Down syndrome and a presenilin 2 variant: dual genetic risk of Alzheimer's disease.

Early onset familial Alzheimer's disease (EOFAD) is rare compared to sporadic AD, but dominant variants in genes involved in amyloid β (Aβ) processing are well-described. One such variant is in the presenilin 2 (PSEN2) gene, N141I, and was first described in a family with Volga German descent. Separately, individuals with Down syndrome (DS) are also at risk for early onset AD, having an extra copy of an amyloid precursor protein gene. While either can drive EOFAD alone, it is extremely rare for both to occur within one individual. Here we describe a unique case of a 48-year-old individual, with both DS and the PSEN2 N141I variant. We investigated whether having two high-risk AD variants results in worsened or distinct pathology compared to single variant carriers. Neuropathologic evaluation, quantitative pathology, and spatial proteomic profiling (NanoString Geomx Digital Spatial Profiling) were performed on post-mortem tissue of the index case compared to individuals with DS and N141I variants alone. Analysis in the index case revealed increased total Aβ burden in multiple brain regions compared to the average levels observed in PSEN2 carriers and DS cases, but not for hyperphosphorylated tau or neuroinflammatory markers. Index case appeared to have more pronounced Aβ pathological burden than the PSEN2 subgroup and was more similar to the DS subgroup in several measurements: the Aβ burden, density of Aβ plaques, fibrillar and dense-core plaques, and the density of ionized calcium binding adaptor molecule 1 (Iba1) labelling in MSTG. As such, it appears that compounded genetic risk for AD was additive for Aβ burden, but not tau or neuroinflammation. This rare case offers new insight into how compounded risk may additively enhance amyloid pathology independently from tau. This underscores the importance of investigating synergistic and additive risk in neurodegenerative disease.