Heterogeneous tissue distribution of a mitochondrial DNA polymorphism in heteroplasmic subjects without mitochondrial disorders

CONTEXT—Several maternally inherited point mutations of the mitochondrial genome cause mitochondrial disorders, but the correlation between genotype and phenotype remains obscure in many cases. The same mutation may cause various diseases, probably because of a different tissue distribution.
OBJECTIVE—To assess the role of random somatic segregation in generating interperson differences by analysis of an apparently neutral polymorphism.
DESIGN—Screening of 81 brain samples from subjects without mitochondrial disorders and selection of five necropsy cases showing a high level of heteroplasmy for the polymorphism.
MAIN OUTCOME MEASURES—A proportion of various distinct genotypes in the mtDNA pool of the tissues, identified by fluorescent PCR products, representing a short polycytosine tract of variable length in the mitochondrial displacement loop.
RESULTS—Differences were found between organs or groups of organs within subjects, pointing towards somatic segregation of mtDNA. In addition, marked differences of this organ distribution occurred between subjects, which cannot be explained by tissue specific selection.
CONCLUSIONS—The observed interperson differences can be explained by somatic segregation, which occurs randomly at various developmental stages. Besides tissue specific selection, this process might participate in the distribution of pathogenic mtDNA mutations.


Keywords: mtDNA; polymorphism; HVR2; heteroplasm

GCF2/LRRFIP1 Represses Tumor Necrosis Factor Alpha Expression

Tumor necrosis factor alpha (TNF-α) is an important mediator of inflammation, apoptosis, and the development of secondary lymphoid structures. Multiple polymorphic microsatellites have been identified in and around the gene, and there are also multiple single-base pair biallelic polymorphisms in the introns and promoter. The TNF-α −308 promoter polymorphism is a G-to-A transition which has been statistically associated with various autoimmune disorders. Some studies have found that it may directly mediate the increased transcription of TNF-α in some circumstances. This study characterizes proteins interacting at the polymorphic promoter site. Affinity purification of binding proteins and confirmatory chromatin immunoprecipitation assays were used to identify the proteins. Electrophoretic mobility shift analyses and surface plasmon resonance were used to define binding characteristics. Proteins interacting at this site include GCF2/LRRFIP1 and Ets-1. GCF2/LRRFIP1 appears to act as a repressor and occupies the −308 site in cells that do not make TNF-α. Cells competent to produce TNF-α have Ets-1 bound to the −308 promoter site. Active transcription is accompanied by NF-κB and c-Jun binding to the proximal promoter. Thus, dynamic changes on the TNF-α promoter, particularly at the −308 site, accompany the transition from repressed to active transcription. GCF2/LRRFIP1 is the first TNF-α repressor identified