In addition to profound neurological disability, MECP2 duplication syndrome is characterized by recurrent pneumonias and other inflammatory complications. We have combined studies of children with MECP2 duplication and mice with a homologous alteration to determine whether overexpression of the MECP2 gene causes immune dysfunction. Our findings confirm that when overexpressed in human and mouse TH cells, MECP2 selectively impairs IFN-γ secretion and TH1 responses by acting as a transcriptional repressor and regulator of ifng locus accessibility. These findings indicate that children with duplication or triplication of the MECP2 gene have immune impairments related to their inability to mount robust TH1 responses, and suggest that the high frequency of pulmonary and other inflammatory complications affecting these children may be due to opportunistic pathogens.
A limitation of these studies is that although reduced TH
1 responses were observed equally in humans and mice with MECP2
duplication, not all human immune abnormalities were reflected in the mouse. The markedly abnormal cell counts from peripheral blood of affected children that were not seen in the transgenic mice most likely represent differences in the environments encountered by humans and inbred research mice: Whereas humans constantly encounter pathogens from birth, specific pathogen–free (SPF) mice do not. We speculate that the relative lack of memory CD4+
T cells, balanced by reciprocal increases in naïve T cells, in children with MECP2
duplication is potentially explained by their impaired TH
1 generation in response to diverse pathogens encountered in infancy and childhood. Chronic, unresolved infections due to lack of TH
1 responses in these children could also explain the marked immaturity and increased abundance of neutrophils, eosinophils, and monocytes (). Relative lack of TH
1 help could also explain the variable defects in antibody responses of some affected children (fig. S1
). Thus, although confirmatory studies are needed, a single defect involving deficient IFN-γ responses involving only TH
cells potentially explains the diverse immunologic anomalies demonstrated in children as well as the discrepancies in observations between humans and mice with MECP2
Additionally, the mechanism by which MeCP2 controls gene expression in neurons and especially immune cells such as T cells remains incompletely understood. MeCP2 binds promiscuously and relatively homogeneously to chromosomal DNA, including methylated and nonmethylated regions, and may either transcriptionally suppress or enhance expression of neuronal genes (27
). Little evidence exists to suggest that a major role of MeCP2 is to target individual genes to regulate their expression. Rather, MeCP2 appears to globally influence gene expression subtly and indirectly in a manner analogous to histone proteins by regulating chromosomal architecture (28
In contrast, our findings suggest that, when overexpressed, MeCP2 interacts specifically with T-bet to inhibit ifng
transcription. The Sin3A-HDAC (histone deacetylase) complex is displaced by T-bet at the ifng
locus, which, although possibly only a correlative observation, may be important for the induction of ifng
transcription and proper TH
1 differentiation (29
). Sin3A-HDAC is recruited by MeCP2 to target gene loci to suppress gene transcription, strongly supporting the importance of the Sin3A-HDAC/MeCP2 complex in the regulation of IFN-γ production (30
). Although speculative, it is possible that increased binding of MeCP2 to T-bet in the setting of MECP2
duplication may be sufficient to preclude the full displacement of Sin3A-HDAC at the IFNG
promoter, leading to DNA hypoacetylation and condensation on histone proteins.
Several human immunodeficiency syndromes are marked by deficiency in IFN-γ activity and involve genetic mutations in the IFN-γ receptor, signal transducer and activator of transcription 1 (STAT1), and the IL-12 receptor β1 (IL-12Rβ1) chain. Patients with these mutations exhibit profound immunodeficiency marked by susceptibility to lethal invasive infections due to intracellular pathogens such as bacteria and viruses (31
). Pneumonia, often due to mycobacteria, is a consistent clinical finding in these patients, but infections due to viruses and other bacteria have also been described (31
). These findings in patients with unrelated genetic anomalies that underlie parallel immune irregularities further suggest that the pneumonias seen in children with MECP2
duplication may be infectious in nature and caused by immune dysfunction, specifically impaired IFN-γ, and not solely by neurological impairments.
Notably, however, IFN-γ secretion and TH
1 responses are not entirely ablated in the setting of MeCP2 overexpression (), perhaps reflecting subtle differences in the intranuclear levels of MeCP2 in immune cells in the setting of gene duplication. Defective IFN-γ responses were further limited to TH
cells, although we cannot exclude the possibility that MeCP2 duplication confers an IFN-γ defect in NK cells. Together, these issues may explain the variable immune phenotypes observed in our cohort with MECP2
duplication and the possibly attenuated infectious disease course of children with MECP2
duplication syndrome as compared to the more severe global IFN-γ deficiencies (such as IL-12Rβ1 deficiency) in which disseminated infections are often seen (35
). IFN-γ further serves as a growth and differentiation factor for NK cells (36
). This suggests that the mild deficiency in NK cells observed in our study subjects with MeCP2 overexpression could be due to the relative lack of the trophic effects of IFN-γ, although the reduction in NK cells could also be related to the depleting effects of recurrent infections.
In summary, we have shown that in addition to neurological phenotypes, duplication of the MECP2 gene causes immune dysfunction due in part to the suppression of IFN-γ production from TH cells. Affected children may be susceptible to infections caused by a wide variety of intracellular pathogens that are capable of causing pneumonia and other infections and possibly contributing to CNS disease. Identifying specific infectious agents that contribute to pneumonia and possibly neurological decline in children with MECP2 duplication will help to clarify the extent and severity of immune dysfunction.