Rett syndrome (RTT) is a progressive neurodevelopmental disorder mainly caused by mutations in the gene encoding the methyl-CpG-binding protein 2 (MeCP2). Although over 200 mutations types have been identified so far, nine of which the most frequent ones. A wide phenotypical heterogeneity is a well-known feature of the disease, with different clinical presentations, including the classical form and the preserved speech variant (PSV). Aim of the study was to unveil possible relationships between plasma proteome and phenotypic expression in two cases of familial RTT represented by two pairs of sisters, harbor the same MECP2 gene mutation while being dramatically discrepant in phenotype, that is, classical RTT versus PSV. Plasma proteome was analysed by 2-DE/MALDI-TOF MS. A significant overexpression of six proteins in the classical sisters was detected as compared to the PSV siblings. A total of five out of six (i.e., 83.3%) of the overexpressed proteins were well-known acute phase response (APR) proteins, including alpha-1-microglobulin, haptoglobin, fibrinogen beta chain, alpha-1-antitrypsin, and complement C3. Therefore, the examined RTT siblings pairs proved to be an important benchmark model to test the molecular basis of phenotypical expression variability and to identify potential therapeutic targets of the disease. 1. Introduction Rett syndrome (RTT; OMIM no. 312750), with a frequency of ~1？:？10000–1？:？15000 females, is a severe and complex neurodevelopmental disorder, as well as the second most common cause of severe mental retardation in the female gender [1]. RTT presents in about 74% of cases in a classical form (typical presentation); after 6–18 months of an apparently normal development girls lose their acquired cognitive, social, and motor skills in a typical 4-stage neurological regression. A wide phenotypical heterogeneity is a well-known feature of the disease, which includes at least four major different clinical presentations, that is, classical, preserved speech (PSV), early seizure (ESV), and congenital variants [2]. Studies have implicated de novo mutations of the X-linked methyl-CpG-binding protein 2 (MECP2) gene (OMIM*300005) in the majority of the RTT cases, while mutations in cyclin-dependent kinase-like 5 (CDKL5) and forkhead box G1 (FOXG1) have been more rarely reported [3–5]. Typical RTT has been described worldwide, whereas PSV is more rarely reported. Girls affected by PSV have been often misreported with various diagnoses ranging from autism to mental retardation [6, 7]. While the available RTT literature is mainly
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