Marianne Kristiansen

Twin study of genetic and aging effects on X chromosome inactivation

To investigate the genetic influence on X chromosome inactivation and on age-related skewing of X inactivation, in particular, we analysed the X inactivation pattern (XIP) in peripheral blood cells from 118 young monozygotic (MZ) twin pairs (18–53 years), 82 elderly MZ twin pairs (55–94 years), 146 young dizygotic (DZ) twin pairs (20–54 years) and 112 elderly DZ twin pairs (64–95 years). Elderly twins had a higher frequency of skewed X inactivation (34%) than young twins (15%) (P<0.001). Our data suggest that the increase in skewing occurs after age 50–60 years. The intraclass correlation was 0.61 and 0.58 in young and elderly MZ twin pairs, and 0.08 and 0.09 in young and elderly DZ twin pairs. Biometric analysis showed that dominant genetic effects accounted for 63 and 58% of the variance of XIP in the young and elderly twin pairs, respectively. The dominant genetic effect and the shared environment for monochorionic MZ twins may explain the high intraclass correlation for the MZ twin pairs compared to the DZ twin pairs. We did not observe a significant decrease in the intraclass correlation in elderly MZ twins compared to young MZ twins, which would be expected if age-related skewing were due to stochastic factors. We conclude that the increased skewing with age implies that a genetically dependent selection of blood cells take place.

High frequency of skewed X inactivation in young breast cancer patients

Introduction: Patients with invasive ovarian cancer were recently shown to have a higher frequency of skewed X chromosome inactivation in peripheral blood cells compared to patients with borderline cancer and controls. In this study, we analysed the X inactivation pattern in peripheral blood from 216 breast cancer patients. Methods: X inactivation analysis was performed using HpaII predigestion of DNA followed by PCR of the highly polymorphic CAG repeat of the androgen receptor gene (AR), which amplifies the undigested inactive X chromosome only. The X inactivation pattern was classified as skewed when 90% or more of the cells preferentially used one X chromosome. Results: Young breast cancer patients (27-45 years) had a higher frequency of skewed X inactivation than young controls (13 and 1%, respectively) (p=0.009), whereas no difference was found for middle aged and older patients compared to controls of a similar age. Conclusions: A germline mutation in an X linked tumour suppressor gene may give a proliferative advantage to cells with this mutation on the active X chromosome, thus causing skewed X inactivation and an increased risk for developing cancer. Another possible explanation could be that females with a constitutionally skewed X inactivation pattern are more susceptible to develop breast cancer because of an X linked low penetrance susceptibility allele that is affected by the inactivation pattern.

Novel splicing mutation in the NEMO (IKK-gamma) gene with severe immunodeficiency and heterogeneity of X-chromosome inactivation.

We report on a family with three stillborn males, three affected males who were small for gestational age and died within 8 months, and one male who died at age 5 years. This boy had cone‐shaped teeth and oligoodontia. He had serious bacterial infections and inflammatory bowel disease. Mutations in the NF‐κB essential modulator (NEMO) gene have recently been shown to be the cause of a group of ectodermal dysplasia and immunodeficiency disorders (EDA‐ID). Analysis of the NEMO gene revealed a nucleotide change in the consensus sequence of the splicing donor site of exon 6 IVS6 + 5G → A(1027 + 5G → A), which has not previously been described in EDA‐ID. RT‐PCR analysis of fibroblast RNA from an aborted affected male fetus demonstrated a skipping of exons 4, 5, and 6 which resulted in a truncated protein of about 35 kDa revealed by NEMO antibody. The skipping of exons 4, 5, and 6 did not affect the ORF of the C‐terminal of NEMO encoded by exons 7, 8, 9, and 10, which contains a coiled‐coil motif (CC2), a leucin‐zipper (LZ), and a zinc finger motif (ZF) sub‐domains of NEMO. IκBα degradation was strongly impaired in the fetal fibroblasts, suggesting an impaired NF‐κB signaling. One healthy carrier had a completely skewed X‐inactivation pattern with the normal X active, whereas the two other carriers had a random X‐inactivation pattern. This family may represent a new phenotype within the EDA‐ID disorders. From the heterogeneity in X‐inactivation phenotype, we conclude that this mutation is not deleterious enough to be lethal for peripheral blood cells.

