Selected aspects of the human gut microbiota

The gut microbiota represents a highly complex assembly of microbes, which interact with each other and with their host. These interactions have various implications in terms of health and disease, and this multi-author review issue will address a number of selected aspects pertaining to gut microbiota research.     

De novo mutations in the actin genes ACTB and ACTG1 cause Baraitser-Winter syndrome

Brain malformations are individually rare but collectively common causes of developmental disabilities. Many forms of malformation occur sporadically and are associated with reduced reproductive fitness, pointing to a causative role for de novo mutations. Here, we report a study of Baraitser-Winter syndrome, a well-defined disorder characterized by distinct craniofacial features, ocular colobomata and neuronal migration defect. Using whole-exome sequencing of three proband-parent trios, we identified de novo missense changes in the cytoplasmic actin-encoding genes ACTB and ACTG1 in one and two probands, respectively. Sequencing of both genes in 15 additional affected individuals identified disease-causing mutations in all probands, including two recurrent de novo alterations (ACTB, encoding p.Arg196His, and ACTG1, encoding p.Ser155Phe). Our results confirm that trio-based exome sequencing is a powerful approach to discover genes causing sporadic developmental disorders, emphasize the overlapping roles of cytoplasmic actin proteins in development and suggest that Baraitser-Winter syndrome is the predominant phenotype associated with mutation of these two genes.     

Mutations in the phospholipid remodeling gene SERAC1 impair mitochondrial function and intracellular cholesterol trafficking and cause dystonia and deafness

Using exome sequencing, we identify SERAC1 mutations as the cause of MEGDEL syndrome, a recessive disorder of dystonia and deafness with Leigh-like syndrome, impaired oxidative phosphorylation and 3-methylglutaconic aciduria. We localized SERAC1 at the interface between the mitochondria and the endoplasmic reticulum in the mitochondria-associated membrane fraction that is essential for phospholipid exchange. A phospholipid analysis in patient fibroblasts showed elevated concentrations of phosphatidylglycerol-34:1 (where the species nomenclature denotes the number of carbon atoms in the two acyl chains:number of double bonds in the two acyl groups) and decreased concentrations of phosphatidylglycerol-36:1 species, resulting in an altered cardiolipin subspecies composition. We also detected low concentrations of bis(monoacyl-glycerol)-phosphate, leading to the accumulation of free cholesterol, as shown by abnormal filipin staining. Complementation of patient fibroblasts with wild-type human SERAC1 by lentiviral infection led to a decrease and partial normalization of the mean ratio of phosphatidylglycerol-34:1 to phosphatidylglycerol-36:1. Our data identify SERAC1 as a key player in the phosphatidylglycerol remodeling that is essential for both mitochondrial function and intracellular cholesterol trafficking.     

Genome-wide association analysis identifies susceptibility loci for migraine without aura

Migraine without aura is the most common form of migraine, characterized by recurrent disabling headache and associated autonomic symptoms. To identify common genetic variants associated with this migraine type, we analyzed genome-wide association data of 2,326 clinic-based German and Dutch individuals with migraine without aura and 4,580 population-matched controls. We selected SNPs from 12 loci with 2 or more SNPs associated with P values of <1 × 10(-5) for replication testing in 2,508 individuals with migraine without aura and 2,652 controls. SNPs at two of these loci showed convincing replication: at 1q22 (in MEF2D; replication P = 4.9 × 10(-4); combined P = 7.06 × 10(-11)) and at 3p24 (near TGFBR2; replication P = 1.0 × 10(-4); combined P = 1.17 × 10(-9)). In addition, SNPs at the PHACTR1 and ASTN2 loci showed suggestive evidence of replication (P = 0.01; combined P = 3.20 × 10(-8) and P = 0.02; combined P = 3.86 × 10(-8), respectively). We also replicated associations at two previously reported migraine loci in or near TRPM8 and LRP1. This study identifies the first susceptibility loci for migraine without aura, thereby expanding our knowledge of this debilitating neurological disorder.     

Genome-wide association analyses identify 13 new susceptibility loci for generalized vitiligo

We previously reported a genome-wide association study (GWAS) identifying 14 susceptibility loci for generalized vitiligo. We report here a second GWAS (450 individuals with vitiligo (cases) and 3,182 controls), an independent replication study (1,440 cases and 1,316 controls) and a meta-analysis (3,187 cases and 6,723 controls) identifying 13 additional vitiligo-associated loci. These include OCA2-HERC2 (combined P = 3.80 × 10(-8)), MC1R (P = 1.82 × 10(-13)), a region near TYR (P = 1.57 × 10(-13)), IFIH1 (P = 4.91 × 10(-15)), CD80 (P = 3.78 × 10(-10)), CLNK (P = 1.56 × 10(-8)), BACH2 (P = 2.53 × 10(-8)), SLA (P = 1.58 × 10(-8)), CASP7 (P = 3.56 × 10(-8)), CD44 (P = 1.78 × 10(-9)), IKZF4 (P = 2.75 × 10(-14)), SH2B3 (P = 3.54 × 10(-18)) and TOB2 (P = 6.81 × 10(-10)). Most vitiligo susceptibility loci encode immunoregulatory proteins or melanocyte components that likely mediate immune targeting and the relationships among vitiligo, melanoma, and eye, skin and hair coloration.     

A genome-wide association meta-analysis identifies new childhood obesity loci

Multiple genetic variants have been associated with adult obesity and a few with severe obesity in childhood; however, less progress has been made in establishing genetic influences on common early-onset obesity. We performed a North American, Australian and European collaborative meta-analysis of 14 studies consisting of 5,530 cases (≥95th percentile of body mass index (BMI)) and 8,318 controls (<50th percentile of BMI) of European ancestry. Taking forward the eight newly discovered signals yielding association with P < 5 × 10(-6) in nine independent data sets (2,818 cases and 4,083 controls), we observed two loci that yielded genome-wide significant combined P values near OLFM4 at 13q14 (rs9568856; P = 1.82 × 10(-9); odds ratio (OR) = 1.22) and within HOXB5 at 17q21 (rs9299; P = 3.54 × 10(-9); OR = 1.14). Both loci continued to show association when two extreme childhood obesity cohorts were included (2,214 cases and 2,674 controls). These two loci also yielded directionally consistent associations in a previous meta-analysis of adult BMI(1).