Methylation of AR locus does not always reflect X chromosome inactivation state

Uterine fibroids at single-cell resolution: unveiling cellular heterogeneity to improve understanding of pathogenesis and guide future therapies

Uterine leiomyomas or fibroids are benign tumors of the myometrium that affect approximately 70% of reproductive-age women. Fibroids continue to be the leading cause of hysterectomy, resulting in substantial healthcare costs. Genetic complexity and lack of cellular and molecular understanding of fibroids have posed considerable challenges to developing noninvasive treatment options. Over the years, research efforts have intensified to unravel the genetic and cellular diversities within fibroids to deepen our understanding of their origins and progression. Studies using immunostaining and flow cytometry have revealed cellular heterogeneity within these tumors. A correlation has been observed between genetic mutations in fibroids and their size, which is influenced by cellular composition, proliferation rates, and extracellular matrix accumulation. Fibroids with mediator complex subunit 12 (MED12) mutation are composed of smooth muscle cells and fibroblasts equally. In contrast, the fibroids with high-mobility group AT-hook 2 (HMGA2) translocation are 90% composed of smooth muscle cells. More recently, single-cell RNA sequencing in the myometrium and MED12 mutation carrying fibroids has identified further heterogeneity in smooth muscle cells and fibroblast cells, identifying 3 different smooth muscle cell populations and fibroblast cell populations. Although both myometrium and fibroids have similar cellular composition, these cells differs in their transcriptomic profile and have specialized roles, underscoring the complex cellular landscape contributing to fibroid pathogenesis. Furthermore, not all smooth muscle cells in MED12-mutant fibroid carry the MED12 mutation, suggesting that MED12-mutant fibroids might not be monoclonal in nature. This review describes the intricacies of fibroid biology revealed by single-cell RNA sequencing. These advances have identified new cellular targets for potential therapies, provided insights into treatment resistance, and laid the groundwork for more personalized approaches to fibroid management. As we continue to unravel the cellular and molecular complexity of fibroids, we anticipate that this knowledge will translate into more effective and less invasive treatments, ultimately improving outcomes for the millions of women affected by this condition.

X-chromosome inactivation pattern and telomere length in recurrent pregnancy loss

Recurrent pregnancy loss is a reproductive disorder affecting about 1 to 5 % of pregnant women worldwide that requires our attention, especially considering that about 50 % of cases are idiopathic. The present study is focused on testing a possible association between extreme skewed X-chromosome inactivation patterns and/or shortened telomeres with idiopathic cases since both are considered non-consensual potential causes underlying recurrent pregnancy loss in the scientific community. For this purpose, two groups of women were analyzed and compared: a group of women with idiopathic recurrent pregnancy loss and a second group of age-matched women with proven fertility, and both X-chromosome inactivation patterns and telomere length were measured and compared from maternal DNA extracted from peripheral blood. Our data showed no statistically significant differences between groups, suggesting no association between extreme skewed X-chromosome inactivation or shortened telomeres with recurrent pregnancy losses. Additionally, the effect of maternal age on both X-chromosome inactivation pattern and telomere length was tested, but no significant correlation was observed between advanced maternal age and extreme skewed X-chromosome inactivation or telomere shortening. This study represents one more valid contribution to the investigation of causes underlying recurrent pregnancy loss suggesting that, new variables may be considered since the pattern of X-chromosome inactivation and telomere length do not seem to be related to this reproductive disorder. Briefly, considering its clinical relevance, it is mandatory a continuous effort in the scientific community to cover new potential recurrent pregnancy loss-related causes.

Out of the Silence: Insights into How Genes Escape X-Chromosome Inactivation

The silencing of all but one X chromosome in mammalian cells is a remarkable epigenetic process leading to near dosage equivalence in X-linked gene products between the sexes. However, equally remarkable is the ability of a subset of genes to continue to be expressed from the otherwise inactive X chromosome-in some cases constitutively, while other genes are variable between individuals, tissues or cells. In this review we discuss the advantages and disadvantages of the approaches that have been used to identify escapees. The identity of escapees provides important clues to mechanisms underlying escape from XCI, an arena of study now moving from correlation to functional studies. As most escapees show greater expression in females, the not-so-inactive X chromosome is a substantial contributor to sex differences in humans, and we highlight some examples of such impact.

X chromosome inactivation skewing is common in advanced carotid atherosclerotic lesions in females and predicts secondary peripheral artery events

BACKGROUND AND AIM

Sex differences in atherosclerosis have been described with female plaques being mostly perceived as stable and fibrous. Sex-specific mechanisms such as mosaic loss of the Y chromosome in men have been linked to cardiovascular health. In women, X-linked mechanisms such as X chromosome inactivation (XCI) skewing is common in several tissues. Yet, information on the role of XCI in female atherosclerotic plaques is lacking. Here, we investigated the presence of XCI skewing in advanced atherosclerotic lesions and its association with cardiovascular risk factors, histological plaque data, and clinical data.

