Neuroscience. 2006 Mar 18; [Epub ahead of print]
Temporal shift in methyl-CpG binding protein 2 expression in a mouse model of Rett syndrome
Metcalf BM, Mullaney BC, Johnston MV, Blue ME.
Abstract
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.
Lay Summary
X-linked inactivation is a process by which one of the two copies of the X chromosome present in females is "inactivated", or "turned-off". The majority of Rett girls have X-chromosome inactivation is random and balanced. However, in some cases, skewing in expression away from the mutant MeCP2-containing X-chromosome and towards the non-mutant-MeCP2-containing X-chromosome correlates with less severity in symptoms. In this study, the investigators looked for MeCP2 expression patterns in the brains of MeCP2-deficient mice of different ages, and compared this to the counts of total neurons. The key finding here is the discovery of a developmental shift in MeCP2-expression not previously observed. That is, the proportion of neurons expressing MeCP2 increased significantly from younger to older adult ages. Briefly, female MeCP2-deficient mice used in this investigation are similar to Rett girls in that they both possess one copy each of non-mutant-MeCP2 and mutant-MeCP2. Here, the investigators found that in the young MeCP2-deficient mice, the X-linked inactivation was balanced. Strikingly, they discovered that in the older MeCP2-deficient mice, a skewing towards the non-mutant-MeCP2 had occurred. How this occurs, remains a mystery. However, the authors suggest that this newly discovered phenomenon may, in part, explain the more neurologically stable period seen in many Rett syndrome patients as they age.