Title: Apoptosis Inhibitor of Macrophages in Cats: A Potential Link Between an Exon 3 Variant Allele and Progression of Naturally Occurring Chronic Kidney Disease (J Vet Intern Med. 2025 Jul-Aug;39(4):e70136. doi: 10.1111/jvim.70136.)

(https://academic.oup.com/jvim/article/39/4/jvim70136/8446823)

Three-sentence Summary

1. Some cats carry a tandem duplication of exon 3 in the fAIM gene, producing a structurally distinct 4-domain AIM protein.
2. Cats homozygous for this variant showed significantly higher odds of CKD progression, though the retrospective design and small sample size warrant further validation.
3. Greycoat Research, in collaboration with Pitter Petter, is exploring AIM exon 3 variant-based genetic testing approaches as part of a broader personalized feline CKD management initiative.

Introduction

This post continues from the previous piece covering the pre-clinical pilot study by Prof. Miyazaki Toru's team on AIM in feline CKD. If you read that post, you already have the foundational context — what AIM is and why it fails to activate sufficiently at the moments it is needed in cats — so those basics won't be repeated here.

If you find this written version a bit dense, a video explanation covering the same content in a more accessible format will be uploaded. That might be a better starting point before coming back to this.

Background Concepts: Exons and Introns

To understand this paper, two terms need to be introduced: exon and intron. While the terminology sounds technical, the underlying concepts are straightforward.

Cells build proteins by reading instructions encoded in DNA. However, the DNA sequence contains two distinct types of content:

• Exon: The coding sequences directly used to produce the final protein.
• Intron: Non-coding sequences that are spliced out during mRNA processing and are therefore absent from the final protein sequence.

A baking recipe is a useful analogy:

• Exon = "200g bread flour, 2 eggs, 100g butter, 80g sugar" — the specific, actionable content needed to make the cake.
• Intron = "This recipe was passed down from my grandmother and pairs especially well with winter evenings" — personal notes with no bearing on the finished product

The cell strips out the introns and joins the exons into a processed mRNA, which is then translated into the final protein. The structure of that protein is therefore determined by which exons are present, how many copies there are, and in what order they appear.

The fAIM gene normally contains six exons encoding a three-domain AIM protein. In some cats, however, exon 3 is present as a tandem duplicate — analogous to "2 eggs" appearing twice in the recipe — producing a structurally distinct four-domain AIM protein with an extra SRCR1 domain.

Background

As covered in the previous post, feline CKD is more than simple age-related renal decline. AIM — apoptosis inhibitor of macrophages — is central to the tubular debris clearance system following renal injury. The structural problem in cats is that feline AIM binds with unusually high affinity to its carrier, the pentameric IgM antibody complex, and consequently fails to dissociate efficiently from IgM when needed — leaving tubular debris uncleared and repair chronically compromised.

Prof. Miyazaki Toru's team addressed this directly by supplementing AIM exogenously, demonstrating suppression of worsening renal biomarkers and a suggestion of prolonged survival in stage 3 CKD cats.

But a question remains. Even among cats at the same CKD stage, progression rates vary considerably. Some remain stable for years; others deteriorate rapidly. Environmental and dietary factors alone cannot fully account for this.

This paper asks whether some cats carry a structurally variant form of the fAIM gene itself, and whether that genomic difference correlates with faster CKD progression. In short: beyond the IgM-binding problem shared by all cats, might certain individuals face an additional layer of disadvantage written into their DNA?

Study Design and Methods

The study was conducted in four stages.

1. Genomic confirmation

PacBio long-read sequencing of DNA from cats carrying 2, 3, or 4 exon 3 copies confirmed that the previously described 4-domain fAIM protein arises from an intragenic tandem duplication of exon 3 at the genomic level.

2. In silico protein modeling

The tertiary structures and physicochemical parameters of the 3-SRCR and 4-SRCR domain fAIM variants were modeled using deep learning algorithms (RaptorX server).

3. Phenotype-genotype association

Medical records from 172 CKD cats at WSU Veterinary Teaching Hospital were screened; 50 met the inclusion criteria. Exon 3 copy number was determined by ddPCR, and the association with renal function change was evaluated using Fisher exact test — a statistical method well-suited to detecting group differences in small samples.

4. Population frequency

100 randomly selected DNA samples from the WSU PrlMe DNA Bank were genotyped by ddPCR to estimate variant allele frequency in the general feline population.

