Segregation of a totally skewed pattern of X chromosome inactivation in four familial cases of Rett syndrome without MECP2 mutation: implications for the disease

BACKGROUND

Rett syndrome is a neurodevelopmental disorder affecting only girls; 99.5% of Rett syndrome cases are sporadic, although several familial cases have been reported. Mutations in the MECP2 gene were identified in approximately 70-80% of sporadic Rett syndrome cases.

METHODS

We have screened the MECP2 gene coding region for mutations in five familial cases of Rett syndrome and studied the patterns of X chromosome inactivation (XCI) in each girl.

RESULTS

We found a mutation in MECP2 in only one family. In the four families without mutation in MECP2, we found that (1) all mothers exhibit a totally skewed pattern of XCI; (2) six out of eight affected girls also have a totally skewed pattern of XCI; and (3) it is the paternally inherited X chromosome which is active in the patients with a skewed pattern of XCI. Given that the skewing of XCI is inherited in our families, we genotyped the whole X chromosome using 32 polymorphic markers and we show that a locus potentially responsible for the skewed XCI in these families could be located on the short arm of the X chromosome.

CONCLUSION

These data led us to propose a model for familial Rett syndrome transmission in which two traits are inherited, an X linked locus abnormally escaping X chromosome inactivation and the presence of a skewed XCI in carrier women.

Two affected boys in a Rett syndrome family: clinical and molecular findings

BACKGROUND

The authors report a family in which two boys had severe neonatal encephalopathy of unknown origin. They both presented with the same condition and died of severe apnea before they were 1 year old. Their sister has a classic form of Rett syndrome.

METHODS

Because mutations in the methyl-CpG-binding protein 2 (MECP2) gene have been identified in 70 to 80% of the sporadic cases of Rett syndrome, the authors looked for a mutation in the MECP2 gene in this family.

RESULTS

The authors identified a missense mutation (T158M) in the affected girl and subsequently showed that one of her affected brothers, for whom DNA was available, carried the same mutation. The mother of the patients is a carrier of the T158M mutation. X-chromosome inactivation studies showed that the mother has a completely skewed X-chromosome inactivation pattern that favors the expression of the normal allele; this explains why she does not exhibit any phenotypic manifestation. In addition, the MECP2 mutation appeared on the grandpaternal X chromosome in this family.

CONCLUSIONS

An MECP2 mutation can be identified in boys, even though they do not present a Rett syndrome phenotype.