In the whole SENIOR cohort, DYRK1A levels were quite disperse. However when stratifying with PiB-PET SUVR, the group with low Aβ had significantly higher DYRK1A than the group with high Aβ, indicating that individuals with low DYRK1A are potentially at-risk for dementia-associated pathologies such as brain Aβ deposition. DYRK1A levels were not correlated with age in the SENIOR cohort, suggesting that the increase of DYRK1A occurred before the age of 60. We observed no correlation with BMI, which was previously reported as a risk factor for dementia 32.
These results are reminiscent of our findings with the INSIGHT cohort of memory complainers without clinical signs of cognitive decline, for which we also observed an association between low DYRK1A levels and high Aβ deposits in the brain 9. Individuals with low Aβ load also had higher DYRK1A levels than young individuals, indicating a potential increase of plasma DYRK1A during aging as Individuals with high Aβ load had DYRK1A levels similar to young individuals. This paralleled our previous observation of increased Dyrk1A levels in the brain of aging mice (4, 12, and 17 months), and in Dyrk1A transgenic mice even though these mice already overexpress Dyrk1A 10. Increase of DYRK1A is present in middle age mice (12 months old). In human, this increase could be observed at 40–50 years, younger than the age of the SENIOR cohort. In addition, lower DYRK1A levels were present in AD and in other dementia-associated pathologies, including frontotemporal lobar degeneration, hippocampal sclerosis, or progressive supranuclear palsy/corticobasal degeneration. We thus hypothesize that age-dependant increase of DYRK1A occurs with normal aging and leads to a lower risk of brain Aβ pathology as observed in the SENIOR cohort. Individuals from the SHATAU7-IMATAU cohort had DYRK1A levels significantly lower than the elderly control group with low PiB-PET SUVR and similar to our young control group. This difference was not observed when comparing the PSP-CBD group with the control group with high PIB-PET-SUVR, suggesting that low DYRK1A level is not a strong risk factor for PSP-CBD.
The decrease of DYRK1A could be due to truncation or degradation of full-length DYRK1A. In 2015, Jin et al. 33 hypothesized that calpain truncates and activates DYRK1A. The authors’ analyses of brain samples revealed that bands characteristic of DYRK1A at 95 kDa appeared at a lower molecular weight in AD patients than in controls, suggesting that DYRK1A truncation occurs in AD. Yet the bands expected to be degradation or truncation products, were not increased in AD patients, indicating that the antibody the authors used might cross-hybridise with other molecular species. Nevertheless, our DYRK1A immunoassay is detecting full-length as well as truncated.
We then investigated DYRK1A in a group of individuals with DS. These individuals develop neuropathological features of AD at > 40 years old, and half of DS patients present cognitive impairment indicative of dementia by 55 years of age 22, 23.
From studies on individuals without dementia and without known genetic abnormalities (INSIGHT and SENIOR cohorts), we hypothesized that diploid control individuals with low Aβ deposition would have increasing DYRK1A levels with age (40–60 years old) (TableII) and might be considered at lower risk of developing dementia (Fig. 7). This critical period might correspond to 20–40 years of age for individuals with DS. Our results with cultured lymphoblastoids showed lower DYRK1A levels in the DSAD group compared to DS group. Lymphoblastoids are established cell lines, indicating that genetic or epigenetic regulation of DYRK1A levels was present before transformation of the cells. Further, plasma from a longitudinal cohort of DS and DSAD individuals showed significant differences between groups, although both DS and DSAD had strongly increased DYRK1A levels compared to controls of a similar age (46 years) (Table II). The DS group had a 2.99-fold increase of DYRK1A (Table II), which is higher than the expected value of 1.5 for trisomy. This elevated ratio may be associated with accelerated aging in DS, thus combining the effect of trisomy (ie x1.5) with the effect of aging (ie x2).
Psychometric data from a cohort of 445 individuals with DS indicate that a single point assessment of acquired mild cognitive impairment, which is expected for the majority of adults with DS, reveals two peaks for age-related prevalence of impairment, suggesting that the risk for AD onset conferred by DS is moderated by other factors than trisomy 34, which could include the APOE ε4 allele or overexpression of regulatory factors encoded by HSA21 genes. Increased DYRK1A could have contradictory effects related to AD: an anti-inflammatory and protective effect 35–37 associated with low homocysteine levels in the periphery as well as a deleterious hyperphosphorylation of Tau protein in the brain 15, 38. Hyperhomocysteinemia is associated with low DYRK1A levels 39 and with a change in inflammatory status 40, 41. Increased plasma DYRK1A levels with aging may exert an anti-inflammatory effect at the beginning of the neuropathological process, thus delaying early signs of neurodegeneration and dementia. Conversely, low plasma DYRK1A levels may be associated with vulnerability to AD and AD-related pathologies. Accordingly, controlling DYRK1A levels during aging may facilitate preventive intervention. However, we cannot exclude the possibility that lack of increase of DYRK1A during aging could be due to modifications of regulatory processes in other pathways involved in neurodegeneration. In that case, DYRK1A levels would be a marker of these alterations, and acting upon pathways regulating DYRK1A and other factors would be a therapeutic target.
Aging is associated with increased risk of dementia in individuals with DS, with a mean age of diagnosis of 55 years 22, 23. However, a fraction of individuals with DS will not develop dementia or develop it later (> 55–60 years old). Our work suggests that the critical point for dementia is not DYRK1A levels alone but a ratio between the levels of DYRK1A and another HSA21 gene that we call X. High DYRK1A/X ratio may delay onset of AD-type dementia in DS. The increased risk for dementia in individuals with DS is associated with trisomy of the APP gene 42. In addition, APP locus duplications cause autosomal dominant early-onset AD 43. APP could thus be a good candidate for X. A low DYRK1A/APP ratio could confer high risk of AD. This hypothesis would be also applicable to diploid elderly individuals with high DYRK1A levels and low AD risk, as these individuals have a high DYRK1A/X ratio, while diploid elderly individuals with low DYRK1A and AD dementia have a low DYRK1A/X ratio.
Our results show that low plasma DYRK1A may indicate at-risk individuals who may benefit from early treatment to prevent AD. Further experiments with genetically engineered mouse models of AD with 1 or 3 copies of Dyrk1A may help unravel the effects of DYRK1A. Additional longitudinal human cohorts are also needed to confirm these findings and determine the timeline of DYRK1A variation compared to Aβ changes.