The SIX2 gene, or Sine oculis homeobox homolog 2, encodes a protein that functions as a transcription factor, controlling which other genes are turned on or off. As a homeobox protein, SIX2 plays a foundational role in developmental processes across various organisms. Its influence extends to organs like the kidney, skull, and stomach.
The Role of six2 in Kidney Formation
Kidney formation, or nephrogenesis, relies on the precise regulation of the SIX2 gene. This process involves the continuous generation of nephrons, the filtering units of the organ. It begins with the metanephric mesenchyme, where SIX2 is expressed. Here, SIX2 maintains a population of undifferentiated nephron progenitor cells.
These progenitor cells self-renew, ensuring a continuous supply for nephron development. SIX2 counteracts signals from the ureteric bud, which would otherwise induce mesenchymal cells to differentiate prematurely. It prevents the early expression of differentiation-related genes like WNT4 and collaborates with pathways like WNT9B to promote progenitor cell proliferation.
Disruptions to SIX2 function during development have significant consequences. Studies in mice show that inactivating SIX2 leads to premature differentiation of mesenchymal cells into epithelial structures. This rapidly depletes the nephron progenitor cell population. As a result, developing kidneys are significantly smaller, a condition known as severe renal hypoplasia, because new nephrons cannot form properly.
SIX2-expressing cells give rise to all cell types of the main body of the nephron throughout nephrogenesis. Its sustained activity ensures the development of a full complement of nephrons, necessary for a functional organ. While SIX2 expression is strong early in development, it becomes much weaker in the peripheral layer of normal kidneys as development progresses, such as from postnatal day 1 to postnatal day 4.
six2’s Importance in Adult Kidney Function
While SIX2 is known for its influence during embryonic kidney development, its role in the adult kidney differs. After birth, particularly after about 36 weeks of gestation, the population of SIX2-positive nephron progenitor cells largely diminishes. Consequently, the adult kidney generally loses the capacity to form new nephrons, limiting its regenerative potential after injury.
Despite this, recent research indicates SIX2 can re-express in adult kidney tissues under specific conditions. Studies administering neonatal kidney stem/progenitor cells (nKSPC) to human donor kidneys during machine perfusion showed SIX2 activation. This re-expression was observed in proximal tubular cells, demonstrating the adult kidney’s capacity to reactivate pathways typically associated with embryonic development.
The re-emergence of SIX2 expression in adult kidneys is significant, as it was previously thought not to occur in response to injury. This activation was accompanied by the upregulation of regeneration markers like SOX9 and VEGF, and a reduction in kidney injury biomarkers. This suggests that while SIX2 is not routinely active in adult kidney homeostasis, its re-activation could be a strategy for kidney repair and regeneration in damaged organs.
How six2 Contributes to Kidney Disorders
Dysregulation of SIX2 activity can lead to non-cancerous kidney diseases, particularly congenital anomalies of the kidney and urinary tract (CAKUT). These conditions involve structural abnormalities from embryonic development, often resulting in underdeveloped or malformed kidneys. Mutations in the SIX2 gene are directly linked to renal hypodysplasia (RHD), where kidney size is reduced or tissue maldevelops.
When SIX2 function is disrupted, the self-renewal and maintenance of nephron progenitor cells are compromised, leading to their premature exhaustion. This results in fewer nephrons forming, contributing to a smaller, less functional kidney. A lack of SIX2 function can cause mesenchymal cells to differentiate too early and in incorrect locations, forming ectopic renal vesicles. This rapidly depletes the progenitor pool, halting proper nephron development and resulting in severe renal hypoplasia. Such altered SIX2 function during kidney formation directly manifests as these congenital disorders, impacting kidney size and architecture.
The Connection Between six2 and Cancer
Beyond its roles in kidney development and non-cancerous disorders, SIX2 is involved in various cancers. Wilms’ tumor, a common pediatric kidney cancer, is a prominent example. This tumor often retains gene expression profiles similar to embryonic kidney nephron progenitor cells, where SIX2 is normally active.
In Wilms’ tumor, SIX2 is highly expressed and contributes to the uncontrolled proliferation of nephron progenitors, which fail to form functional nephrons. SIX2-positive cells within tumors exhibit characteristics of cancer stem cells, possessing tumorigenic abilities. Overexpression of SIX2 in Wilms’ tumor cell lines enhances cell survival and can alter signaling pathways, such as WNT/β-catenin, to favor a stem cell-like state over differentiation.
SIX2 also influences adult kidney cancers, notably renal cell carcinoma (RCC). Studies show SIX2 expression is increased in RCC tissues and cells compared to normal kidney tissue. Elevated SIX2 levels in RCC correlate with poorer survival for patients. It promotes a “stemness” phenotype in RCC cells, contributing to their self-renewal capacity and aggressive behavior.
SIX2 directly binds to and promotes the activity of SOX2, another gene associated with cancer stem cell progression in RCC. Knocking down SIX2 in RCC cells reduces their stem-like properties, including spheroid formation and expression of stemness markers. This suggests SIX2 acts as an oncogene in RCC, potentially enhancing cancer metastasis and offering a target for future treatments.