22q11.2 Deletion Syndrome (DiGeorge) and Mutations in Forkhead Box N1 (FOXN1) cause a Thymic Hypoplasia through distinct Developmental Processes

Abstract

Patients with 22q11.2 deletion syndrome and those with mutations in the Forkhead Box N1(FOXN1) transcription factor (Nude/SCID) can both present with a thymic hypoplasia that results in a severe T cell lymphopenia. In both clinical conditions, the thymic anlage fails to develop properly within the 3rdpharyngeal pouch during embryogenesis. We characterized the development of the thymus in mouse models of 22q11.2 deletion syndrome (22q11.2del) and a new set of mice with mutations in Foxn1that genocopied a SCID patient with novel compound heterozygous mutations in FOXN1. Both sets of mice develop hypoplastic thymic lobes. An analysis of thymopoiesis in embryos revealed distinct development problems. The hypoplastic thymii from the 22q11.2del mice were primarily sized restricted, with normal percentages of all thymocyte subsets apparent. This contrasted a severe deficiency of thymocytes due to the Foxn1mutations, which primarily affected the development and expansion of thymic epithelial cells. Comparative gene expression analyses of e13.5 fetal thymii revealed differentially regulated transcripts that define the basis of the hypoplasia. A dysregulated mesenchymal cell signature was apparent in the 22q11.2del model, which contrasted the epithelial transcript disruption due to the Foxn1mutations. These results suggest different strategies are necessary to correct the thymic tissue abnormalities in patients who present with thymic hypoplasias due to their congenital disorders.