Key Results

1. Genomic architecture

PacBio sequencing confirmed the 4-domain fAIM protein arises from a large intragenic tandem duplication of exon 3. Three genotypes were identified: homozygous wild-type (2 copies), heterozygous variant (3 copies), and homozygous variant (4 copies).

2. Protein physiochemical properties

In silico modeling predicted that the 4-SRCR domain fAIM variant is larger, more negatively charged, and has altered solubility compared to the 3-SRCR wild-type. Direct experimental verification is still needed, but if the simulation holds, these properties could impair glomerular filtration efficiency and reduce accessibility to the tubular lumen, where AIM-mediated debris clearance occurs..

3. Variant frequency

Of 100 randomly selected DNA samples, 38% were homozygous wild-type, 42% heterozygous, and 20% homozygous variant. At least one variant allele was present in 62% of cats screened.

4. Phenotype-genotype association

Cats homozygous for the exon 3 variant showed significantly higher odds of:

• IRIS stage worsening: OR = 17 (Odds Ratio — the relative likelihood of a given outcome occurring; 95% CI: 1.55–196), p = 0.01
• Serum creatinine ≥20% increase: OR = 9.6 (95% CI: 1.38–67), p = 0.03

Heterozygous cats did not reach statistical significance under either approach, though a subset showed SCr worsening, suggesting partial compensation from wild-type fAIM expression.

Interpretation and Limitations

The key clinical implication is that individual variability in CKD progression may be partially explained by fAIM genotype. The variant is common across breeds (62%), meaning it is not a rare or breed-specific finding but a broadly prevalent genetic variant potentially associated with CKD progression risk. If confirmed in larger studies, fAIM exon 3 genotyping could contribute to CKD risk stratification and personalized monitoring strategies.

Regarding limitations: this is a retrospective study — meaning it analyzed existing medical records rather than being prospectively designed — which makes it difficult to fully control for confounding variables. The relatively small sample size (n=50) also means that while the OR values are striking, the wide confidence intervals (e.g., 1.55–196 for OR=17) reflect genuine uncertainty that larger studies would need to resolve.

Reliance on serum creatinine as the sole phenotyping criterion is also worth noting in interpretation. As a retrospective study, it was the only consistently available marker, but creatinine alone does not fully represent renal function. Inclusion of SDMA and other biomarkers in future prospective studies would substantially improve phenotypic resolution.

Finally, the protein analysis remains computational. The prediction that the 4-SRCR variant is larger and more negatively charged — and therefore less efficiently filtered into the tubular lumen — is a modeling output, not a directly measured result. Studies comparing fAIM protein isolated from genotyped cats will be needed to verify these predictions.

A large, prospectively designed multicenter study incorporating direct protein functional analysis and a broader biomarker panel would substantially strengthen the evidence base for this hypothesis.

Summary

This study provides the first genomic confirmation that the 4-domain fAIM protein arises from an exon 3 tandem duplication, and demonstrates a statistically significant association between the homozygous variant genotype and accelerated CKD progression in a retrospective cohort. The variant is prevalent across the feline population and is not breed-specific. While causality remains to be established, these findings represent a meaningful early step toward genotype-informed management of feline CKD.

Personal Thoughts

The shift toward genetic risk stratification — well-established in human oncology — is increasingly relevant to veterinary medicine. Feline CKD management may similarly benefit from moving beyond outcome-based markers toward an approach that incorporates genetic context. It should be noted, however, that the current evidence positions fAIM exon 3 genotyping closer to an exploratory biomarker than a validated clinical tool; sufficient prospective validation must precede any clinical application.

Greycoat Research is currently exploring these possibilities and has recently initiated early-stage discussions with Pitter Petter, a companion animal genetic analysis company, regarding the development of AIM-related genetic testing services. While this work remains firmly in the research and exploratory phase, the long-term aim is to enable more precise identification of which cats are at higher risk of rapid CKD progression and which may respond preferentially to specific management strategies.

Ultimately, this points toward a future in which personalized feline CKD management — informed by both clinical biomarkers and genetic profiling — becomes a meaningful part of standard care.

About the author Hocheol Shin, Ph.D. B.S./M.S./Ph.D. in Biological Sciences, KAIST (Korea Advanced Institute of Science and Technology) Research background in cancer immunology, tumor microenvironment, and translational oncology. Currently engaged in companion animal health and longevity research.