HELIOS: Open-Label, Long-Term Extension Study to Investigate the Safety, Tolerability, and Efficacy of DISC-1459 (Bitopertin) in Participants With EPP or XLP

Inclusion Criteria: * Participants with diagnosis of EPP who are participating (or who have participated) in a prior Disc Medicine bitopertin study and who have completed the randomized treatment phase and End-of-Study visit * Aged ≥12 years upon study consent * Body weight ≥32 kg for participants \<18 years of age and BMI ≥18.5 kg/m2 for adult participants * Willing to practice highly effective methods of birth control (both males who have partners of childbearing potential and females of childbearing potential during the study, while taking study drug, and for at least 30 days after the last dose of study drug. * Negative urine or serum pregnancy test (females of childbearing potential). * Able to understand the study aims, procedures, and requirements, and provide written informed consent (and assent if necessary). * Able to comply with all study procedures. Exclusion Criteria: * Participants who have an ongoing SAE from a clinical study that is assessed by the investigator as related to bitopertin * Other medical or psychiatric condition or laboratory finding not specifically noted above that, in the judgement of the Investigator or Sponsor, would put the participant at unacceptable risk or otherwise preclude participation in the study * Condition or concomitant medication that would confound the ability to interpret clinical, clinical laboratory, or participant diary data, including a major psychiatric condition that has had an exacerbation or required hospitalization in the last 6 months * Planned treatment with afamelanotide or dersimelagon during the study * Planned use of any drugs or herbal remedies known to be strong inhibitors or inducers of cytochrome p450 (CYP)3A4 enzymes throughout the study * If female, pregnant, or breastfeeding * Participation in any other clinical protocol or investigational trial, other than Disc Medicine bitopertin trials, that involves administration of experimental therapy and/or therapeutic devices within 30 days of Day 1 * Score of PHQ-8 ≥10 at screening or imminent suicidal risk identified by the C-SSRS as defined as suicidal ideation with intent (Grade 4 or 5) within the last year or any suicidal behavior within the last 5 years. * Consumption of grapefruit/Seville orange and products containing these for 14 days prior to first dose of study drug and throughout the study