METHODS

XCI skewing was quantified in 154 atherosclerotic plaque and 55 blood DNA samples of women included in the Athero-Express study. The skewing status was determined performing the HUMARA assay. Then, we studied the relationship of XCI skewing in female plaque and cardiovascular risk factors using regression models. In addition, we studied if plaque XCI predicted plaque composition, and adverse events during 3-years follow-up using Cox proportional hazard models.

RESULTS

XCI skewing was detected in 76 of 154 (49.4%) plaques and in 27 of 55 (67%) blood samples. None of the clinical risk factors were associated with plaque skewing. Plaque skewing was more often detected in plaques with a plaque hemorrhage (OR [95% CI]: 1.44 [1.06-1.98], P = 0.02). Moreover, skewed plaques were not associated with a higher incidence of composite and major events but were specifically associated with peripheral artery events during a 3-year follow-up period in a multivariate model (HR [95%CI]: 1.46 [1.09-1.97]; P = 0.007).

CONCLUSIONS

XCI skewing is common in carotid plaques of females and is predictive for the occurrence of peripheral artery events within 3 years after carotid endarterectomy.

A View on Uterine Leiomyoma Genesis through the Prism of Genetic, Epigenetic and Cellular Heterogeneity

Uterine leiomyomas (ULs), frequent benign tumours of the female reproductive tract, are associated with a range of symptoms and significant morbidity. Despite extensive research, there is no consensus on essential points of UL initiation and development. The main reason for this is a pronounced inter- and intratumoral heterogeneity resulting from diverse and complicated mechanisms underlying UL pathobiology. In this review, we comprehensively analyse risk and protective factors for UL development, UL cellular composition, hormonal and paracrine signalling, epigenetic regulation and genetic abnormalities. We conclude the need to carefully update the concept of UL genesis in light of the current data. Staying within the framework of the existing hypotheses, we introduce a possible timeline for UL development and the associated key events—from potential prerequisites to the beginning of UL formation and the onset of driver and passenger changes.

Single-cell sequencing reveals novel cellular heterogeneity in uterine leiomyomas

STUDY QUESTION

What are the cellular composition and single-cell transcriptomic differences between myometrium and leiomyomas as defined by single-cell RNA sequencing?

SUMMARY ANSWER

We discovered cellular heterogeneity in smooth muscle cells (SMCs), fibroblast and endothelial cell populations in both myometrium and leiomyoma tissues.

WHAT IS KNOWN ALREADY

Previous studies have shown the presence of SMCs, fibroblasts, endothelial cells and immune cells in myometrium and leiomyomas. However, there is no information on the cellular heterogeneity in these tissues and the transcriptomic differences at the single-cell level between these tissues.

STUDY DESIGN, SIZE, DURATION

We collected five leiomyoma and five myometrium samples from a total of eight patients undergoing hysterectomy. We then performed single-cell RNA sequencing to generate a cell atlas for both tissues. We utilized our single-cell sequencing data to define cell types, compare cell types by tissue type (leiomyoma versus myometrium) and determine the transcriptional changes at a single-cell resolution between leiomyomas and myometrium. Additionally, we performed MED12-variant analysis at the single-cell level to determine the genotype heterogeneity within leiomyomas.

PARTICIPANTS/MATERIALS, SETTING, METHODS

We collected five MED12-variant positive leiomyomas and five myometrium samples from a total of eight patients. We then performed single-cell RNA sequencing on freshly isolated single-cell preparations. Histopathological assessment confirmed the identity of the samples. Sanger sequencing was performed to confirm the presence of the MED12 variant in leiomyomas.

MAIN RESULTS AND ROLE OF CHANCE

Our data revealed previously unknown heterogeneity in the SMC, fibroblast cell and endothelial cell populations of myometrium and leiomyomas. We discovered the presence of two different lymphatic endothelial cell populations specific to uterine leiomyomas. We showed that both myometrium and MED12-variant leiomyomas are relatively similar in cellular composition but differ in cellular transcriptomic profiles. We found that fibroblasts influence the leiomyoma microenvironment through their interactions with endothelial cells, immune cells and SMCs. Variant analysis at the single-cell level revealed the presence of both MED12 variants as well as the wild-type MED12 allele in SMCs of leiomyomatous tissue. These results indicate genotype heterogeneity of cellular composition within leiomyomas.

LARGE SCALE DATA

The datasets are available in the NCBI Gene Expression Omnibus (GEO) using GSE162122.

LIMITATIONS, REASONS FOR CAUTION

Our study focused on MED12-variant positive leiomyomas for single-cell RNA sequencing analyses. Leiomyomas carrying other genetic rearrangements may differ in their cellular composition and transcriptomic profiles.

