Genetic associations with age at dementia onset in the PSEN1 E280A Colombian kindred.

INTRODUCTION: Genetic associations with Alzheimer's disease (AD) age at onset (AAO) could reveal genetic variants with therapeutic applications. We present a large Colombian kindred with autosomal dominant AD (ADAD) as a unique opportunity to discover AAO genetic associations. METHODS: A genetic association study was conducted to examine ADAD AAO in 340 individuals with the PSEN1 E280A mutation via TOPMed array imputation. Replication was assessed in two ADAD cohorts, one sporadic early-onset AD study and four late-onset AD studies. RESULTS: 13 variants had p<1×10-7 or p<1×10-5 with replication including three independent loci with candidate associations with clusterin including near CLU. Other suggestive associations were identified in or near HS3ST1, HSPG2, ACE, LRP1B, TSPAN10, and TSPAN14. DISCUSSION: Variants with suggestive associations with AAO were associated with biological processes including clusterin, heparin sulfate, and amyloid processing. The detection of these effects in the presence of a strong mutation for ADAD reinforces their potentially impactful role.

A neurodegenerative disease landscape of rare mutations in Colombia due to founder effects.

BACKGROUND: The Colombian population, as well as those in other Latin American regions, arose from a recent tri-continental admixture among Native Americans, Spanish invaders, and enslaved Africans, all of whom passed through a population bottleneck due to widespread infectious diseases that left small isolated local settlements. As a result, the current population reflects multiple founder effects derived from diverse ancestries. METHODS: We characterized the role of admixture and founder effects on the origination of the mutational landscape that led to neurodegenerative disorders under these historical circumstances. Genomes from 900 Colombian individuals with Alzheimer's disease (AD) [n = 376], frontotemporal lobar degeneration-motor neuron disease continuum (FTLD-MND) [n = 197], early-onset dementia not otherwise specified (EOD) [n = 73], and healthy participants [n = 254] were analyzed. We examined their global and local ancestry proportions and screened this cohort for deleterious variants in disease-causing and risk-conferring genes. RESULTS: We identified 21 pathogenic variants in AD-FTLD related genes, and PSEN1 harbored the majority (11 pathogenic variants). Variants were identified from all three continental ancestries. TREM2 heterozygous and homozygous variants were the most common among AD risk genes (102 carriers), a point of interest because the disease risk conferred by these variants differed according to ancestry. Several gene variants that have a known association with MND in European populations had FTLD phenotypes on a Native American haplotype. Consistent with founder effects, identity by descent among carriers of the same variant was frequent. CONCLUSIONS: Colombian demography with multiple mini-bottlenecks probably enhanced the detection of founder events and left a proportionally higher frequency of rare variants derived from the ancestral populations. These findings demonstrate the role of genomically defined ancestry in phenotypic disease expression, a phenotypic range of different rare mutations in the same gene, and further emphasize the importance of inclusiveness in genetic studies.

Genetic and nongenetic factors associated with CADASIL: A retrospective cohort study.

OBJECTIVE: To explore the role of cardiovascular risk factors and the different NOTCH-3 mutations to explain the variability observed in the clinical presentation of CADASIL. METHODS: This was a retrospective cohort study of 331 individuals, 90 were carriers of four mutations in the NOTCH3 gene. These four mutations are the ones identified in our region from the genetic evaluation of probands. Cox proportional hazards models were fitted to estimate the effect of genetic and cardiovascular factors on the onset of migraine, first stroke, and dementia. Competing risk regression models considered death as risk. RESULTS: Noncarriers (healthy controls from the same families without NOTCH3 mutations) and NOTCH3 mutation carriers had similar frequencies for all cardiovascular risk factors. Diabetes (SHR 2.74, 95% CI 1.52-4.94) was associated with a younger age at onset of strokes among carriers. Additionally, a genotype-phenotype relationship was observed among C455R mutation carriers, with higher frequency of migraines (100%), younger age at onset of migraine (median age 7 years, IQR 8) and strokes (median age 30.5 years, IQR 26). Moreover, fewer carriers of the R141C mutation exhibited migraines (20%), and it was even lower than the frequency observed in the noncarrier group (44.8%). CONCLUSIONS: This study characterizes extended family groups, allowing us a comparison in the genotype-phenotype. The results suggest a complex interplay of genetic and cardiovascular risk factors that may help explain the variability in the clinical presentation and severity of CADASIL.

Genetic origin of a large family with a novel PSEN1 mutation (Ile416Thr).

INTRODUCTION: A small percentage of Alzheimer's disease (AD) cases are caused by genetic mutations with autosomal dominant inheritance. We report a family with a novel variant in PSEN1. METHODS: We performed clinical and genetic evaluation of 93 related individuals from a Colombian admixed population. 31 individuals had whole-genome sequencing. RESULTS: Genetic analysis revealed a missense variant in PSEN1 (NM_000021.3: c.1247T>C p.Ile416Thr), which originated on an African haplotype and segregated with AD logarithm of the odds score of 6. Their clinical phenotype is similar to sporadic AD except for earlier age at onset: the mean age at onset for mild cognitive impairment was 47.6 years (standard deviation 5.83) and for dementia 51.6 years (standard deviation 5.03). DISCUSSION: Ile416Thr is a novel pathogenic variant that causes AD in the sixth decade of life. The history of the region that included slave importation and admixtures within a confined geographic locale represents a "mini-population bottleneck" and subsequent emergence of a rare dominant mutation.