Testing for a surprising antigen enables insight into a chronic kidney illness

Eye on Innovation

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In the microscopic world of renal filtration structures known as glomeruli, there exists a patchwork of protein expression that tells a story about kidney health.

Some expressed proteins, or antigens, such as PLA2R, are well-established biomarkers for membranous nephropathy (MN), a condition in which the glomeruli become damaged and allow protein leakage into the urine. Other antigens are less common, but equally important for understanding the cause and pathology of MN.

Sanjeev Sethi, M.D, Ph.D.

“Each antigen has a story to tell,” says Mayo Clinic renal pathologist Sanjeev Sethi, M.D., Ph.D., who’s fostered a decades-long hobby of glomeruli dissection and examination via mass spectrometry to detect antigens expressed in different disease states. “That’s why I think it’s extremely important for us to find out what the underlying antigen is.”

Dr. Sethi uncovered the story of NELL-1, or neural epidermal growth factor-like 1 protein, in 2019 as part of his quest to identify additional antigens for MN, which affects approximately 300,000 individuals in the U.S. At the time, nearly 40% of patients lacked disease etiology.

“We dissected glomeruli that were PLA2R-negative, and then did the proteomics, and the first protein that stood out in many of these patients, to my surprise, was something called NELL-1,” Dr. Sethi says. “I didn’t have a clue about it, but it was very highly expressed in many of these PLA2R-negative patients, so I said, ‘Okay, if this is true, let’s start screening.’”

Sanjeev Sethi, M.D., Ph.D

A common biomarker with serious implications

Referring back to Mayo Clinic Laboratories’ archive of previously tested MN patients who were negative for PLA2R or THSD7A (another known MN antigen), Dr. Sethi and his team identified and screened more than 200 cases. Among the specimens tested, 10% to 15% were positive for NELL-1.

“NELL-1 became this common antigen that we were detecting on mass spectrometry of these cases, and the obvious thing, because I cannot do mass spectrometry on every (MN) case, was to develop an immunohistochemistry test,” Dr. Sethi says.

To that end, Dr. Sethi and his colleagues in the Renal Pathology Laboratory developed a test called Neural Epidermal Growth Factor-Like 1 Protein Immunostain (Test ID: NELL1 - coming soon), which is conducted on paraffin-embedded tissue samples collected through biopsy.

“The way we’ve worked this into our system is when we get a case of MN, the first thing we do is stain for PLA2R,” Dr. Sethi says. “Once it is negative, then the next step is to stain for NELL-1.”

Dr. Sethi’s research revealed another facet of NELL-1’s story: among the cohort of patients with NELL-1associated MN, 10% to 30% had an underlying malignancy.

“So if a physician gets a diagnosis of Nell-1 membranous nephropathy, they need to be extra vigilant in screening for hidden malignancies,” Dr. Sethi says.

For NELL-1 associated patients who present with malignancy, diagnostic confirmation could also offer insight into the relationship between treatment and disease expression, Dr. Sethi says.

“NELL-1 is relatively new and studies need to be done to prove this, but I can go ahead and make a small prediction that maybe patients who have NELL-1 and an underlying malignancy — and if the treatment of the malignancy is successful — treating the malignancy would also result in cure of MN as well, because the hypothesis would be that the malignancy itself is driving overproduction of NELL-1,” Dr. Sethi says.

Enhanced outcomes through biomarker evaluation

Prior to discovering NELL-1, PLA2R, THSD7A, and several other recently identified MN antigens, including semaphorin-3B (Sema3B) and exostosin (EXT), the cause of MN was unknown, Dr. Sethi says. “We just knew there was an antibody attacking some antigen in the glomerulus.”

Identification of the antigens in MN triggering the antibody buildup responsible for the glomeruli breakdown provided an alternative metric on how to diagnose and monitor patients.

“Now you had a marker and you could either identify the antigen-PLA2R in kidney biopsy, or you could follow the anti-PLA2R antibody titer in the serum,” Dr. Sethi says. “If the patient got treatment for PLA2R-MN, and the titer of anti-PLA2R antibodies started going down, you knew you were on the right path. If the anti-PLA2R antibodies disappeared, the patient was cured.”

The same follows for NELL-1, Dr. Sethi explains. Detection of NELL-1 biomarkers allows physicians to use precision laboratory test results, as opposed to less specific markers like proteinuria, to follow patients along their treatment journeys.  

No matter the causative antigen for MN, treatment often entails long-term, high-dose immunosuppression that can cause serious side effects, including heightened risk for infection. For that reason, accurate diagnosis is imperative to optimize treatment outcomes.

“To be able to make an etiology-based diagnosis and be able to follow these patients — versus giving them treatments for a long, long time — is critical,” Dr. Sethi says. “This is a long, drawn-out disease. There are relapses and remissions. So as a result, there’s a lot of patients with MN. The discovery on MN with specific antigens such as NELL1, EXT, Sema3B, and PCDH7, all discovered at the Mayo Clinic, likely represent distinct diseases that are in their infancy. As time goes on and we start making diagnoses based on IHC of NELL-1-, EXT-, Sema3B-, and PCHD7-associated MN, then we’ll know a lot more about the story in detail.”

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Robin Huiras-Carlson

Robin Huiras-Carlson is a senior marketing specialist at Mayo Clinic Laboratories and a Mayo Clinic employee since 2015. Her writing focuses on specialty testing, innovation, and patient-focused initiatives.