidation?