Its most likely caused by dropsy or stress through poor water quality.
Dropsy can be cured if treated early but usually its fatal.
quarantine the fish asap to avoid infecting others.
Spine bending can also be caused by fish Tuberculosis though its rare in tropical aquariums. unfortunately once its got to a stage when you can visibly see the fish is deformed its fatal.
Fish TB can also be passed onto humans so be careful when handling the fish and wash every thing thoroughly.
Either way fish disease is usually caused by poor water quality which leads to fish stress which in turn drops the fishes immune system making them more vulnerable to disease.
Do large water changes every day for a week and add dechlorinator with stress relief, some thing like API stress coat.
After a week drop down to a water change every other day.
then after another week go to a regular large water change once a week.
Do NOT rely on water test kits, they are inaccurate and inconsistent and will only lead you up the garden path.
overfeeding is a common cause of poor water quality in conjunction with infrequent water changes. how often do you feed your fish and how much?
Deformed bodies can also be caused by a vitamin or mineral deficiency but if you're feeding a quality flake food this is unlikely.
It's also possible that the fish has always had this abnormality and its getting progressively worse, however if you've already lost a fish to the same symptoms I'd say its a disease.