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Background The disintegrin metalloproteinase 10 (ADAM10) is the main alpha-secretase acting in the non-amyloidogenic processing of APP. Some ADAM10 gene variants have been associated with higher susceptibility to develop late-onset AD, though clear clinical-genetic correlates remain elusive. Methods Clinical-genetic and biomarker study of a first family with early- and late-onset AD associated with a nonsense ADAM10 mutation (p.Tyr167*). CSF analysis included AD core biomarkers, as well as Western blot of ADAM10 species and sAPP alpha and sAPP beta peptides. We evaluate variant's pathogenicity, pattern of segregation, and further screened for the p.Tyr167* mutation in 197 familial AD cases from the same cohort, 200 controls from the same background, and 274 AD cases from an independent Spanish cohort. Results The mutation was absent from public databases and segregated with the disease. CSF A beta 42, total tau, and phosphorylated tau of affected siblings were consistent with AD. The predicted haploinsufficiency effect of the nonsense mutation was supported by (a) ADAM10 isoforms in CSF decreased around 50% and (b) 70% reduction of CSF sAPP alpha peptide, both compared to controls, while sAPP beta levels remained unchanged. Interestingly, sporadic AD cases had a similar decrease in CSF ADAM10 levels to that of mutants, though their sAPP alpha and sAPP beta levels resembled those of controls. Therefore, a decreased sAPP alpha/sAPP beta ratio was an exclusive feature of mutant ADAM10 siblings. The p.Tyr167* mutation was not found in any of the other AD cases or controls screened. Conclusions This family illustrates the role of ADAM10 in the amyloidogenic process and the clinical development of the disease. Similarities between clinical and biomarker findings suggest that this family could represent a genetic model for sporadic late-onset AD due to age-related downregulation of alpha-secretase. This report encourages future research on ADAM10 enhancers.