WIDER IMPLICATIONS FOR THE FINDINGS

Our study provides a cellular atlas for myometrium and MED12-variant positive leiomyomas as defined by single-cell RNA sequencing. Our analysis provides significant insight into the differences between myometrium and leiomyomas at the single-cell level and reveals hitherto unknown genetic heterogeneity in multiple cell types within human leiomyomas. Our results will be important for future studies into the origin and growth of human leiomyomas.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by funding from the National Institute of Child Health and Human Development (HD098580 and HD088629). The authors declare no competing interests.

Pitfalls of X-chromosome inactivation testing in females with Fabry disease

Fabry disease (FD) is an X-linked lysosomal storage disorder caused by mutations in the GLA gene encoding alpha-galactosidase A (AGAL). The impact of X-chromosome inactivation (XCI) on the phenotype of female FD patients remains unclear. In this study we aimed to determine pitfalls of XCI testing in a cohort of 35 female FD patients. XCI was assessed by two methylation-based and two allele-specific expression assays. The results correlated, although some variance among the four assays was observed. GLA transcript analyses identified crossing-over in three patients and detected mRNA instability in three out of four analyzed null alleles. AGAL activity correlated with XCI pattern and was not influenced by the mutation type or by reduced mRNA stability. Therefore, AGAL activity may help to detect crossing-over in patients with unstable GLA alleles. Tissue-specific XCI patterns in six patients, and age-related changes in two patients were observed. To avoid misinterpretation of XCI results in female FD patients we show that (i) a combination of several XCI assays generates more reliable results and minimizes possible biases; (ii) correlating XCI to GLA expression and AGAL activity facilitates identification of cross-over events; (iii) age- and tissue-related XCI specificities of XCI patterning should be considered.

Not Just Loss-of-Function Variations: Identification of a Hypermorphic Variant in a Patient With a CDKL5 Missense Substitution

Background and Objectives

CDKL5 deficiency disorder (CDD) is a neurodevelopmental encephalopathy characterized by early-onset epilepsy and impaired psychomotor development. Variations in the X-linked CDKL5 gene coding for a kinase cause CDD. Molecular genetics has proved that almost all pathogenic missense substitutions localize in the N-terminal catalytic domain, therefore underlining the importance for brain development and functioning of the kinase activity. CDKL5 also features a long C-terminal domain that acts as negative regulator of the enzymatic activity and modulates its subcellular distribution. CDD is generally attributed to loss-of-function variations, whereas the clinical consequences of increased CDKL5 activity remain uncertain. We have identified a female patient characterized by mild epilepsy and neurologic symptoms, harboring a novel c.2873C>G nucleotide substitution, leading to the missense variant p.(Thr958Arg). To increase our comprehension of genetic variants in CDKL5-associated neurologic disorders, we have characterized the molecular consequences of the identified substitution.

Methods

MRI and video EEG telemetry were used to describe brain activity and capture seizure. The Bayley III test was used to evaluate the patient development. Reverse transcriptase PCR was used to analyze whether the identified nucleotide variant affects messenger RNA stability and/or splicing. The X chromosome inactivation pattern was analyzed determining the DNA methylation status of the androgen receptor (AR) gene and by sequencing of expressed alleles. Western blotting was used to investigate whether the novel Thr958Arg substitution affects the stability and/or enzymatic activity of CDKL5. Immunofluorescence was used to define whether CDKL5 subcellular distribution is affected by the Thr958Arg substitution.

Results

Our data suggested that the proband tends toward a skewed X chromosome inactivation pattern in favor of the novel variant. The molecular investigation revealed that the p.(Thr958Arg) substitution leads to a significant increase in the autophosphorylation of both the TEY motif and residue Tyr¹⁷¹ of CDKL5, as well as in the phosphorylation of the target protein MAP1S, indicating an hyperactivation of CDKL5. This occurs without evidently affecting the kinase subcellular distribution.

Discussion

Our data provide a strong indication that the c.2873C>G nucleotide substitution represents an hypermorphic pathogenic variation of CDKL5, therefore highlighting the importance of a tight control of CDKL5 activity in the brain.

Use of the FMR1 Gene Methylation Status to Assess the X-Chromosome Inactivation Pattern: A Stepwise Analysis

X-chromosome inactivation (XCI) is a developmental process to compensate the imbalance in the dosage of X-chromosomal genes in females. A skewing of the XCI pattern may suggest a carrier status for an X-linked disease or explain the presence of a severe phenotype. In these cases, it is important to determine the XCI pattern, conventionally using the gold standard Human Androgen-Receptor Assay (HUMARA), based on the analysis of the methylation status at a polymorphic CAG region in the first exon of the human androgen receptor gene (AR). The aim of this study was to evaluate whether the methylation status of the fragile mental retardation protein translational regulator gene (FMR1) can provide an XCI pattern similar to that obtained by HUMARA. A set of 48 female carriers of FMR1 gene normal-sized alleles was examined using two assays: HUMARA and a FMR1 methylation PCR (mPCR). Ranges were defined to establish the XCI pattern using the methylation pattern of the FMR1 gene by mPCR. Overall, a 77% concordance of the XCI patterns was obtained between the two assays, which led us to propose a set of key points and a stepwise analysis towards obtaining an accurate result for the XCI pattern and to minimize the underlying pitfalls.

