Hi..it could be because decalcification of enamel in a specific zone. Go for a cosmetic treatment to make area natural by composites. Meantime visit physician for blood calcium and Vitamin D evaluation Regards

Hi! The white lines could be because of multiple reasons. Could be because of decalification of the tooth, very initial decay, an occlusal problem whereby your bite could be damaging the teeth. It needs to be checked and diagnosed for the best treatment options . It should be easily treatable with good cosmetic work without damaging any more tooth structure