Maxwell P Westerman

Uppsala University, Uppsala, Uppsala, Sweden

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Publications (5)22.91 Total impact

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    ABSTRACT: Although historically considered as junk-DNA, tandemly repeated sequence motifs can affect human phenotype. For example, variable number tandem repeats (VNTR) with embedded enhancers have been shown to regulate gene transcription. The post-zygotic variation is the presence of genetically distinct populations of cells in an individual derived from a single zygote, and this is an understudied aspect of genome biology. We report somatically variable VNTR with sequence properties of an enhancer, located upstream of IFNAR1. Initially, SNP genotyping of 63 monozygotic twin pairs and multiple tissues from 21 breast cancer patients suggested a frequent post-zygotic mosaicism. The VNTR displayed a repeated 32 bp core motif in the center of the repeat, which was flanked by similar variable motifs. A total of 14 alleles were characterized based on combinations of segments, which showed post-zygotic and inter-individual variation, with up to 6 alleles in a single subject. Somatic variation occurred in ∼24% of cases. In this hypervariable region, we found a clustering of transcription factor binding sites with strongest sequence similarity to mouse Foxg1 transcription factor binding motif. This study describes a VNTR with sequence properties of an enhancer that displays post-zygotic and inter-individual genetic variation. This element is within a locus containing four related cytokine receptors: IFNAR2, IL10Rβ, IFNAR1 and IFNGR2, and we hypothesize that it might function in transcriptional regulation of several genes in this cluster. Our findings add another level of complexity to the variation among VNTR-based enhancers. Further work may unveil the normal function of this VNTR in transcriptional control and its possible involvement in diseases connected with these receptors, such as autoimmune conditions and cancer.
    PLoS ONE 09/2013; 8(9):e67752. DOI:10.1371/journal.pone.0067752 · 3.53 Impact Factor
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    ABSTRACT: Patients with sickle cell trait (STr) are usually considered to be asymptomatic. However, complications, including hypercoagulability, increased risk of venous thromboembolism and the exertional exercise syndrome with rhabdomyolysis and sudden death, have been described. The exact cause of these adverse events is unclear. We have investigated two patients, a set of monozygotic twins with STr, to establish their procoagulant activity status as a potential indicator of thrombotic risk. In-vivo thrombin generation was assessed by the measurement of prothrombin fragment 1 + 2 (F1 + 2) and thrombin-antithrombin complexes (TAT). D-dimer was used as a marker of fibrinolytic activity. The potential to generate thrombin was determined using an ex-vivo thrombin generation test (TGT). The impact of red blood cell (RBC)-derived microparticle shedding and RBC rheology were examined. TAT (>60 μg/l) and F1 + 2 (948 pmol/l) were markedly elevated in patient 2 but within the normal reference range in patient 1 (TAT = 2.5 μg/l; F1 + 2 = 138 pmol/l). D-dimer levels (0.9 mg/l FEU) were similarly elevated in both patients. TGT peak thrombin and endogenous thrombin potential (ETP) were elevated to similar degrees in both patients. Flow cytometric analysis for RBC-derived microparticles showed that both patients had elevated levels on two occasions. RBC deformability, blood viscosity and RBC aggregation were normal and similar in both patients. The results demonstrated different coagulation activity in the patients with one patient in a prothrombotic state, suggesting that there may be two levels of hypercoagulability in STr. Measurement of such differences would allow for separation of high and low-risk patients from serious complications.
    Blood coagulation & fibrinolysis: an international journal in haemostasis and thrombosis 02/2012; 23(4):268-70. DOI:10.1097/MBC.0b013e32835187f8 · 1.38 Impact Factor
