X chromosome-inactivation patterns in patients with Rett syndrome

Rett syndrome (RS) is a complex and severely disabling neurologic disorder, restricted to females. As non-random X inactivation could indicate that the X chromosome has a role in the etiology of the syndrome, we performed molecular analysis based on the differential methylation of the active and inactive X chromosomes with probe M27β, taking into account the parental origin of the two Xs, in 24 RS girls (including a pair of concordant monozygote twins), 22 mothers, and a control group of 30 normal women. The results showed a significantly (Fisher’s exact test) increased frequency of skewed X inactivation in lymphocytes from 15/23 RS compared with 4/22 mothers (P = 0.0031) and 6/30 controls (P = 0.0021). Our results, together with those from the literature, showed that as a group, RS patients are apparently more prone to skewed X inactivation than their mothers and normal controls, and this suggests that the X chromosome is somehow involved in RS etiology. Received: 13 February 1997 / Accepted: 5 November 1997     

X chromosome-inactivation patterns of 1,005 phenotypically unaffected females

X-chromosome inactivation is widely believed to be random in early female development and to result in a mosaic distribution of cells, approximately half with the paternally derived X chromosome inactive and half with the maternally derived X chromosome inactive. Significant departures from such a random pattern are hallmarks of a variety of clinical states, including being carriers for severe X-linked diseases or X-chromosome cytogenetic abnormalities. To evaluate the significance of skewed patterns of X inactivation, we examined patterns of X inactivation in a population of >1,000 phenotypically unaffected females. The data demonstrate that only a very small proportion of unaffected females show significantly skewed inactivation, especially during the neonatal period. By comparison with this data set, the degree of skewed inactivation in a given individual can now be quantified and evaluated for its potential clinical significance.     

X chromosome-inactivation patterns confirm the late timing of monoamniotic-MZ twinning.

Autosomal dominant brachydactyly type B (BDB) is characterized by nail aplasia with rudimentary or absent distal and middle phalanges. We describe two unrelated families with BDB. One family is English; the other family is Canadian but of English ancestry. We assigned the BDB locus in the Canadian family to an 18-cM interval on 9q, using linkage analysis (LOD score 3.5 at recombination fraction [theta] 0, for marker D9S938). Markers across this interval also cosegregated with the BDB phenotype in the English family (LOD score 2.1 at straight theta=0, for marker D9S277). Within this defined interval is a smaller (7.5-cM) region that contains 10 contiguous markers whose disease-associated haplotype is shared by the two families. This latter result suggests a common founder among families of English descent that are affected with BDB.     

The association of skewed X chromosome inactivation with aneuploidy in humans

Recently, we reported that skewed X chromosome inactivation (XCI) was more common in women who had experienced a trisomic pregnancy as compared to control women. Rather than an overall shift in the distribution of skewing there appears to only be an excess of extreme (= 95%) skewing. Further analysis of our data reveals that the increase in skewed XCI is dependent on which chromosome is involved in the trisomy and how many trisomies the woman has experienced, although sample sizes in each group are small. In this review we discuss limitations of the commonly used assays of XCI, which use measurements of DNA methylation to infer skewing patterns, and review the data based on current knowledge of the causes of XCI skewing. Gonadal mosaicism, premature aging, loss of methylation at some CpGs, and X-linked mutations can all be considered as potential mechanisms explaining both increased risk of trisomy and skewed XCI. While further research is needed to evaluate the role of each of these, the association of trisomy with apparent skewed XCI in the mother offers new opportunities to clarify the risk factors for and causes of the high incidence of aneuploidy in human females.     

DNA methylation age is associated with mortality in a longitudinal Danish twin study

An epigenetic profile defining the DNA methylation age (DNAm age) of an individual has been suggested to be a biomarker of aging, and thus possibly providing a tool for assessment of health and mortality. In this study, we estimated the DNAm age of 378 Danish twins, age 30–82 years, and furthermore included a 10‐year longitudinal study of the 86 oldest‐old twins (mean age of 86.1 at follow‐up), which subsequently were followed for mortality for 8 years. We found that the DNAm age is highly correlated with chronological age across all age groups (r = 0.97), but that the rate of change of DNAm age decreases with age. The results may in part be explained by selective mortality of those with a high DNAm age. This hypothesis was supported by a classical survival analysis showing a 35% (4–77%) increased mortality risk for each 5‐year increase in the DNAm age vs. chronological age. Furthermore, the intrapair twin analysis revealed a more‐than‐double mortality risk for the DNAm oldest twin compared to the co‐twin and a ‘dose–response pattern’ with the odds of dying first increasing 3.2 (1.05–10.1) times per 5‐year DNAm age difference within twin pairs, thus showing a stronger association of DNAm age with mortality in the oldest‐old when controlling for familial factors. In conclusion, our results support that DNAm age qualifies as a biomarker of aging.     