Lack of MECP2 gene transcription on the duplicated alleles of two related asymptomatic females with Xq28 duplications and opposite X-chromosome inactivation skewing

Xq28 duplication syndrome (MIM# 300815) is a severe neurodevelopmental disorder in males due to MeCP2 overexpression. Most females with MECP2 duplication are asymptomatic carriers, but there are phenotypic heterogeneities. Skewed X-chromosome inactivation (XCI) can protect females from exhibiting clinical phenotypes. Herein we reported two asymptomatic females (mother and grandmother) with interstitial Xq28 duplication. AR and RP2 assays showed that both had extremely skewed XCI, the Xq28 duplicated chromosome was inactivated in the mother, but was surprisingly activated in the grandmother. Interestingly, by combining RNA sequencing and whole-exome sequencing, we confirmed that XIST only expressed in the Xq28 duplication chromosomes of the two females, indicating that the Xq28 duplication chromosomes were inactive. Meanwhile, MECP2 and most XCI genes in the duplicated X-chromosomes were not transcriptionally expressed or upregulated, precluding major clinical phenotypes in the two females, especially the grandmother. We showed that XCI status detected using RNA sequencing was more relevant for establishing the clinical phenotype of MECP2 duplication in females. It suggested that there were other factors maintaining the XCI status in addition to DNA methylation, a possible additional inhibition mechanism occurred at the transcriptional level in the unmethylated X-chromosome, counter balancing the MECP2 duplication's detrimental phenotype effects.

Dosage Compensation in Females with X-Linked Metabolic Disorders

Through the use of new genomic and metabolomic technologies, our comprehension of the molecular and biochemical etiologies of genetic disorders is rapidly expanding, and so are insights into their varying phenotypes. Dosage compensation (lyonization) is an epigenetic mechanism that balances the expression of genes on heteromorphic sex chromosomes. Many studies in the literature have suggested a profound influence of this phenomenon on the manifestation of X-linked disorders in females. In this review, we summarize the clinical and genetic findings in female heterozygotic carriers of a pathogenic variant in one of ten selected X-linked genes whose defects result in metabolic disorders.

Polymorphisms of IRAK1 Gene on X Chromosome Is Associated with Hashimoto Thyroiditis in Korean Children

Autoimmune thyroid disease (AITD) is predominant in females and has been focused on the sexual diploid in immune response. The IL-1 receptor-associated kinase 1 (IRAK1) gene on the X chromosome was recently suggested as strong autoimmune disease-susceptible loci, second to the major histocompatibility complex region. We investigated the frequency of IRAK1 single-nucleotide polymorphisms (SNPs) in children with AITD. In this study, we observed that SNPs of IRAK1 including rs3027898, rs1059703, and rs1059702 in 115 Korean AITD pediatric patients (Graves' disease = 74 [females = 52/males = 22]; Hashimoto disease [HD] = 41 [females = 38/males = 3]; thyroid-associated ophthalmopathy [TAO] = 40 (females = 27/males = 13); without TAO = 75 (females = 63/males = 12); total males = 25, total females = 90; mean age = 11.9 years) and 204 healthy Korean individuals (males = 104/females = 100). The data from cases and controls were analyzed from separate sex-stratified or all combined by χ 2 test for categorical variables and Student t test for numerical variables. Our study revealed that SNPs of IRAK1-associated HD and without TAO but Graves' disease and TAO were not found significant. When cases and controls were analyzed by separate sex, we found that rs3027898 AA, rs1059703 AA, and rs1059702 GG showed disease susceptibility in female AITD, HD, and without TAO. Also, all rs3027898, rs1059703, and rs1059702 were found to be in strong linkage disequilibrium (D' = 0.96-0.98, r2 = 0.83-0.97). The haplotype of 3 SNPs was higher in AITD than in controls (CGA, r2 = 5.42, P = 0.019). Our results suggest that IRAK1 polymorphisms may contribute to the pathogenesis of HD, AITD, and without thyroid-associated ophthalmopathy for females.

