A new case series published in the Journal of Neurology is broadening the medical understanding of disorders linked to pathogenic variants in MECP2, the gene most famously associated with Rett syndrome. The report describes five unrelated individuals whose genetic changes produced neurological conditions that differed substantially from the classic Rett profile. Instead of the characteristic early development followed by regression, repetitive hand movements, loss of spoken language, and severe motor impairment, some patients showed only mild neurodevelopmental difficulties, while others developed predominantly cerebellar or spastic-ataxic syndromes with relatively preserved cognition. The findings suggest that MECP2 variants should not be considered synonymous with Rett syndrome alone, and that the gene may contribute to a wider range of developmental, epileptic, movement, and coordination disorders than current diagnostic pathways routinely recognize.
Rett syndrome is an X-linked neurodevelopmental disorder caused in most affected individuals by pathogenic changes in MECP2, which encodes methyl-CpG-binding protein 2. MeCP2 is a chromatin-associated protein that helps regulate gene expression by binding methylated DNA and influencing the activity of numerous other genes. Because the protein is highly abundant in mature neurons, disturbances in its function can alter neuronal maturation, synaptic communication, network stability, and circuit plasticity. The clinical outcome, however, is not determined by the gene variant alone. The type and location of the variant, the amount of residual MeCP2 activity, biological sex, and—among females—the proportion of cells in which the altered X chromosome remains active can all influence disease severity. This biological complexity helps explain why people carrying pathogenic variants in the same gene may present with markedly different neurological patterns.
The Italian research team examined five unrelated individuals identified through a combination of molecular diagnostic methods, including multiplex ligation-dependent probe amplification, chromosomal microarray analysis, and next-generation sequencing. Two patients were girls with large, de novo deletions involving the entire MECP2 region on chromosome Xq28. Both had mild neurodevelopmental impairment and epilepsy, yet neither experienced the developmental regression typically regarded as a defining feature of Rett syndrome. Their cognitive performance was in the borderline range, and brain magnetic resonance imaging showed no structural abnormalities. These cases are clinically important because the absence of regression or classic Rett signs could easily lead clinicians to pursue other diagnostic explanations, even though a substantial deletion affecting MECP2 was the underlying cause.
The lack of regression in these girls adds to growing evidence that developmental decline is not universal among individuals with pathogenic MECP2 variants. In classic Rett syndrome, children often appear to develop relatively normally during the first months or years of life before losing acquired language, purposeful hand use, and social or motor abilities. Yet milder or atypical presentations may involve developmental delay from the outset, persistent but less severe intellectual disability, autism-related traits, seizures, or subtle motor abnormalities without a clearly recognizable regression phase. The authors emphasize that genetic testing should therefore not be restricted to patients who satisfy every established clinical criterion for Rett syndrome. A broader view may be especially valuable when epilepsy and unexplained developmental difficulties occur alongside otherwise nonspecific neurological findings.
The third case involved a 9-year-old boy carrying a maternally inherited MECP2 frameshift variant. Frameshift changes disrupt the normal reading frame of the gene and commonly produce an abnormally shortened protein or trigger cellular mechanisms that destroy the faulty messenger RNA. The boy had intellectual disability, autism spectrum disorder, and focal epilepsy. His clinical course differed from the severe neonatal encephalopathy that can occur in males with complete loss of MeCP2 function. Several female relatives who carried the same familial variant showed milder neuropsychiatric manifestations, illustrating the influence of X-chromosome biology. In females, random or skewed X-chromosome inactivation can create a mosaic pattern in which some cells express the normal copy of MECP2 while others express the altered copy. In males, who generally have only one X chromosome, the consequences can be more direct, although residual protein function and variant-specific effects remain critical.
The final two patients highlighted an even less familiar part of the MECP2-related spectrum. A 44-year-old man with a missense variant—meaning that one amino acid in the MeCP2 protein had been substituted for another—developed an early-onset spastic-ataxic syndrome. His symptoms included increased muscle tone caused by corticospinal tract involvement, impaired balance and coordination, peripheral neuropathy, and cerebellar dysfunction. Rather than presenting primarily with profound developmental disability, he exhibited a progressive movement and motor disorder. By contrast, a 16-year-old girl with a distinct de novo missense variant had preserved cognitive functioning and only mild motor incoordination, accompanied by subtle cerebellar signs. The contrast between these two patients suggests that some missense changes may selectively affect neural circuits involved in coordination and motor control while leaving higher cognitive functions comparatively intact.
