Genetic testing finds few aHUS-linked variants in 167 suspected cases
Identified variants are "often of unknown significance," scientists note
Genetic screening of a large group of people suspected of having atypical hemolytic uremic syndrome (aHUS) detected very few gene variants likely to be associated with the condition, a study reported.
Despite signs of aHUS, around two-thirds of those tested had no evidence of any known disease-causing variants.
These results highlight the ongoing challenges in classifying variants after genetic testing, and the need for clear guidance with such testing in a clinic, the researchers noted. “Due to the rarity of aHUS and limited numbers of case reports and functional studies, identified variants are often of unknown significance, with uncertain biologic or clinical relevance,” they wrote.
Their study, “Genotypic analysis of a large cohort of patients with suspected atypical hemolytic uremic syndrome,” was published in the Journal of Molecular Medicine.
167 people with thrombotic microangiopathy tested for possible aHUS
Hemolytic uremic syndrome (HUS) is marked by thrombotic microangiopathy (TMA), or damage to small blood vessels due to blood clots that lead to anemia (low red blood cell count), low platelets, and kidney failure. While Shiga toxin-producing E. coli infection causes most HUS cases, a small proportion of patients present with familial aHUS.
aHUS is caused by the abnormal activation of the complement cascade, a group of immunological proteins that normally helps to fight infection. Such activation results in inflammation and clotting in small blood vessels, particularly those in the kidneys, eventually leading to kidney failure.
Variants in genes that encode proteins regulating the complement cascade or blood clotting have been linked with aHUS susceptibility. Because screening for these variants can help to confirm aHUS and inform treatment options, scientists at the London Health Sciences Centre, in Canada, established a next-generation sequencing (NGS) panel to test patients with suspected aHUS. NGS allows for the rapid, large-scale screening of multiple genes to detect disease-related variants.
“To our knowledge, this is the first time molecular genetic testing was implemented in the clinical setting as a first-tier assessment, aimed at streamlining aHUS diagnosis,” the scientists wrote.
A total of 167 individuals (89 women), most with suspected aHUS due to signs of TMA, underwent panel screening.
Genetic tests found at least one reportable variant in 28 (17%) people. This was defined as a pathogenic (disease-causing) or likely pathogenic variant or a variant of uncertain significance, meaning it was unclear whether the variant was actually associated with aHUS.
Atypical hemolytic uremic syndrome diagnosed in 3 people based on variants
In three patients (1.8%), a diagnosis was based on carrying at least one pathogenic or likely variant, “a rate consistent with the expected prevalence of genetic aHUS in all HUS cases,” the researchers wrote. Four variants were found in these patients, including in the CD46 (MCP), CFH, CFI, and DGKE genes.
Overall, 31.7% had variants with an uncertain significance, including 25 individuals (15%) with these variants in an aHUS-associated gene; there were 28 candidate variants, or those without a well-established association. No clinically significant or candidate variants were detected in 111 individuals (66.5%).
The team noted that “a lack of literature, clinical reports, and/or functional studies makes it difficult to interpret most variants.”
To illustrate an aHUS screening example, researchers described the case of a 27-year-old man with acute kidney injury, anemia, and a low platelet count. Due to a rapid decline in kidney function, he underwent renal replacement therapy, given to filter the blood when the kidneys cannot. Plasma exchanges improved his blood test results but not kidney function, so he was started on Soliris (eculizumab).
After 56 months (nearly 4.7 years) of treatment, his kidney function improved such that renal replacement therapy was discontinued. No obvious triggers for his disease were found.
Genetic tests detected two variants in his CD46 and CFI genes, both of which harbor instructions for proteins that regulate the complement cascade.
The CD46 gene variant, reported in two case reports, was not at all in databases. The team classified it as a likely pathogenic variant. The variant in the CFI gene, also reported in two aHUS cases, occasionally was found in a population database. It, too, was classified as likely pathogenic.
Further investigation found that the man’s mother, who showed no signs of aHUS, carried the CFI variant but not the CD46 variant. A DNA sample from his father was not available.
According to the scientists, these findings suggested that the CFI variant may not contribute to disease in this family, or it may have an additive effect with the CD46 gene. Likewise, the asymptomatic mother demonstrated incomplete penetrance, meaning she carried a disease-causing variant but did not develop the disease.
Researchers noted that incomplete penetrance is common across most complement-mediated genes and remains an “important parameter to consider in all individuals undergoing genetic testing.”
“In this cohort of patients with suspected aHUS, using clinical pipelines for genetic testing and variant classification, pathogenic/likely pathogenic variants occurred in a very small percentage of patients,” the researchers concluded.
“Our results highlight the ongoing challenges in variant classification following NGS panel testing in patients with suspected aHUS, alongside the need for clear testing guidance in the clinical setting,” they added.
Alexion, the company marketing Soliris, partly funded this study.