Transmission-ratio distortion of X chromosomes among male offspring of females with skewed X-inactivation

We have begun a search for heritable variation in X-chromosome inactivation pattern in normal females to determine whether there is a genetic effect on the imprinting of X-chromosome inactivation in humans. We have performed a quantitative analysis of X-chromosome inactivation in lymphocytes from mothers in normal, three-generation families. Eight mothers and 12 grandmothers exhibited evidence of highly skewed patterns of X-chromosome inactivation. We observed that the male offspring of females with skewed X-inactivation patterns were three times more likely to inherit alleles at loci that were located on the inactive X chromosome (Xi) than the active X chromosome (Xa). The region of the X chromosome for which this phenomenon was observed extends from XP11 to -Xq22. We have also examined X-chromosome inactivation patterns in 21 unaffected mothers of male bilateral sporadic retinoblastoma patients. Six of these mothers had skewed patterns of X-chromosome inactivation. In contrast to the tendency for male offspring of skewed mothers from nondisease families to inherit alleles from the inactive X chromosome, five of the six affected males inherited the androgen receptor alleles from the active X chromosome of their mother. © 1995 Wiley-Liss, Inc.     

Local estimation of age-dependent variance components from longitudinal twin data

In the study of longitudinal twin and family data, interest is often in the covariance structure of the data and the decomposition of this covariance structure into genetic and environmental components rather than in estimating the mean function. Various parametric models for covariance structures have been proposed but, e.g., in studies of children where growth spurts occur at various ages, it is difficult to a priori determine an appropriate parametric model for the covariance structure. In particular, there is a general lack of the visualization procedures, such as lowess, that are invaluable in the initial stages of constructing a parametric model for a mean function. Here we use kernel smoothing to modify a cross-sectional approach based on the sample covariance matrices to obtain smoothed estimates of the genetic and environmental variances and correlations for longitudinal twin data. The methods are proposed to be exploratory as an aid to parametric modeling rather than inferential, although approximate asymptotic standard errors are derived in the Appendix.     

White matter development in early puberty: a longitudinal volumetric and diffusion tensor imaging twin study

White matter microstructure and volume show synchronous developmental patterns in children. White matter volume increases considerably during development. Fractional anisotropy, a measure for white matter microstructural directionality, also increases with age. Development of white matter volume and development of white matter microstructure seem to go hand in hand. The extent to which the same or different genetic and/or environmental factors drive these two aspects of white matter maturation is currently unknown. We mapped changes in white matter volume, surface area and diffusion parameters in mono- and dizygotic twins who were scanned at age 9 (203 individuals) and again at age 12 (126 individuals). Over the three-year interval, white matter volume (+6.0%) and surface area (+1.7%) increased, fiber bundles expanded (most pronounced in the left arcuate fasciculus and splenium), and fractional anisotropy increased (+3.0%). Genes influenced white matter volume (heritability ~85%), surface area (~85%), and fractional anisotropy (locally 7% to 50%) at both ages. Finally, volumetric white matter growth was negatively correlated with fractional anisotropy increase (r = -0.62) and this relationship was driven by environmental factors. In children who showed the most pronounced white matter growth, fractional anisotropy increased the least and vice-versa. Thus, white matter development in childhood may reflect a process of both expansion and fiber optimization.     

A twin study of structural chromosome aberrations in lymphocytes

Structural chromosome aberrations were analyzed in peripheral lymphocytes of eight monozygotic (MZ) and seven dizygotic (DZ) pairs of male twins. There was no significant intrapair difference in the variance of aberration frequencies among the MZ and DZ twins. Thus, there was no evidence of a major genetic influence on the development of structural chromosome aberrations. Although a genetic component could not be excluded, it was concluded that any chromosome aberrations observed were probably due mainly to environmental influences.     