Clonal hematopoiesis in hematological disorders: Three different scenarios

Clonality studies can establish the single-cell origin of tumors and thus differentiate clonal malignant and premalignant processes from reactive polyclonal processes. Detection of clonal cells may be based on direct tracking of cell lineage-specific sequences or disease-specific somatic mutations identifying the clonal population. Historically, clonal hematopoiesis was defined using the principle of X-chromosome inactivation based on observation that in circulating clonal cells, only one of the active chromosomes was expressed. In myeloproliferative neoplasms (MPNs) virtually all circulating erythrocytes, platelets, and granulocytes are products of single mutated stem cells that preferentially differentiate into the myeloid rather than lymphoid lineage. Thus, clonal differentiated myeloid cells co-exist in circulation with polyclonal long-lived T lymphocytes that originated before the MPN-initiating somatic clonal event. Chronic lymphocytic leukemia (CLL) starts in a differentiating B cell, but other lymphoid lineages and myeloid cells remain polyclonal. Normal T and B cells co-exist with the CLL clone, but are diluted by the massively expanded CLL population, which outnumbers the residual normal cells. Clonal hematopoiesis of undetermined potential (CHIP) has been identified by whole-genome sequencing of healthy individuals. These clones contain a specific somatic mutation previously considered to be disease defining but are detected in only a small proportion of circulating leukocytes, and there is no obvious suppression of normal hematopoietic stem cells. However, more studies are needed to properly define these clones, their persistence or disappearance, and their relative propensity for transforming into leukemias, myeloproliferative neoplasms, or other clonal hematological malignancies.

Random X chromosome inactivation in patients with Klinefelter syndrome

Background

X chromosome inactivation (XCI) is an indispensable process in the development of human female embryos. Reportedly, XCI occurs when a blastocyst contains 10–12 embryonic progenitor cells. To date, it remains unclear whether XCI ratios are normally preserved in Klinefelter syndrome (KS) patients with 47,XXY karyotype.

Methods

We examined XCI ratios in 18 KS patients through DNA methylation analysis for the polymorphic trinucleotide locus in the AR gene. The results of the KS patients were compared to previous data from healthy young women.

Results

XCI ratios in KS patients followed a normal distribution. Skewed XCI was observed in two patients, one of whom exhibited extremely skewed XCI. The frequencies of skewed and extremely skewed XCI in the KS cohort were comparable to those in healthy women.

Conclusion

This study confirmed the rarity of skewed XCI in KS patients. These results indicate that the presence of a supernumerary X chromosome during the cleavage and early blastocyst stages does not affect the developmental tempo of embryos. Our data deserve further validation.

JAK2 ex13InDel drives oncogenic transformation and is associated with chronic eosinophilic leukemia and polycythemia vera

The V617F mutation in the JH2 domain of Janus kinase 2 (JAK2) is an oncogenic driver in several myeloproliferative neoplasms (MPNs), including essential thrombocythemia, myelofibrosis, and polycythemia vera (PV). Other mutations in JAK2 have been identified in MPNs, most notably exon 12 mutations in PV. Here, we describe a novel recurrent mutation characterized by a common 4-amino-acid deletion and variable 1-amino-acid insertion (Leu583-Ala586DelInsSer/Gln/Pro) within the JH2 domain of JAK2. All 4 affected patients had eosinophilia, and both patients with Leu583-Ala586DelInsSer fulfilled diagnostic criteria of both PV and chronic eosinophilic leukemia (CEL). Computational and functional studies revealed that Leu583-Ala586DelInsSer (herein referred to as JAK2ex13InDel) deregulates JAK2 through a mechanism similar to JAK2V617F, activates signal transducer and activator of transcription 5 and extracellular signal-regulated kinase, and transforms parental Ba/F3 cells to growth factor independence. In contrast to JAK2V617F, JAK2ex13InDel does not require an exogenous homodimeric type 1 cytokine receptor to transform Ba/F3 cells and is capable of activating β common chain family cytokine receptor (interleukin-3 receptor [IL-3R], IL-5R, and granulocyte-macrophage colony stimulating factor receptor) signaling in the absence of ligand, with the maximum effect observed for IL-5R, consistent with the clinical phenotype of eosinophilia. Recognizing this new PV/CEL-overlap MPN has significant clinical implications, as both PV and CEL patients are at high risk for thrombosis, and concomitant cytoreduction of red cells, neutrophils, and eosinophils may be required for prevention of thromboembolic events. Targeted next-generation sequencing for genes recurrently mutated in myeloid malignancies in patients with unexplained eosinophilia may reveal additional cases of Leu583-Ala586DelInsSer/Gln/Pro, allowing for complete characterization of this unique MPN.

X chromosome inactivation does not necessarily determine the severity of the phenotype in Rett syndrome patients

Rett syndrome (RTT) is a severe neurological disorder usually caused by mutations in the MECP2 gene. Since the MECP2 gene is located on the X chromosome, X chromosome inactivation (XCI) could play a role in the wide range of phenotypic variation of RTT patients; however, classical methylation-based protocols to evaluate XCI could not determine whether the preferentially inactivated X chromosome carried the mutant or the wild-type allele. Therefore, we developed an allele-specific methylation-based assay to evaluate methylation at the loci of several recurrent MECP2 mutations. We analyzed the XCI patterns in the blood of 174 RTT patients, but we did not find a clear correlation between XCI and the clinical presentation. We also compared XCI in blood and brain cortex samples of two patients and found differences between XCI patterns in these tissues. However, RTT mainly being a neurological disease complicates the establishment of a correlation between the XCI in blood and the clinical presentation of the patients. Furthermore, we analyzed MECP2 transcript levels and found differences from the expected levels according to XCI. Many factors other than XCI could affect the RTT phenotype, which in combination could influence the clinical presentation of RTT patients to a greater extent than slight variations in the XCI pattern.

