Overview
Autosomal dominant Alzheimer’s disease (ADAD) is a rare inherited form of Alzheimer’s disease caused by pathogenic variants in one of three genes: PSEN1, PSEN2, or APP. Unlike the more common sporadic form of Alzheimer’s disease, ADAD often begins earlier in life and can appear to run strongly through families. Even among people who carry the same disease-causing mutation, symptoms, age at onset, and speed of progression can differ substantially. That variability suggests that other genetic factors, called modifiers, may influence how the disease develops.
This genome-wide association study set out to identify new genetic modifiers of ADAD and to explore how those variants might affect biological pathways linked to Alzheimer’s disease. The researchers also examined whether the same variants influenced age at onset in sporadic Alzheimer’s disease, helping to determine whether the findings may extend beyond inherited disease.
Why genetic modifiers matter
In inherited diseases, the main causal mutation does not always explain the full range of outcomes. Some people develop symptoms earlier, some later, and some have different patterns of memory loss, language impairment, behavioral change, or biomarker abnormalities. Genetic modifiers may help explain these differences.
Identifying modifiers in ADAD is important for several reasons. First, it improves understanding of disease biology. Second, it may help predict when symptoms will begin or how quickly they may progress. Third, these modifiers can point to biological pathways that might be targeted by future treatments or preventive strategies. In families affected by ADAD, such information may also improve genetic counseling and help guide clinical trial design.
Study design and participants
The investigators analyzed data from three major research resources: the Knight Alzheimer Disease Research Center (Knight-ADRC), the Dominantly Inherited Alzheimer Network observational study (DIAN), and the Alzheimer Disease Sequencing Project (ADSP) R4. Together, these cohorts provided a strong foundation for studying both rare inherited variants and broader genetic patterns.
The main genetic analysis used whole-genome sequencing data from 101 unrelated, non-Hispanic White participants with symptomatic ADAD and 5,050 asymptomatic, unrelated control participants. Sensitivity analyses also included related participants, expanding the sample to 148 cases and 5,813 controls. The team then assessed whether identified variants had downstream effects on gene regulation, blood protein levels, cerebrospinal fluid (CSF) Alzheimer’s biomarkers, and brain imaging measures.
The study also looked at whether the same variants were associated with age at onset in people with ADAD and in 6,177 participants with sporadic Alzheimer’s disease from ADSP R5.
Main genetic findings
The analysis identified three genome-wide loci that were significantly associated with ADAD risk, regardless of which ADAD gene mutation caused the disease. These loci were near or within CNIH4, CCNG1, and RHOJ.
The CNIH4 signal was driven by a missense variant, meaning the genetic change altered the amino acid sequence of the protein. Specifically, the variant involved Gly54Ser, and it showed a very strong association with disease risk. The odds ratio was 11.99, indicating a large increase in risk among carriers compared with non-carriers. In practical terms, this means the variant may have an important biological effect, although it likely remains rare.
At the CCNG1 locus, the risk allele also strongly increased the odds of Alzheimer’s disease, with an odds ratio of 9.56. In addition, this allele was associated with an earlier age at dementia onset by about 10 years on average. That finding is notable because it suggests not only increased susceptibility but also faster clinical emergence of disease.
The RHOJ risk allele was also significantly associated with higher risk of Alzheimer’s disease, with an odds ratio of 5.96. Beyond disease risk, it was linked to biomarker changes in the CSF that are consistent with more active Alzheimer’s pathology.
Biological interpretation of the variants
To better understand what these loci might be doing, the researchers studied several biological readouts.
For CCNG1, the risk allele was positively associated with higher plasma levels of TDP-43, a protein increasingly recognized as relevant to neurodegeneration. TDP-43 abnormalities are common in several brain diseases and may contribute to neuronal injury. The same allele was also associated with a larger gap between chronological age and MRI-predicted brain age, suggesting accelerated structural brain aging.
For RHOJ, the risk allele was associated with higher CSF total tau and phosphorylated tau 181, both markers of neuronal injury and tau pathology. It was also associated with a lower Aβ42/Aβ40 ratio, which typically reflects abnormal amyloid processing and is a hallmark of Alzheimer’s disease. These biomarker shifts support the idea that the variant may promote the classic amyloid-tau cascade seen in AD.