Evidence that skewed X inactivation is not needed for the phenotypic expression of Aicardi syndrome

Aicardi syndrome is a rare disorder characterized by absent corpus callosum, infantile spasms, and chorioretinal lacunae. It is sporadic in nature and affects only females, resulting in severe mental and physical handicap. It has been suggested that the disease is caused by a dominant X-linked mutation which occurs de novo in females, and is lethal in hemizygous male embryos. This mode of inheritance has been observed in a number of other rare syndromes. In these syndromes, when X inactivation is studied, a non-random pattern is usually found. We have studied the X inactivation pattern in ten female patients with Aicardi syndrome and their parents using the highly polymorphic, differentially methylated androgen receptor gene. The results showed an unexpected random X-inactivation pattern in these patients. Previous clinical and cytogenetic evidence suggests that Aicardi syndrome is caused by an X-linked dominant mutation, de novo in females and lethal in males. However, unlike most other known X-linked disorders inherited in this fashion, Aicardi syndrome patients have a normal (i.e., random) X-inactivation pattern. A number of possible explanations is proposed for this apparently contradictory evidence. Received: 20 December 1996 / Accepted: 30 April 1997     

High-resolution chromosomal microarray analysis of early-stage human embryonic stem cells reveals an association between X chromosome instability and skewed X inactivation

X chromosome inactivation (XCI) is a dosage compensation mechanism that silences the majority of genes on one X chromosome in each female cell via a random process. Skewed XCI is relevant to many diseases, but the mechanism leading to it remains unclear. Human embryonic stem cells (hESCs) derived from the inner cell mass (ICM) of blastocyst-stage embryos have provided an excellent model system for understanding XCI initiation and maintenance. Here, we derived hESC lines with random or skewed XCI patterns from poor-quality embryos and investigated the genome-wide copy number variation (CNV) and loss of heterozygosity (LOH) patterns at the early passages of these two groups of hESC lines. It was found that the average size of CNVs on the X chromosomes in the skewed group is twice as much as that in the random group. Moreover, the LOH regions of the skewed group covered the gene locus of either XIST or XACT, which are master long non-coding RNA (lncRNA) effectors of XCI in human pluripotent stem cells. In conclusion, our work has established an experimentally tractable hESC model for study of skewed XCI and revealed an association between X chromosome instability and skewed XCI.     

A high risk twin study of combat-related PTSD comorbidity.

Combat-related posttraumatic stress disorder (PTSD) is highly comorbid with other mental disorders. However, the nature of the relationship between PTSD and other mental disorders remains unclear. A discordant high-risk twin design was used on data from a sub-sample of the male-male twin pair members of the Vietnam Era Twin Registry to examine whether patterns of comorbidity are consistent with a psychopathological response to combat exposure or reflect familial vulnerability to psychopathology. Mental disorders were assessed via the Mental Health Diagnostic Interview Schedule Version III - Revised. Discordant monozygotic within-pair comparisons revealed that PTSD probands had higher symptom counts and diagnostic prevalences of mood and anxiety disorders than their non-combat exposed co-twins. Monozygotic co-twins of PTSD probands had significantly more mood disorder symptoms than monozygotic co-twins of combat controls or dizygotic co-twins of veterans with PTSD. These findings suggest that a) major depression, generalized anxiety disorder and panic disorder are part of a post-combat response syndrome; b) a shared familial vulnerability also contributes to the association between PTSD and major depression, PTSD and dysthymia, and c) this shared vulnerability is mediated by genetic factors.     

A twin titration microcalorimeter for the study of biochemical reactions

A small-volume (200 microliter) titration calorimeter of high sensitivity (1 mu cal ) has been developed for the purpose of studying biochemical reactions where the amounts of material are limited to a few nanomoles. High sensitivity is achieved by calorimetric twining , use of glass cells, elimination of vapor space, effective low-energy stirring, and reduction of measurement time. The calorimeter operates using the heat conduction principal with computer-controlled electrical compensation, which reduces the measurement time of each point from 10 to 3 min. This reduction in time is accompanied by a corresponding increase in the precision of measurement. The use of the calorimeter is demonstrated by a measurement of the heat of oxygenation of hemocyanin.