Aberrant X chromosome skewing and acquired clonal hematopoiesis in adult-onset common variable immunodeficiency

Advances in genomic medicine have elucidated an increasing number of genetic etiologies for patients with common variable immunodeficiency (CVID). However, there is heterogeneity in clinical and immunophenotypic presentations and a limited understanding of the underlying pathophysiology of many cases. The primary defects in CVID may extend beyond the adaptive immune system, and the combined defect in both the myeloid and lymphoid compartments suggests the mechanism may involve bone marrow output and earlier progenitors. Using the methylation profile of the human androgen receptor (AR) gene as a surrogate epigenetic marker for bone marrow clonality, we examined the hematopoietic compartments of patients with CVID. Our data show that clonal hematopoiesis is common among patients with adult-onset CVID who do not have associated noninfectious complications. Nonblood tissues did not show a skewed AR methylation status, supporting a model of an acquired clonal hematopoietic event. Attenuation of memory B cell differentiation into long-lived plasma cells (CD20^–CD27⁺CD38⁺CD138⁺) was associated with marked changes in the postdifferentiation methylation profile, demonstrating the functional consequence of clonal hematopoiesis on humoral immunity in these patients. This study sheds light on a potential etiology of a subset of patients with CVID, and the findings suggest that it is a stage of an acquired lymphocyte maturation disorder.

Clonal hematopoiesis is common among a subset of patients with common variable immunodeficiency (CVID), suggesting that CVID may be a stage of lymphoid dysplasia.

Development of canine X-chromosome inactivation pattern analysis for the detection of cell clonality by incorporating the examination of the SLIT and NTRK-like family member 4 (SLITRK4) gene

X-chromosome inactivation pattern (XCIP) analysis can be used to assess the clonality of cell populations of various origin by distinguishing the methylated X chromosome from the unmethylated X chromosome. In this study, the utility of XCIP analysis was improved by incorporating the examination of AC dinucleotide repeats in SLIT and NTRK-like family member 4 (SLITRK4) gene into the previously reported CAG repeat examination of androgen receptor (AR) gene in dogs. The rate of heterozygosity when both genes were analysed (125/150, 83.3%) was higher than AR gene examination alone (86/150, 57.3%). Blood samples from heterozygous dogs in either AC-1 or AC-2 of SLITRK4 gene were examined for the corrected inactivation allele ratio (CIAR), resulting in the determination of a reference range of CIAR <3.8 in non-neoplastic cell/tissue samples. Using this analytical method, 49% (21/43) of neoplastic tissue samples from dogs showed a CIAR >3.8, indicating the presence of a clonal population. Through the present study, the availability of canine XCIP analysis was improved by incorporating the examination of the SLITRK4 gene, providing a highly useful laboratory examination system for the detection of the clonality of various cell/tissue samples in dogs.

Skewed X-inactivation is common in the general female population

X-inactivation is a well-established dosage compensation mechanism ensuring that X-chromosomal genes are expressed at comparable levels in males and females. Skewed X-inactivation is often explained by negative selection of one of the alleles. We demonstrate that imbalanced expression of the paternal and maternal X-chromosomes is common in the general population and that the random nature of the X-inactivation mechanism can be sufficient to explain the imbalance. To this end, we analyzed blood-derived RNA and whole-genome sequencing data from 79 female children and their parents from the Genome of the Netherlands project. We calculated the median ratio of the paternal over total counts at all X-chromosomal heterozygous single-nucleotide variants with coverage ≥10. We identified two individuals where the same X-chromosome was inactivated in all cells. Imbalanced expression of the two X-chromosomes (ratios ≤0.35 or ≥0.65) was observed in nearly 50% of the population. The empirically observed skewing is explained by a theoretical model where X-inactivation takes place in an embryonic stage in which eight cells give rise to the hematopoietic compartment. Genes escaping X-inactivation are expressed from both alleles and therefore demonstrate less skewing than inactivated genes. Using this characteristic, we identified three novel escapee genes (SSR4, REPS2, and SEPT6), but did not find support for many previously reported escapee genes in blood. Our collective data suggest that skewed X-inactivation is common in the general population. This may contribute to manifestation of symptoms in carriers of recessive X-linked disorders. We recommend that X-inactivation results should not be used lightly in the interpretation of X-linked variants.

