Genetic epidemiologic methods to screen for matrilineal inheritance in mitochondrial disorders

The patterns of inheritance in early-onset dementia: Alzheimer's disease and frontotemporal dementia

AIM

To investigate the patterns of inheritance and gene mutation status in early-onset dementia (EOD).

METHODS

Data were collected on 202 consecutive patients presenting to an EOD clinic. Early-onset Alzheimer's disease (EOAD, n = 120) and early-onset frontotemporal dementia (EOFTD, n = 82) were studied.

RESULTS

The majority of participants, 72.5% with EOAD and 74.4% with EOFTD, did not have a positive family history of dementia. An autosomal dominant pattern of inheritance was observed in 14.2% of patients with EOAD and 13.4% of patients with FTD. Of those with an autosomal dominant pattern of inheritance, 11.8% of EOAD and 45.5% of FTD probands had known pathogenic mutations. Only 1.6% of the total population of EOAD and 7.3% of EOFTD possessed known gene mutations.

CONCLUSION

Early-onset dementia does not appear to be a strongly inherited autosomal dominant condition. The majority of patients were sporadic. Known mutations were uncommon and do not explain the total autosomal dominant burden.

A novel class of tests for the detection of mitochondrial DNA-mutation involvement in diseases

We develop a novel class of tests to detect mitochondrial DNA (mtDNA)-mutation involvement in complex diseases by the study of affected pedigree members. For a pedigree, affected individuals are first considered and are then connected through their relatives. We construct a reduced pedigree from an original pedigree. Each configuration of a reduced pedigree is given a score, with high scores given to configurations that are consistent with mtDNA-mutation involvement and low scores given to configurations that are not consistent with mtDNA-mutation involvement. For many pedigrees, the weighted sum of scores of the pedigrees is calculated. The tests are formed by comparing the observed score with the expected score under the null hypothesis that only nuclear autosomal mutations are involved. We study the optimality of score functions and weights under the heterogeneity model without phenocopies. We also develop a method to estimate the contribution that mtDNA mutations make if they are involved under a heterogeneity model. Finally, we apply our methods to three data sets: Leber hereditary optic neuropathy, a disease that has been proved to be caused by mtDNA mutations; non-insulin-dependent diabetes mellitus (NIDDM); and hypertension (HTN). We find evidence of mtDNA-mutation involvement in all three diseases. The estimated fraction of patients with NIDDM due to mtDNA-mutation involvement is 22% (95% confidence interval [CI] 6%-38%). The fraction of patients with HTN potentially due to mtDNA-mutation involvement is estimated at 55% (95% CI 45%-65%).

Testing for contributions of mitochondrial DNA mutations to complex diseases

Several complex disorders are suspected of being associated with mitochondrial DNA (mtDNA) mutations. We studied the statistical properties of a test based on proband-relative pairs to identify potential mtDNA mutation involvement in a complex disorder. The test compares the recurrence risk of relatives of probands along the mitochondrial lineage with that of relatives along the nonmitochondrial lineage. If mtDNA mutations are involved, the recurrence risk will be higher among relatives in the mitochondrial lineage. The form of the test is independent of the assumed models of inheritance and interaction of the nuclear autosomal mutations with mtDNA mutations. The power of the test, however, differs among the different models and by the type of proband-relative pairs used in the test. We considered heterogeneity models with and without phenocopies, a three-state heteroplasmic mtDNA transmission model, and a multiplicative epistasis model. Under the heterogeneity model, the power of the test increases as the relationship between the proband and the relative becomes more distant. Under the multiplicative epistasis model, the power of the test decreases as the relationship between the proband and the relative becomes more distant.