The cerebellar findings are consistent with a growing body of experimental and clinical research indicating that MeCP2 has important functions beyond the cerebral cortex. The cerebellum fine-tunes movement, posture, timing, motor learning, and aspects of cognitive processing. Purkinje cells, the principal output neurons of the cerebellar cortex, depend on tightly regulated gene expression and synaptic signaling to coordinate these activities. Disruption of MeCP2 in such cells may interfere with the precise network dynamics required for motor learning and balance. Although the new case series cannot establish a direct molecular mechanism for the patients’ cerebellar signs, it reinforces the possibility that particular MECP2 variants may produce phenotypes dominated by cerebellar dysfunction, tremor, ataxia, or spasticity rather than the severe global impairment traditionally associated with Rett syndrome.
The investigators caution that the findings come from only five patients and should not be interpreted as a definitive map of genotype–phenotype relationships. Even within families, people carrying the same variant can differ substantially, and X-chromosome inactivation does not always predict clinical severity reliably. Nevertheless, the cases have practical implications for genetic diagnosis. The authors propose that clinicians consider MECP2 analysis in selected individuals with unexplained neurodevelopmental disorders, autism, epilepsy, intellectual disability, progressive neurological syndromes, spasticity, tremor, or atypical movement disorders. In females with large deletions, assessing X-chromosome inactivation may provide useful context, although it should not be used as the sole predictor of outcome. In males, testing may be relevant when developmental disability is accompanied by pyramidal signs, parkinsonism, macroorchidism, psychiatric symptoms, or an unusual progressive motor syndrome.
The proposed diagnostic approach begins with careful clinical phenotyping and may include targeted MECP2 sequencing followed by deletion and duplication analysis when no sequence variant is detected. For patients with atypical presentations, chromosomal microarray, multigene panels, exome sequencing, or genome sequencing can provide a broader search for copy-number changes and alternative genetic diagnoses. The researchers stress that a negative result should not end the evaluation: variants may be missed by particular technologies, and genomic data can become more informative as databases and interpretation tools improve. At the same time, isolated cerebellar or movement phenotypes are common consequences of many other genetic conditions, so MECP2 testing should be considered selectively and alongside more frequent causes. The study’s central message is not that every unexplained coordination problem reflects a Rett-related disorder, but that the diagnostic spectrum should remain open when clinical features and molecular evidence point toward MeCP2 dysfunction.
By documenting mild developmental impairment without regression, male neurodevelopmental disease, and predominantly spastic-ataxic or cerebellar presentations, the report challenges a diagnosis-driven approach based solely on recognizable Rett syndrome. It also highlights the value of integrating molecular genetics with long-term neurological observation. A pathogenic MECP2 variant may explain epilepsy in one patient, autism and intellectual disability in another, and progressive motor dysfunction in a third. Recognizing these distinctions could improve genetic counseling, surveillance for seizures and movement complications, and access to appropriate supportive therapies. The authors conclude that MECP2 belongs in the diagnostic conversation for a carefully selected group of atypical neurodevelopmental and movement disorders. Their small series does not redefine Rett syndrome, but it makes clear that the biological reach of MeCP2 extends well beyond its most familiar clinical label.
Subject of Research: Neurological disorders associated with pathogenic MECP2 variants beyond classic Rett syndrome
Article Title: Beyond Rett syndrome: a case series expanding the neurological spectrum associated with pathogenic MECP2 variants
Article References: Meossi C, De Falco A, Rinaldi D, et al. “Beyond Rett syndrome: a case series expanding the neurological spectrum associated with pathogenic MECP2 variants.” Journal of Neurology 273, Article 552 (2026).
Image Credits: AI Generated
DOI: 10.1007/s00415-026-14092-6
Keywords: MECP2, Rett syndrome, atypical neurodevelopmental disorders, epilepsy, autism spectrum disorder, movement disorders, cerebellar syndrome, spastic ataxia, X-chromosome inactivation, genetic diagnosis