Clonality analysis of multifocal ipsilateral breast carcinomas using X-chromosome inactivation patterns

The definition of multifocal breast cancer is ambiguous, and its incidence varies depending on the definition and detection methods. Multifocal breast cancers either have the same clonal origin or arise from completely distinct progenitor cells. The current American Joint Committee on Cancer Staging system and College of American Pathologists breast tumor guidelines state that only the largest tumor needs to be staged and studied immunohistochemically, on the assumption that they are of the same origin. However, some multifocal tumors have been proved to have arisen from different clones. In the present study, 71 cases of surgically resected multifocal breast cancers were selected. To detect and characterize the tumors of each clonal origin, a human androgen receptor gene (HUMARA) assay to compare the X-chromosome inactivation patterns of multiple tumors was conducted. Twenty-nine of 71 (40.8%) patients were revealed to be heterozygous for HUMARA. Sixty-four (90.1%) patients had the same X chromosome inactivated in different tumors. Seven (9.9%) cases had different inactivated X chromosomes between multifocal tumors, indicating that those tumors were from separate progenitor cells. Five (7.0%) cases showed identical histologic features but had different inactivated HUMARA alleles. According to these results, 2 separate tumors might be synchronous primary tumors, although their histopathologic characteristics are similar. Furthermore, multifocal tumors can be of different origins despite being closely located to each other. These findings suggest that separate grouping of multiple breast tumors based on their clonal origin is needed for future studies.

Familial MPN Predisposition

Chronic myeloproliferative neoplasms (MPN) characteristically arise from a somatic mutation in the pluripotent hematopoietic stem cell, and most common recurring mutations are in the JAK2, CALR, and cMPL genes. However, these mutations are not founder mutations, but mainly drive the disease phenotype and a pre-existing germline predisposition has been long speculated, but has not been clearly defined to date. Genome-wide association studies in family clusters of MPN have identified a number of genetic variants that are associated with increased germline risk for developing clonal MPN. The strongest association discovered so far is the presence of JAK2 46/1 haplotype, and subsequently, many studies have found additional variants in other genes, most notably in TERT gene. However, these still account for a small fraction of familial MPN, and more in-depth studies including whole genome sequencing are needed to gain better insight into familial genetic predisposition of clonal MPNs.

ICUS/CCUS/CHIP: basics & beyond

INTRODUCTION

Patients presenting with idiopathic cytopenia with non-diagnostic marrow morphology and a normal karyotype pose a diagnostic and therapeutic challenge. Additional diagnostic information from mutation analysis could provide important clinical insights. However, one has to be cautious during such diagnostic interpretations in view of the recent documentation of clonal somatic mutations in healthy elder individuals. Whether to regard clonality synonymous with malignant proliferation or a manifestation of ageing process is to be judged carefully. Areas covered: The review covers defining criteria and diagnostic work up for Idiopathic cytopenia of undetermined significance (ICUS), Clonal cytopenia of undetermined significance (CCUS), Clonal hematopoiesis of indeterminate potential (CHIP). It also presents the results from previous reports on this subject. In addition the evolution and potential impact of these entities is discussed. Expert commentary: Current evidence does not support the use of somatic mutations as presumptive evidence of myelodysplastic syndrome (MDS). Including CCUS under the category of MDS requires further insight on natural disease course. Longitudinal follow up study on ICUS, CCUS, CHIP may eventually identify the pathological significance of the clonal mutations. An absence of mutation however may still be useful as good predictor of not having MDS.

Microsatellites within the feline androgen receptor are suitable for X chromosome-linked clonality testing in archival material

Objectives A hallmark of neoplasms is their origin from a single cell; that is, clonality. Many techniques have been developed in human medicine to utilise this feature of tumours for diagnostic purposes. One approach is X chromosome-linked clonality testing using polymorphisms of genes encoded by genes on the X chromosome. The aim of this study was to determine if the feline androgen receptor gene was suitable for X chromosome-linked clonality testing. Methods The feline androgen receptor gene was characterised and used to test clonality of feline lymphomas by PCR and polyacrylamide gel electrophoresis, using archival formalin-fixed, paraffin-embedded material. Results Clonality of the feline lymphomas under study was confirmed and the gene locus was shown to represent a suitable target in clonality testing. Conclusions and relevance Because there are some pitfalls of using X chromosome-linked clonality testing, further studies are necessary to establish this technique in the cat.

X-Chromosome Inactivation Analysis in Different Cell Types and Induced Pluripotent Stem Cells Elucidates the Disease Mechanism in a Rare Case of Mucopolysaccharidosis Type II in a Female

Mucopolysaccharidosis type II (MPS II) is an X-linked lysosomal storage disorder resulting from deficiency of iduronate-2-sulphatase activity. The disease manifests almost exclusively in males; only 16 symptomatic heterozygote girls have been reported so far. We describe the results of X-chromosome inactivation analysis in a 5-year-old girl with clinically severe disease and heterozygous mutation p.Arg468Gln in the IDS gene. X inactivation analysed at three X-chromosome loci showed extreme skewing (96/4 to 99/1) in two patient's cell types. This finding correlated with exclusive expression of the mutated allele. Induced pluripotent stem cells (iPSC) generated from the patient's peripheral blood demonstrated characteristic pluripotency markers, deficiency of enzyme activity, and mutation in the IDS gene. These cells were capable of differentiation into other cell types (cardiomyocytes, neurons). In MPS II iPSC clones, the X inactivation ratio remained highly skewed in culture conditions that led to partial X inactivation reset in Fabry disease iPSC clones. Our data, in accordance with the literature, suggest that extremely skewed X inactivation favouring the mutated allele is a crucial condition for manifestation of MPS II in females. This suggests that the X inactivation status and enzyme activity have a prognostic value and should be used to evaluate MPS II in females. For the first time, we show generation of iPSC from a symptomatic MPS II female patient that can serve as a cellular model for further research of the pathogenesis and treatment of this disease.

