Carolina Valderrama-Hincapie

Methyl-CpG binding protein 2 (MeCP2) is a chromatin-associated regulator essential for neuronal gene regulation, and pathogenic mutations in MeCP2 cause Rett syndrome (RTT). However, the mechanisms governing MeCP2 engagement on chromatin and the effects of RTT mutations on nucleosome interactions remain poorly understood. We determined cryo-EM structures of MeCP2 bound to mono-nucleosomes with or without linker DNA methylation. In the absence of linker DNA methylation, the methyl-CpG binding domain (MBD) engages nucleosomal DNA near superhelical location +/- 7, whereas a methylated linker CpG redirects MBD to the linker methylation site. Quantitative EMSA, MNase footprinting, and fluorescence polarization show that both MBD and AT-hook regions cooperate to stabilize nucleosome binding, and that six common RTT variants (R133C, T158M, R306C, R168X, R255X, R270X) fall into four mechanistic classes. Loss-of-function truncations progressively weaken binding, whereas missense variants show near-WT DNA affinity. Among the variants, R133C shows near wild-type affinity for naked DNA but loses methylation-directed nucleosome engagement. Our analysis of human neuronal transcriptomic and chromatin occupancy datasets showed that RTT variants R133C and R168X lose CpG-island specificity and redistribute in the genome through distinct biochemical routes. Together, this work establishes a framework linking variant-specific defects in nucleosome binding to chromatin-targeting failure and transcriptional dysregulation in RTT.

The role of antibodies in protection against food allergy is debated. Here, we designed a longitudinal birth cohort study recruiting participants from an Old Order Mennonite (OOM) community, a traditional farm lifestyle considered to be at low risk for allergic diseases, and participants from urban and suburban Rochester, NY, USA, that were at higher risk for developing allergic diseases. We collected cord and peripheral blood, stool, saliva, and maternal milk samples longitudinally during pregnancy and during the first year of the infant’s life to evaluate B cell and antibody responses. Farm exposure led to an accrual of memory and immunoglobulin G–positive (IgG ⁺ ) B cells, higher titers of plasma IgG and IgA and salivary and fecal IgA in infants, and higher IgA titers in maternal milk. Moreover, OOM infants and their mothers had higher titers of plasma IgG ₄ and IgA antibodies to egg ovalbumin in maternal milk, respectively, a feature that we found to be associated with lower incidence of egg allergy. Infant systemic IgG ₄ and IgA were associated with early introduction or more frequent ingestion of lightly cooked egg, respectively, but there was also an effect independent of egg exposure. Several food antigens were detected in cord blood, with in utero exposure potentially explaining the unexpected presence of antigen-specific IgA at birth. These analyses suggest earlier maturation of B cell immunity in farming lifestyle communities.