{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,11]],"date-time":"2026-09-11T23:48:03Z","timestamp":1789170483424,"version":"build-2803163510"},"reference-count":50,"publisher":"MDPI AG","issue":"3","license":[{"start":{"date-parts":[[2026,2,27]],"date-time":"2026-02-27T00:00:00Z","timestamp":1772150400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Genes"],"abstract":"<jats:p>Background\/Objectives: Factors modulating phenotypic variability in Rett syndrome (RTT, OMIM 312750) include X chromosome inactivation (XCI), type of MECP2 variant, and\/or disease modifiers. Emerging evidence also points to multi-locus genetic variants. Understanding the phenotypic variability associated with multi-locus genetic diagnoses in individuals with RTT and MECP2-related disorders would be important not only for accurate diagnosis, risk stratification and clinical management but also to explain symptoms that might not be typically associated with RTT. Methods: We present a case series of five individuals with a diagnosis of RTT or an MECP2-related disorder with co-occurring genetic findings, including pathogenic variants, variants of unknown significance and chromosome duplications. Clinical features such as neurodevelopmental history and comorbid medical conditions were assessed alongside the genetic findings. Results: A review of 200 cases with RTT identified five cases (all females aged 7\u201327 years) with a co-occurring genetic finding. Each case harboured at least one additional genetic variant that included a beta thalassaemia trait, Calmodulin 3 (CALM3) missense variant, maternally inherited 22q12.3 to q13.1 duplication, 7p14.3 and Dynein Cytoplasmic 1 Heavy Chain 1 (DYNC1H1) variants of uncertain significance and a pathogenic Set Domain-containing protein 5 (SETD5) variant. A rare triple genetic finding was illustrated in a single case, combining MECP2, CALM3, and DYNC1H1 variants. Conclusions: This case series supports the premise that RTT and MECP2-related disorders exist in a more complex neurogenetic spectrum than previously defined. It also emphasises the complexity within MECP2-related disorders. They are not static, and in the context of severe treatment resistant epilepsy, MECP2 disorders can evolve over time, necessitating diagnostic reclassification. Although the co-occurrence of multiple genetic disorders in RTT and MECP2-related disorders is rare, these cases underscore the importance of considering cumulative genetic burden when evaluating individuals with atypical features or evolving neurodevelopmental phenotypes.<\/jats:p>","DOI":"10.3390\/genes17030274","type":"journal-article","created":{"date-parts":[[2026,2,27]],"date-time":"2026-02-27T08:31:19Z","timestamp":1772181079000},"page":"274","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Co-Occurring Genetic Mutations in Rett Syndrome and MECP2-Related Disorders\u2014Clinical and Diagnostic Implications from a Case Series"],"prefix":"10.3390","volume":"17","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-0205-856X","authenticated-orcid":false,"given":"Jatinder","family":"Singh","sequence":"first","affiliation":[{"name":"Department of Child and Adolescent Psychiatry, Institute of Psychiatry, Psychology and Neuroscience, King\u2019s College London, London SE5 8AF, UK"},{"name":"Centre for Interventional Paediatric Psychopharmacology and Rare Diseases (CIPPRD), South London and Maudsley NHS Foundation Trust, London SE5 8AZ, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Samiya","family":"Chishti","sequence":"additional","affiliation":[{"name":"Department of Child and Adolescent Psychiatry, Institute of Psychiatry, Psychology and Neuroscience, King\u2019s College London, London SE5 8AF, UK"},{"name":"Centre for Interventional Paediatric Psychopharmacology and Rare Diseases (CIPPRD), South London and Maudsley NHS Foundation Trust, London SE5 8AZ, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4830-5893","authenticated-orcid":false,"given":"Paramala","family":"Santosh","sequence":"additional","affiliation":[{"name":"Department of Child and Adolescent Psychiatry, Institute of Psychiatry, Psychology and Neuroscience, King\u2019s College London, London SE5 8AF, UK"},{"name":"Centre for Interventional Paediatric Psychopharmacology and Rare Diseases (CIPPRD), South London and Maudsley NHS Foundation Trust, London SE5 8AZ, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,2,27]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"944","DOI":"10.1002\/ana.22124","article-title":"Rett syndrome: Revised diagnostic criteria and nomenclature","volume":"68","author":"Neul","year":"2010","journal-title":"Ann. 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