Expression of Dlx and Lhx family homeobox genes in fetal thymus and thymocytes.

Homeobox genes comprise a nearly ubiquitous and highly conserved superfamily of developmental regulatory genes that encode transcription factors involved in the determination of axis and tissue identity. While homeobox gene expression has been well characterized in a variety of embryonic tissues, their expression has not been extensively studied in lymphoid progenitor cells or in sites of lymphogenesis. To examine homeobox gene expression in the developing thymus, we screened an embryonic day 13.5 thymus cDNA library by polymerase chain reaction (PCR) using degenerate oligonucleotides within the highly conserved homeodomain region of eight homeobox gene families. The resulting PCR products were then cloned and sequenced. Transcripts for multiple Dlx family members and Lhx2 were repeatedly detected in this screen. Screening of embryonic day 16.5 and adult murine thymus and Thy1+ thymocytes was performed for selected members of these homeobox gene families. Transcripts encoding Lhx2, Lhx3, and Lhx9, as well as Dlx1 and Dlx2 were detected in both thymus and purified thymocytes. Dlx1 is a member of the distal-less homeobox gene family that has been shown to regulate embryonic craniofacial development. Significantly, Dlx1 is expressed in the third branchial arch, which contributes to the thymus. Although Dlx1 knockout mice did not display any obvious developmental defects in thymus or thymocyte development, the expression of these homeobox genes in neural crest derivatives suggests a possible role in cell migration and development that may overlap with other homeobox genes.