Project Summary/Abstract
The thymus is the primary immune organ responsible for the generation of T cells, and its functional decline with
age (involution) is considered a significant contributor to aging-associated immunosenescence. There is strong
evidence that aging-related thymic involution is initiated during youth by mechanisms that operate in thymic
epithelial cells (TECs) to result in their depletion and loss of compartmental organization. Despite its central role
in the formation and maintenance of T cell adaptive immunity and the pleiotropic negative impacts of thymus
involution during aging, surprisingly few molecular regulators of thymus organ function, homeostasis, and
involution have been identified. A key transcription factor, FOXN1, is known to be a primary regulator of both
thymus development and postnatal maintenance. However, the mechanisms of its regulation and the pathways
through which it affects TEC development, function, and aging-related involution remain largely unknown. TECs
are a small subset of total thymus cells and are notoriously difficult to isolate experimentally. Thus, identifying
regulators and effectors by gene expression-based or biochemical analysis is difficult, and in any case would be
specifically targeted to a TEC subset or specific age. Given the gradual nature of thymic involution, designing
such experiments to identify key regulators of thymus function and maintenance with aging would be doubly
challenging. Forward genetic screens are powerful tools for identifying genes based on their function, thus
allowing unbiased discovery of novel pathways and mechanisms, but are not generally practical for aging-related
phenotypes. In a recent R21-funded project, my lab performed a dominant modifier screen using ENU
mutagenesis to identify novel mutations that, when heterozygous, either enhance (increase the severity) or
suppress (rescue/reduce the severity) the Foxn1Z/Z thymic involution phenotype. We identified 18 dominant
modifier lines, including both enhancers and suppressors. Because they impact Foxn1 expression and/or
function in the Foxn1Z/Z model of premature thymic involution, our hypothesis is that these modifiers represent
previously unrecognized genes that normally act to promote or restrain aging-related thymic involution. In the
current proposal, we will follow up on our successful initial project to identify the modifiers found in our pilot
screen, and test whether they affect the trajectory of involution during normal aging. This novel approach to
investigating the molecular basis of thymus involution represents a potentially transformative approach to
identifying new genes, pathways, and mechanisms that control thymus function and aging-related involution.