Original articleX chromosome inactivation patterns in brain in Rett syndrome: implications for the disease phenotype
Introduction
Rett syndrome (RTT) is an X linked dominant neurodevelopmental disorder that predominantly affects females. A period of apparently normal development in the first 6–18 months of life is followed by loss of previously acquired skills (such as speech and purposeful hand movements) to varying degrees, and is accompanied by acquired microcephaly and stereotypic hand movements. The phenotypic spectrum is very broad, ranging from the milder ‘speech preserved’ cases to severe intellectual disability [1].
In up to 80% of clinically diagnosed RTT cases a mutation in the X linked methyl-CpG binding protein 2 (MECP2) gene is identified [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. MECP2 is expressed widely and is particularly abundant in the central nervous system. Not surprisingly then, the clinical presentation of RTT is one of the neurological dysfunction. The RTT brain shows regional pathology, with areas such as the frontal, motor and inferior temporal cortices affected to a greater extent than the occipital (visual) cortex, which appears to escape neuropathological changes [12], [13], [14].
Skewed X chromosome inactivation (XCI) has been implicated in easing the severity of specific X linked mental retardation disorders due to preferential inactivation of the X chromosome that harbours the mutant allele [15]. Consequently, if the normal allele is primarily expressed this may lead to a lessening of the severity of the disease phenotype. Alternatively, skewed XCI resulting in the X chromosome harbouring the mutant allele being predominantly active, could lead to a more severe disease phenotype.
To date, most researchers have examined XCI patterns in peripheral blood DNA with variable results [10], [11], [16], [17]. There are at least two lines of evidence supporting the hypothesis that skewed X-inactivation might modulate the clinical severity of RTT. Ishii and colleagues reported identical twins with the R294X mutation who had very disparate clinical features, with the more mildly affected twin having skewed XCI, whilst the more severely affected twin had random XCI [18]. Secondly, there have been a number of reports of obligate heterozygotes with only mild or no clinical features of RTT and who had skewed (presumably protective) XCI, whilst their affected daughters had balanced XCI [3], [5], [19], [20], [21].
There have been a small number of studies examining XCI patterns in brain, but they have looked at most one or two brain regions [22], [23], [24], [25]. In this study we have characterised the XCI pattern in various neuroanatomical regions of nine RTT brains and non-neural tissue in two patients. This was undertaken in order to examine whether or not variable XCI patterns occur in different brain regions or somatic tissues of the same patient.
Section snippets
Tissue samples
Post mortem neural and somatic tissue was obtained from various sources including The Harvard Brain Tissue Resource Center, Boston, USA, the New South Wales Tissue Resource Centre, NSW, Australia, Baylor College of Medicine, Texas, USA, Telethon Institute for Child Health Research, Perth, Western Australia. Relevant information about the samples is summarised in Table 1. All specimens were retained and used for research with appropriate consent from the families. They were stored at −70 °C from
Mutation analysis
Pathogenic mutations were present in the coding region of MECP2 in seven out of nine of the RTT patient samples examined (Table 1). This included two common missense mutations in the methyl-binding domain (MBD) in three patients (c.316C>T and c.473C>T), two common nonsense mutations in the nuclear localisation signal that lies within the transcription repression domain (TRD-NLS) in three patients (c.763C>T and c.808C>T) and one rare frameshift mutation in the TRD in one patient (c.750insC).
General observations
XCI
Discussion
Rett syndrome is an X linked disorder with a markedly broad range of clinical severity. The contribution of skewing of XCI in RTT patients has therefore been of considerable interest to researchers as a possible explanation for this variable expressivity. A number of studies have characterised the XCI pattern of RTT patients in peripheral blood samples [10], [11], [16], [17]. However, as RTT manifests itself primarily as a disorder of the central nervous system, it would be of interest to more
Acknowledgements
We are grateful to the Harvard Brain Tissue Resource Center, Boston, USA; the New South Wales Tissue Resource Centre, Sydney; NSW, Australia; Professor Dawna Armstrong, Baylor College of Medicine, Houston, USA; Dr Anthony Tannenberg, Mater Misericordiae Hospitals, Brisbane, Australia; Dr Ian Andrews, Sydney Children's Hospital, Sydney, NSW, Australia and Dr Helen Leonard, Telethon Institute for Child Health Research, Perth, Australia for provision of the brain samples used in these studies.
References (30)
- et al.
Rett syndrome and beyond: recurrent spontaneous and familial MECP2 mutations at CpG hotspots
Am J Hum Genet
(1999) - et al.
Skewed X-chromosome inactivation is a common feature of X-linked mental retardation disorders
Am J Hum Genet
(2002) - et al.
Patterns of X chromosome inactivation in the Rett syndrome
Brain Dev
(1990) - et al.
Rett syndrome—clinical studies and pathophysiological consideration
Brain Dev
(1984) - et al.
X-chromosome inactivation patterns are unbalanced and affect the phenotypic outcome in a mouse model of Rett syndrome
Am J Hum Genet
(2004) - et al.
Rett syndrome: clinical characteristics and recent genetic advances
Disabil Rehabil
(2001) - et al.
Rett Syndrome is caused by mutations in X-linked MECP2, encoding methyl-CpG-binding protein 2
Nat Genet
(1999) - et al.
Rett syndrome: methyl-CpG-binding protein 2 mutations and phenotype–genotype correlations
Am J Med Genet
(2000) - et al.
MECP2 mutations account for most cases of typical forms of Rett syndrome
Hum Mol Genet
(2000) - et al.
Rett syndrome: a surprising result of mutation in MECP2
Hum Mol Genet
(2000)