The study also considered cis-regulatory effects, which refer to how a variant may influence the expression of nearby genes. Such regulatory changes can affect cellular behavior without changing the protein sequence directly. Although the detailed molecular mechanisms will need further study, the findings point to gene regulation as a likely contributor to disease risk.
What the results suggest about Alzheimer’s biology
Taken together, the findings support the idea that multiple biological systems contribute to ADAD risk and symptom timing. The study points particularly to amyloid beta (Aβ), tau, TDP-43, astrocytes, and angiogenesis.
Aβ and tau are the core pathological proteins in Alzheimer’s disease. Aβ is involved in plaque formation, while tau is linked to tangles and neuronal dysfunction. The association with TDP-43 suggests that Alzheimer’s may overlap with other neurodegenerative proteinopathies more than previously appreciated. The mention of astrocytes points to glial cell involvement, which is increasingly recognized as important in brain inflammation, metabolism, and synaptic support. Angiogenesis, the formation of new blood vessels, may also play a role in maintaining or disrupting brain health in dementia.
This broader view matters because Alzheimer’s disease is not caused by a single pathway. Instead, it likely reflects interacting processes that affect neurons, glia, vascular function, and protein clearance. Genetic modifiers can highlight which of these processes are especially important in specific forms of the disease.
Clinical implications
Although these findings are not yet ready for direct clinical use, they have several important implications.
For families affected by ADAD, knowledge of modifiers may eventually help explain why one person develops symptoms decades earlier than another despite carrying a similar causal mutation. This could improve genetic counseling and family planning discussions. It may also help researchers better stratify participants in clinical trials, ensuring that differences in disease progression do not obscure treatment effects.
From a drug development standpoint, the genes and pathways identified here may represent new targets. If future studies confirm that these loci influence Alzheimer’s biology in a causal way, therapies designed to modulate their activity could be explored. The association with biomarker changes also suggests possible ways to monitor treatment response using blood, CSF, or imaging measures.
It is important to emphasize that this study identifies associations, not proof of causation. Additional functional work in cell and animal models, as well as replication in larger and more diverse cohorts, will be needed before these modifiers can be translated into clinical practice.
Relevance to sporadic Alzheimer’s disease
The investigators also tested whether the risk variants affected age at onset in sporadic Alzheimer’s disease. This step is important because it can show whether the same biology applies across both inherited and non-inherited forms of the illness.
By examining 6,177 participants with sporadic Alzheimer’s disease, the study broadened the possible relevance of the findings. If certain modifiers influence both ADAD and sporadic disease, they may point to shared mechanisms that are broadly relevant across Alzheimer’s subtypes. That could make them especially valuable therapeutic targets.
Strengths and limitations
One major strength of this work is the use of whole-genome sequencing and deep phenotyping across multiple research cohorts. The investigators did not stop at finding associations; they also linked genetic variants to biomarkers and imaging findings, which adds biological depth and strengthens the interpretation.
The study also has limitations. The sample of symptomatic ADAD cases was relatively small, which is understandable given the rarity of the condition but still limits statistical power. The analyses were restricted to non-Hispanic White participants in the main discovery set, so the findings may not generalize to all populations. Some biomarker analyses included very small numbers of participants, especially in the DIAN subset, so those results should be interpreted cautiously until replicated.
Despite these limitations, the study provides compelling evidence that inherited Alzheimer’s disease risk is shaped by more than the primary pathogenic mutation alone.
Conclusion
This genome-wide association study identified three loci associated with autosomal dominant Alzheimer’s disease risk: CNIH4, CCNG1, and RHOJ. The findings suggest that these variants may influence disease through effects on protein biology, gene regulation, tau and amyloid biomarkers, TDP-43, brain aging, astrocytes, and vascular processes.
By uncovering potential genetic modifiers, the study advances understanding of why ADAD can vary so widely even within the same family. It also offers promising clues for future genetic counseling, biomarker development, and clinical trial design. Most importantly, it opens new directions for research into the biology of Alzheimer’s disease and the pathways that may be targeted to slow or prevent its progression.
