Mutations in or near the Transmembrane Domain Alter PMEL Amyloid Formation from Functional to Pathogenic

PMEL is a pigment cell-specific protein that forms physiological amyloid fibrils upon which melanins ultimately deposit in the lumen of the pigment organelle, the melanosome. Whereas hypomorphic PMEL mutations in several species result in a mild pigment dilution that is inherited in a recessive manner, PMEL alleles found in the Dominant white (DW) chicken and Silver horse (HoSi)—which bear mutations that alter the PMEL transmembrane domain (TMD) and that are thus outside the amyloid core—are associated with a striking loss of pigmentation that is inherited in a dominant fashion. Here we show that the DW and HoSi mutations alter PMEL TMD oligomerization and/or association with membranes, with consequent formation of aberrantly packed fibrils. The aberrant fibrils are associated with a loss of pigmentation in cultured melanocytes, suggesting that they inhibit melanin production and/or melanosome integrity. A secondary mutation in the Smoky chicken, which reverts the dominant DW phenotype, prevents the accumulation of PMEL in fibrillogenic compartments and thus averts DW–associated pigment loss; a secondary mutation found in the Dun chicken likely dampens a HoSi–like dominant mutation in a similar manner. We propose that the DW and HoSi mutations alter the normally benign amyloid to a pathogenic form that antagonizes melanosome function, and that the secondary mutations found in the Smoky and Dun chickens revert or dampen pathogenicity by functioning as null alleles, thus preventing the formation of aberrant fibrils. We speculate that PMEL mutations can model the conversion between physiological and pathological amyloid

An APOE haplotype associated with decreased ε4 expression increases the risk of late onset Alzheimer's disease.

This paper addresses a tenet of the literature on APOE, i.e., the relationship between the effects of the 4, one of the established genetic risk factor for Alzheimer's disease (AD), and its expression levels as determined by APOE promoter polymorphisms. Five polymorphisms (-491 rs449647,-427 rs769446,-219 rs405509, and rs429358-rs7412) were studied in 1308 AD patients and 1082 control individuals from the Central-Northern Italy. Major findings of the present study are the following: 1) the variants-219T and 4 increase the risk for late onset AD (LOAD) when they are both present in cis on the same chromosome (in phase); 2) the correlation between the haplotype (-219T/4) and AD risk persists when the data are stratified by age; 3) this haplotype likely anticipates the age of onset of the disease. These data, while confirming the association between-219T and AD, highlight the importance of the phase of the alleles for the observed effects on AD risk, suggesting that this information has to be taken into account when assessing the AD genetic risk. Moreover, the data help to clarify the apparent discrepancy that emerges from the genetic analysis where an SNP characterizing the haplotype responsible for an increased risk for LOAD is coherently associated with a reduced expression of ApoE levels. Our data are compatible with the hypothesis of a complex role of ApoE in the AD pathogenesis, with positive and negative effects occurring concomitantly according to its expression levels and its protein-protein interactions largely unclarified. © 2011 - IOS Press and the authors. All rights reserved