Sex chromosome contributions to sex differences in multiple sclerosis susceptibility and progression

Abstract

Background: Why are women more susceptible to multiple sclerosis, but men have worse disability progression? Sex differences in disease may be due to sex hormones, sex chromosomes, or both.

Objective: Determine whether differences in sex chromosomes can contribute to sex differences in multiple sclerosis using experimental autoimmune encephalomyelitis.

Methods: Sex chromosome transgenic mice, which permit the study of sex chromosomes not confounded by differences in sex hormones, were used to examine an effect of sex chromosomes on autoimmunity and neurodegeneration, focusing on X chromosome genes.

Results: T-lymphocyte DNA methylation studies of the X chromosome gene Foxp3 suggested that maternal versus paternal imprinting of X chromosome genes may underlie sex differences in autoimmunity. Bone marrow chimeras with the same immune system but different sex chromosomes in the central nervous system suggested that differential expression of the X chromosome gene Toll-like receptor 7 in neurons may contribute to sex differences in neurodegeneration.

Conclusion: Mapping the transcriptome and methylome in T lymphocytes and neurons in females versus males could reveal mechanisms underlying sex differences in autoimmunity and neurodegeneration.

Keywords: Multiple sclerosis; animal model; genetics; immunology; neurodegeneration; sex differences.

Figure 1. The maternal X imprint, compared to the paternal X imprint, has less Foxp3 methylation

DNA methylation was quantified in a CG-rich Foxp3 upstream enhancer region using bisulfite sequencing, as described in autoantigen specific T lymphocytes from mice with a maternal X chromosome (X_mY^*x) or a paternal X chromosome (X_pY^*x). The position of each CG site is given on the x-axis using the mouse genome version mm10.

Figure 2. Less methylation of the maternal X chromosome is protective for autoimmunity in XY mice since T lymphocytes of XY mice express more FOXP3 due to having the maternal X allele exclusively, as compared to XX who have the maternal X allele expression in only half of their T lymphocytes

Maternal X (X_m): lower Foxp3 DNA methylation, less silencing, higher FOXP3 protein expression, less autoimmunity in X_mY males. Paternal X (X_p): higher Foxp3 DNA methylation, more silencing, less FOXP3 protein expression, more autoimmunity, in X_mX_p females. Blue crossing over one allele in females indicates random X-inactivation.

Figure 3. Mice with XY- CNS, compared with XX CNS, have more Tlr7 expressing cortical neurons in EAE

Cerebral motor cortical layers I-VI stained with Tlr7 antibody, n = 5 mice per group. Double labeling of Tlr7 co-localizing with neuronal (NeuN) markers, previously quantified. DAPI = nuclear stain. Bone marrow chimeras: XX→XY- indicates XX donor cells into XY- recipient, XX→XX indicates XX donor cells into XX recipient. Shown are representative 10x capture of coronal sections.

Figure 4. Less methylation of the maternal X chromosome is deleterious for neurodegeneration in XY mice since neurons of XY mice express more TLR7 due to having the maternal X allele exclusively, as compared to XX who have the maternal X allele expression in only half of their neurons

Tlr7 on Xm, lower methylation, less silencing, higher TLR7 expression, worse neurodegeneration in X_mY males. Tlr7 on Xp, higher methylation, more silencing, less TLR7 expression, less neurodegeneration in X_mX_p females. Blue crossing over one allele in females indicates random X-inactivation.