Hidden rare mutations may help explain mystery cases of aHUS
Study IDs gene variations in overlooked parts of body's complement system
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People with atypical hemolytic uremic syndrome (aHUS) without a clear genetic cause may have rare mutations in overlooked parts of complement system genes, a new study from Europe suggests.
The researchers found that one such mutation in the CFI gene — a known aHUS risk gene — greatly reduced the levels of a protein called factor I, which may lead to complement system overactivation. The complement system is part of the immune response.
While this study initially focused on a single mutation, the scientists noted that “growing evidence supports the [potentially disease-causing] relevance of [different] variants … in several inherited diseases,” starting with aHUS.
“Our findings underscore the importance of analyzing [gene sequences] in complement-mediated diseases with unexplained genetic predisposition,” the team wrote.
The study, “Functional Characterization of a Novel Intronic Complement Factor I Variant in Factor I Deficiency and aHUS,” was published online in the journal Kidney International Reports.
Overactivation of the immune system in aHUS leads to thrombotic microangiopathy (TMA), characterized by the formation of small blood clots in tiny blood vessels. This causes red blood cell destruction, acute kidney injury, and low levels of platelets, the small cell fragments that help blood to clot.
Gene variations in CFI gene known to increase risk of aHUS
Certain genetic mutations increase the likelihood of having aHUS. One such example involves the gene CFI, which provides instructions for making complement factor I. Factor I helps break down complement proteins C3b and C4b, preventing excessive complement activation that could damage the body’s own cells.
However, about 40% of aHUS cases remain without a known genetic cause. This suggests that some mutations may occur in regions of the genome — the complete set of genetic instructions, or DNA, found in a person or other living thing — that do not code for proteins. That includes introns, which are DNA segments that do not directly encode proteins but can still affect how genetic instructions are processed.
In this work, researchers in Hungary and Austria focused on a rare intronic CFI mutation known as c.328+42G>A. It had been found in a young patient with DEFICIT syndrome, a severe condition linked with factor I deficiency and brain inflammation.
The scientists aimed to identify additional carriers of the c.328+42G>A mutation and to test whether it altered RNA splicing, a process in which cells edit immature RNA molecules by removing introns and assembling protein-coding segments to create instructions for making a protein.
To do this, they screened a genetic registry of people with suspected complement-related kidney diseases or factor I deficiency who had undergone CFI sequencing between 2010 and 2024. The registry included 674 people with suspected aHUS. It also involved 292 individuals with C3 glomerulopathy, and 10 with factor I deficiency.
The researchers identified three people carrying the c.328+42G>A mutation. All three were of Austrian descent and had been referred after a clinical diagnosis of aHUS.
Rare mutation found at much higher rate than expected
When the researchers focused on 410 people with confirmed aHUS, the mutation was found at a much higher rate than would be expected based on a large European population database. This suggested the mutation may be enriched among people with aHUS.
All carriers of the c.328+42G>A mutation had consistently reduced levels of factor I, regardless of whether their disease was active at the time of testing. In the original patient with complete factor I deficiency, no factor I protein was detected, while the other carriers had lower levels than healthy controls.
To understand why the mutation reduced factor I levels, the researchers used computer-based tools. These showed that the mutation created an abnormal splice site — essentially, a faulty signal that caused the cell to process the CFI gene message incorrectly.
Instead of producing a normal CFI message, the mutant gene contained 43 extra building blocks from an intron that would normally be spliced out. This created an early stop signal for the cellular machinery that produces proteins, which would be expected to prevent normal factor I from being made.
The faulty RNA message also appeared to be unstable. In laboratory experiments, its levels rose after the researchers used a compound that interferes with a cellular quality-control process that normally destroys defective RNA messages.
Together, the findings indicate that the c.328+42G>A mutation disrupts CFI splicing and causes a near-complete loss of factor I production from the affected gene cop, according to the researchers.
The results may be especially relevant for people with aHUS whose standard genetic testing has not identified a cause, the team noted. Two of the c.328+42G>A carriers in the study had previously been classified as having aHUS of unknown cause, or genetically unexplained complement-mediated TMA, per the researchers.
According to the researchers, these results highlight the importance of analyzing intronic regions in complement-mediated diseases among people with a genetic predisposition for which no explanation exists.
The team noted as a study limitation the small number of patients involved. As such, the findings will need “validation in larger, independent patient cohorts for all CFI-associated conditions,” the researchers wrote.
