SPI1

SPI1 Full Name

spleen focus forming virus (SFFV) proviral integration oncogene

SPI1 Introduction

Introduction
The SPI1 gene, also widely known as PU.1, encodes a master transcription factor that orchestrates the development of the entire hematopoietic system. As a member of the ETS-domain transcription factor family, SPI1 binds to a purine-rich DNA sequence called the PU-box, found near the promoters of target genes, to regulate their expression. SPI1 stands at the apex of a regulatory hierarchy, controlling cell fate decisions that determine whether hematopoietic stem cells commit to myeloid or lymphoid lineages, and its function is indispensable for the generation of B cells, macrophages, and other critical immune cells. Disruptions in SPI1 activity, whether through mutation or dysregulated expression, lead to severe immunodeficiency or contribute to the development of leukemia, underscoring its central role in both normal physiology and disease.

Figure 1. Strcuture of SPI1.

Expression Pattern and Tissue Distribution

SPI1 exhibits a highly restricted expression pattern that is essentially limited to the hematopoietic system. The gene is broadly expressed in bone marrow (RPKM 36.5), appendix (RPKM 27.0), spleen, lymph node, and other immune tissues. Within the bone marrow, SPI1 is concentrated in hematopoietic stem cells, lymphoid progenitors, myeloid lineage cells (including granulocyte-macrophage progenitors, classical dendritic cells, and monocytes), and B-cell clusters. Among B cells, expression is particularly high in pre-B1 cells, highlighting its importance during early B lymphopoiesis. Transcriptomic profiling of in vitro hematopoietic differentiation has revealed dynamic SPI1 expression during development. In human pluripotent stem cell (hPSC) differentiation systems, SPI1 expression progressively increases as cells transition from endothelial cells to hematopoietic progenitor cells (HPCs), with HPC clusters showing prominent SPN, SPI1, and GFI1B expression. This pattern mirrors in vivo embryonic development, where SPI1 expression rises during the endothelial-to-hematopoietic transition (EHT) in the aorta-gonad-mesonephros (AGM) region. Single-cell analyses confirm that SPI1 is one of the key hematopoietic-related transcription factors that exhibit an overall upward trend during EHT, alongside RUNX1 and GATA2.

Molecular Function as a Master Hematopoietic Regulator
SPI1 functions as a pioneer transcription factor that controls hematopoietic cell fate by decompacting stem cell heterochromatin, enabling other transcription factors to enter otherwise inaccessible genomic sites. Once open chromatin is established, SPI1 directly activates gene expression by binding to PU-box sequences in target gene promoters and enhancers, and can also recruit additional transcription factors, such as interferon regulatory factors (IRFs), to further amplify transcriptional responses.The protein's activity is tightly regulated through interactions with multiple partners. SPI1 cooperates with IRF4 and IRF8 to activate genes important for myeloid and lymphoid development. It interacts with RUNX1/AML1, GATA1, GATA2, CEBP family members, and numerous other transcription factors, forming complex regulatory networks that specify lineage identity. The transcriptional activity of SPI1 at macrophage-specific genes is inhibited by interaction with GFI1, which blocks SPI1-induced macrophage differentiation while permitting granulocyte development, illustrating how protein-protein interactions fine-tune lineage outcomes.

Alternate Names for SPI1

SPI1; spleen focus forming virus (SFFV) proviral integration oncogene; OF; PU.1; SFPI1; SPI-1; SPI-A; transcription factor PU.1; SPI-1 proto-oncogene; 31 kDa transforming protein