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    ABSTRACT: Structural variations are among the most frequent interindividual genetic differences in the human genome. The frequency and distribution of de novo somatic structural variants in normal cells is, however, poorly explored. Using age-stratified cohorts of 318 monozygotic (MZ) twins and 296 single-born subjects, we describe age-related accumulation of copy-number variation in the nuclear genomes in vivo and frequency changes for both megabase- and kilobase-range variants. Megabase-range aberrations were found in 3.4% (9 of 264) of subjects ≥60 years old; these subjects included 78 MZ twin pairs and 108 single-born individuals. No such findings were observed in 81 MZ pairs or 180 single-born subjects who were ≤55 years old. Recurrent region- and gene-specific mutations, mostly deletions, were observed. Longitudinal analyses of 43 subjects whose data were collected 7-19 years apart suggest considerable variation in the rate of accumulation of clones carrying structural changes. Furthermore, the longitudinal analysis of individuals with structural aberrations suggests that there is a natural self-removal of aberrant cell clones from peripheral blood. In three healthy subjects, we detected somatic aberrations characteristic of patients with myelodysplastic syndrome. The recurrent rearrangements uncovered here are candidates for common age-related defects in human blood cells. We anticipate that extension of these results will allow determination of the genetic age of different somatic-cell lineages and estimation of possible individual differences between genetic and chronological age. Our work might also help to explain the cause of an age-related reduction in the number of cell clones in the blood; such a reduction is one of the hallmarks of immunosenescence.
    The American Journal of Human Genetics 02/2012; 90(2):217-28. DOI:10.1016/j.ajhg.2011.12.009 · 10.99 Impact Factor
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    ABSTRACT: Somatic genetic variation in health and disease is poorly explored. Monozygotic (MZ) twins are a suitable model for studies of somatic mosaicism since genetic differences in twins derived from the same zygote represent an irrefutable example of somatic variation. We report the analysis of a pair of generally healthy female MZ twins, discordant for somatic mosaicism for aneuploidy of chromosomes X and Y. Both twins are heterozygous carriers of sickle cell disease mutation. Genotyping of blood DNA from both twins using Illumina Human 610 SNP array revealed a copy number imbalance for chromosome X in a proportion of cells in one twin. Fluorescent in situ hybridization (FISH) analysis confirmed monosomy X (45,X) in 7% of proband nucleated blood cells. Unexpectedly, FISH analysis of cells from the other twin revealed 45,X and 46,XY lineages, both present in 1% of cells. The mechanism behind formation of these aneuploidies suggests several aberrant chromosome segregation events in meiosis and mitoses following conception. Our report contributes to the delineation of the frequency of somatic structural genomic variation in normal MZ twins. These results also illustrate the plasticity of the human genome for tolerating large copy number changes in healthy subjects and show the sensitivity of the Illumina platform for detection of aberrations that are present in a minority of the studied cells.
    American Journal of Medical Genetics Part A 10/2010; 152A(10):2595-8. DOI:10.1002/ajmg.a.33604 · 2.05 Impact Factor
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    ABSTRACT: Levels of circulating red blood cell (RBC)-derived vesicles are increased in sickle cell anaemia (SCA) and thalassaemia intermedia (TI) but the mechanisms, effects and controlling factors may differ. This study found that levels of vesicles and intravascular haemolysis were linked as shown by the correlation between levels of vesicles and plasma Hb. Vesicle levels were 6-fold greater in SCA and 4-fold greater in TI than in controls. The proportion of plasma Hb within vesicles was increased in SCA and TI with a significantly higher proportion in TI. We examined whether subpopulations of RBC expressing phosphatidylserine (PS) were a source of PS(+) vesicles and observed a significant association. Thrombin generation was promoted by the vesicles in which 40-50% expressed PS. In TI, markers of thrombin generation were significantly related to PS(+) RBC. Splenectomy in TI had significant effects including greater increases in vesicle levels, plasma Hb, PS(+) RBCs and thrombin generation markers than in unsplenectomised patients. In hydroxycarbamide (HC)-treated SCA patients these measures were decreased compared with untreated controls. The relationship between vesicle levels and plasma Hb suggests a mechanism linking vesiculation to haemolysis and consequently nitric oxide (NO) bioavailability and suggests a means by which HC treatment improves NO bioavailability.
    British Journal of Haematology 08/2008; 142(1):126-35. DOI:10.1111/j.1365-2141.2008.07155.x · 4.96 Impact Factor