Early and severe presentation of X-linked myotubular myopathy in a girl with skewed X-inactivation

X-linked myotubular myopathy is a severe congenital myopathy in males, caused by mutations in the myotubularin (MTM1) gene on chromosome Xq28. In heterozygous carriers of MTM1 mutations, clinical symptoms are usually absent or only mild. We report a 6-year-old girl presenting at birth with marked hypotonia and associated feeding and respiratory difficulties. A muscle biopsy performed at 5 months suggested a diagnosis of myotubular myopathy. On examination at 6 years she had marked facial weakness with bilateral ptosis and external ophthalmoplegia, severe axial and proximal weakness and a mild scoliosis. Muscle magnetic resonance imaging showed a distinctive pattern of muscle involvement. Molecular genetic investigation of the MTM1 gene identified a heterozygous mutation in exon 12. X-inactivation studies in lymphocytes showed an extremely skewed pattern (97:3). This case emphasizes that investigation of the MTM1 gene and X-inactivation studies are indicated in isolated females with histopathological and clinical findings suggestive of myotubular myopathy.

X-inactivation patterns in carriers of X-linked myotubular myopathy

X-linked myotubular myopathy is a rare severe muscle disorder in affected male neonates. Most female carriers are free from symptoms. Skewed X inactivation has been proposed to be responsible for the affected phenotype seen in some carriers. We have compared the X inactivation patterns in blood DNA with the clinical phenotype in carriers of X-linked myotubular myopathy. The X-inactivation analysis was performed using HpaII predigestion of DNA followed by polymerase chain reaction of the highly polymorphic CAG repeat of the androgen receptor (AR) gene. The frequency of skewed X inactivation was similar in the X-linked myotubular myopathy carriers (22%) and in 235 controls (18%). Three overtly affected carriers had skewed X inactivation with the mutated X as the predominantly active X in at least two of them. Four females with mild symptoms had random X inactivation. The unaffected X-linked myotubular myopathy carriers had either skewed X inactivation in favour of expression from the normal X or random X-inactivation. Thus, there was a tendency for females with a more severe phenotype to have a skewed pattern of X inactivation, while females with an intermediate phenotype had a random pattern of X-inactivation.

High incidence of skewed X chromosome inactivation in young patients with familial non-BRCA1/BRCA2 breast cancer

Background: A higher frequency of skewed X chromosome inactivation has been reported in a consecutive series of young patients with breast cancer compared with controls of a similar age. Objective: To investigate the X inactivation pattern in patients with familial non-BRCA1/BRCA2 breast cancer (n = 272), BRCA1/BRCA2 germline mutations (n = 35), and sporadic breast cancer (n = 292). Methods: X inactivation pattern was determined by polymerase chain reaction analysis of the highly polymorphic CAG repeat in the androgen receptor (AR) gene. The X inactivation pattern was classified as skewed when 90% or more of the cells preferentially expressed one X chromosome. Results: Young patients with familial breast cancer had a significantly higher frequency of skewed X inactivation (11.2%) than young controls (2.7%) (p = 0.001). There was also a strong tendency for middle aged patients with sporadic breast cancer to be more skewed than middle aged controls (13.6% v 4.4%) (p = 0.02). No association between skewed X inactivation and breast cancer was found for the BRCA1/BRCA2 patients . Conclusions: Skewed X inactivation may be a risk factor for the development of breast cancer in both sporadic and familial breast cancer and may indicate an effect of X linked genes.

X chromosome inactivation in cervical cancer patients

Development of cervical carcinomas is strongly associated with presence of human papilloma virus (HPV). Recently we found that young patients with breast cancer had a higher frequency of skewed X inactivation in peripheral blood cells, indicating an effect of X-linked genes on breast cancer development. In this study, we investigated the frequency of skewed X-inactivation pattern in blood and tumor biopsies from patients with cervical cancer. No difference in the frequency of skewed X inactivation in blood was found between 142 patients and 437 age-matched controls. Elderly females have a higher frequency of skewed X inactivation in blood cells than younger females. An age effect was confirmed in this study for blood cells in both patients and controls. A tendency to an age effect was also found in the tumor biopsies. The correlation between X inactivation in blood and biopsies was 0.39 (P<0.001), showing that the X inactivation in biopsies to some degree reflects skewing in blood. Furthermore, of eight patients with a skewing of > or =75% in biopsies, seven patients had a skewing in the same direction in their blood cells (P=0.03). Our results indicate that if X-inactivation analysis is to be used in clonality studies of cervical cancers, it is essential to consider both the age and the X-inactivation pattern in blood cells.