Unexpected gene therapy finding may help treat two rare kidney diseases
Mouse study links liver gene delivery to lower complement activity
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Researchers have identified a potential gene therapy approach for atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy (C3G), two kidney diseases driven by overactive complement activity, according to a new mouse study.
In mouse models of both diseases, delivering the gene for the complement protein factor D to liver cells shut down the overactive alternative complement pathway, a part of the immune system that contributes to both conditions. This improved survival and brought several disease-related blood markers closer to normal, with no major safety concerns reported in the animals.
“Expression of mature [factor D] in the liver is an effective way to inhibit … [alternative] complement activity with potential therapeutic applications in diseases such as aHUS and [C3 glomerulopathy],” the researchers wrote.
The findings were reported in the study, “Treatment of atypical hemolytic uremic syndrome and C3 glomerulopathy in mice by hepatic expression of factor D,” published in Blood Advances.
How complement overactivity drives kidney damage
aHUS and C3 glomerulopathy are rare diseases caused by the uncontrolled activation of the alternative complement pathway, part of the immune system that normally helps defend the body against infections. Both conditions can lead to kidney damage and, in severe cases, kidney failure.
The alternative pathway involves several proteins, including C3, factor B, and factor D. The pathway becomes active when factor D activates factor B, helping drive a chain reaction that amplifies complement activity.
Unlike most complement proteins in blood — including C3 and factor B, which are mainly produced in the liver — factor D is primarily made in fat tissue. Factor D is first produced in an inactive form called pro-factor D, which must be activated by an enzyme called MASP3.
Researchers at the University of Pennsylvania found that expressing mature factor D — rather than inactive pro-factor D — in liver cells, instead of fat tissue where it is normally produced, shut down alternative complement activity in mouse models.
Unexpected discovery while studying factor D biology
The team initially set out to create a “humanized” factor D mouse, meaning mice engineered to produce human factor D instead of the mouse version. They first confirmed that giving mature human factor D to mice lacking the protein restored alternative complement activity, showing the human and mouse proteins worked similarly.
To create these humanized mice, the researchers delivered the gene for mature human factor D into factor D-deficient mice. They used an AAV8 vector, which is commonly used for gene delivery to the liver. As expected, higher doses led to higher levels of mature human factor D in the bloodstream.
However, human factor D produced in mouse liver cells did not restore alternative complement activity, unlike direct factor D supplementation. Further analysis showed that factor B levels in the bloodstream were greatly reduced, helping explain why complement activity was lost.
To investigate further, the team tested an AAV8-based gene therapy carrying the gene for mature mouse factor D. Like the human version, this approach also shut down alternative complement activity in mice.
Even though treatment produced only a fraction of normal factor D levels, it still caused factor B depletion. When researchers instead delivered the gene for inactive pro-factor D, only mature mouse factor D was detected in the blood, suggesting it had been activated by MASP3.
Lab cell experiments reveal why factor B levels drop
Experiments using cultured mouse liver cells showed that when C3, factor B, and factor D were produced together, factor B became activated inside cells and its levels dropped. This helps explain why factor B levels were low in mice treated with the AAV8 gene therapy.
The gene therapy did not appear to affect other complement pathways, such as the classical or lectin pathways, which are not linked to aHUS or C3 glomerulopathy. The researchers also did not observe major treatment-related abnormalities in liver or blood tests in treated healthy mice.
The researchers then tested the gene therapy in a mouse model of aHUS. Treatment depleted factor B in the bloodstream and prevented disease development in this model. Treated mice survived, showed significant benefits in kidney disease markers, and had normalized platelet and hemoglobin levels.
In a mouse model of C3 glomerulopathy, treatment also prevented disease features “with remarkable efficacy,” normalizing blood C3 levels and extending survival, with no signs of kidney disease in treated animals.
“A gene therapy approach to reduce [factor B] expression as illustrated here, either alone or in combination with other biological or chemical therapies, may have the potential to reduce treatment burden and offer more convenience to the patients,” the researchers wrote.
