Murat Coskun 1 , Seda Erbilgin 2 , Ibrahim Akalin 3 , Ilyas Kaya 1 , Zeynep Nur Gulle 1 , Afig Berdeli 4 DOI: 10.14744/DAJPNS.2019.00067 Dusunen Adam The Journal of Psychiatry and Neurological Sciences 2020;33:99-103 How to cite this article: Coskun M, Erbilgin S, Akalin I, Kaya I, Gulle ZN, Berdeli A. Typical Rett Syndrome in a young boy with hemizygous c.316C>T mutation in MECP2 gene. Dusunen Adam The Journal of Psychiatry and Neurological Sciences 2020;33:99-103. Typical Rett Syndrome in a young boy with hemizygous c.316C>T mutation in MECP2 gene 1 Istanbul University, Istanbul Faculty of Medicine, Department of Child and Adolescent Psychiatry, Istanbul - Turkey 2 Istanbul Okmeydanı Training and Research Hospital, Department of Child and Adolescent Psychiatry, Istanbul - Turkey 3 Istanbul Medeniyet University, Faculty of Medicine, Department of Medical Genetics, Istanbul - Turkey 4 Ege University, Faculty of Medicine, Department of Pediatric Rheumatology, Izmir - Turkey Correspondence: Zeynep Nur Gulle, Department of Child and Adolescent Psychiatry, Istanbul Faculty of Medicine, Istanbul University, Istanbul 34390, Turkey Phone: +90 538 625 90 60 - 212 414 20 23 E-mail: zeynepnur.gulle@gmail.com Received: May 02, 2019; Revised: August 03, 2019; Accepted: November 21, 2019 ABSTRACT Mutations in the Methyl-CpG-binding protein 2 (MECP2) gene have been implicated in the etiology of Rett syndrome (RTT), a neurodevelopmental disorder that primarily affects girls. MECP2 mutations in males, once thought to be lethal, are now recognized with a broad spectrum of clinical manifestations. Here we report a 3-year-old boy who presented with developmental problems and regression and eventually was diagnosed with RTT that genetic analysis revealed to be a hemizygous c.316C>T missense mutation in the MECP2 gene suggesting somatic mosaicism with the normal 46,XY karyotype. DNA analysis of the patient’s mother showed this either to be a de novo mutation or a case of gonadal mosaicism. To the best of our knowledge, this is the first case report of RTT in a young boy with a hemizygous c.316C>T mutation in the MECP2 gene. Keywords: c.316C>T, male, MECP2 mutation, rett syndrome, somatic mosaicism CASE REPORT INTRODUCTION Rett Syndrome (RTT, MIM 312750) is an X-linked neurodevelopmental disorder characterized by loss of spoken language and volitional hand use with the development of repetitive hand stereotypes that occur after an apparently normal initial six to eighteen months of development (1). RTT is almost exclusively seen in girls (1) and rarely reported in male patients (2-8). In 1999, Amir et al. (9) discovered that mutations in the gene encoding Methyl-CpG-binding protein 2 (MECP2) were associated with the most common sporadic occurrences of typical RTT in addition to rare familial cases. Mutations in MECP2 can be detected in 95-97% of subjects with typical RTT, while a lower rate of mutations, 50-70%, has been reported in atypical cases (1). Although the precise function of MECP2 has not been fully understood, dysfunction or loss of MECP2, as observed in RTT, would be predicted to give rise to inappropriate activation of genes (10). Typically, affected girls follow an apparently normal development during the first months of life. Between six months and two years of age, developmental regression becomes evident through the loss of speech, purposeful hand movements, and social as well as cognitive abilities (1). In the past, MECP2 mutations in male patients have been thought to be lethal. However, today, it is supposed to present with a broad spectrum of clinical manifestations from death in infancy to developmental delay associated with seizures and