ATM encodes a tumor suppressor which helps repair double strand breaks of DNA. Mutation of ATM can be seen in 4% to 15% of CLL patients. MAY lead to shorter time to first treatment but a lot hinges on what the mutational variant of the gene is.
So many different things can affect the coarse of a patients CLL journey that worrying about them all is not worth it. A person can get too caught up in worrying and forget to live. I just try and live my life the best I can and let the Universe take care of the rest.