Temporal shift in methyl-CpG binding protein 2 expression in a mouse model of Rett syndrome

Rett syndrome is an X-linked neurodevelopmental disorder caused by mutations in methyl-CpG binding protein 2. Females with identical mutations in the methyl-CpG binding protein 2 gene can display varying severity of symptoms, suggesting that other factors such as X-chromosome inactivation affect phenotypic expression in Rett syndrome. Although X-chromosome inactivation is random and balanced in the blood and brain of the majority of girls with classic Rett syndrome, skewing in the ratio of expression of the mutant methyl-CpG binding protein 2-X to the wildtype-X affects the severity of symptoms. In this study, the pattern of immunostaining for methyl-CpG binding protein 2 was compared with that of neuronal nuclei specific protein, a pan-neuronal marker, to assess X-chromosome inactivation in a Rett syndrome mouse model. The number of cortical neurons and cortical volume were assessed by unbiased stereological measurements in younger adult (7–9 week old) wildtype (wildtype/methyl-CpG binding protein 2+/+), female heterozygous (heterozygous/methyl-CpG binding protein 2+/−), and null (methyl-CpG binding protein 2−/y) male mice and in older adult (24–95 week old) wildtype and heterozygous mice. The results showed that the number of neuronal nuclei specific protein–positive cells and cortical volume did not differ by genotype or age. However, younger adult heterozygous mice had significantly fewer methyl-CpG binding protein 2 cells and the pattern of methyl-CpG binding protein 2 staining was less distinct than in younger adult wildtype mice. However, in older adult heterozygous mice, the number and pattern of methyl-CpG binding protein 2-expressing neurons were similar to the wildtype. The ratio of methyl-CpG binding protein 2 to neuronal nuclei specific protein–stained neurons, a potential measure of X-chromosome inactivation, was close to 50% in the younger adult heterozygous mice, but nearly 70% in the older adult heterozygous mice. These results suggest that X-chromosome inactivation status changes with age. Such a change may underlie the more stable neurological function in older Rett syndrome patients.