Buccal swab as a reliable predictor for X inactivation ratio in inaccessible tissues

Background

As a result of the epigenetic phenomenon of X chromosome inactivation (XCI) every woman is a mosaic of cells with either an inactive paternal X chromosome or an inactive maternal X chromosome. The ratio between inactive paternal and maternal X chromosomes is different for every female individual, and can influence an X-encoded trait or disease. A multitude of X linked conditions is known, and for many of them it is recognised that the phenotype in affected female carriers of the causative mutation is modulated by the XCI ratio. To predict disease severity an XCI ratio is usually determined in peripheral blood samples. However, the correlation between XCI ratios in peripheral blood and disease affected tissues, that are often inaccessible, is poorly understood. Here, we tested several tissues obtained from autopsies of 12 female individuals for patch size and XCI ratio.

Methods

XCI ratios were analysed using methyl-sensitive PCR-based assays for the AR, PCSK1N and SLITRK4 loci. XCI patch size was analysed by testing the XCI ratio of tissue samples with decreasing size.

Results

XCI patch size was analysed for liver, muscle, ovary and brain samples and was found too small to confound testing for XCI ratio in these tissues. XCI ratios were determined in the easily accessible tissues, blood, buccal epithelium and hair follicle, and compared with ratios in several inaccessible tissues.

Conclusions

Buccal epithelium is preferable over peripheral blood for predicting XCI ratios of inaccessible tissues. Ovary is the only inaccessible tissue showing a poor correlation to blood and buccal epithelium, but has a good correlation to hair follicle instead.

X Chromosome Inactivation in Opioid Addicted Women

INTRODUCTION

X chromosome inactivation (XCI) is a process during which one of the two X chromosomes in female human is silenced leading to equal gene expression with males who have only one X chromosome. Here we have investigated XCI ratio in females with opioid addiction to see whether XCI skewness in women could be a risk factor for opioid addiction.

METHODS

30 adult females meeting DSM IV criteria for opioid addiction and 30 control females with no known history of addiction were included in the study. Digested and undigested DNA samples which were extracted from blood were analyzed after amplification of the polymorphic androgen receptor (AR) gene located on the X chromosome. XCI skewness was studied in 3 ranges: 50:50-64:36 (random inactivation), 65:35-80:20 (moderately skewed) and >80:20 (highly skewed).

RESULTS

XCI from informative females in control group was 63% (N=19) random, 27% (N=8) moderately skewed and 10% (N=3) highly skewed. Addicted women showed 57%, 23% and 20%, respectively. The distribution and frequency of XCI status in women with opioid addiction was not significantly different from control group (P=0.55).

DISCUSSION

Our data did not approve our hypothesis of increased XCI skewness among women with opioid addiction or unbalanced (non-random) expression of genes associated with X chromosome in female opioid addicted subjects.

Characterization of X chromosome inactivation using integrated analysis of whole-exome and mRNA sequencing

In females, X chromosome inactivation (XCI) is an epigenetic, gene dosage compensatory mechanism by inactivation of one copy of X in cells. Random XCI of one of the parental chromosomes results in an approximately equal proportion of cells expressing alleles from either the maternally or paternally inherited active X, and is defined by the XCI ratio. Skewed XCI ratio is suggestive of non-random inactivation, which can play an important role in X-linked genetic conditions. Current methods rely on indirect, semi-quantitative DNA methylation-based assay to estimate XCI ratio. Here we report a direct approach to estimate XCI ratio by integrated, family-trio based whole-exome and mRNA sequencing using phase-by-transmission of alleles coupled with allele-specific expression analysis. We applied this method to in silico data and to a clinical patient with mild cognitive impairment but no clear diagnosis or understanding molecular mechanism underlying the phenotype. Simulation showed that phased and unphased heterozygous allele expression can be used to estimate XCI ratio. Segregation analysis of the patient's exome uncovered a de novo, interstitial, 1.7 Mb deletion on Xp22.31 that originated on the paternally inherited X and previously been associated with heterogeneous, neurological phenotype. Phased, allelic expression data suggested an 83∶20 moderately skewed XCI that favored the expression of the maternally inherited, cytogenetically normal X and suggested that the deleterious affect of the de novo event on the paternal copy may be offset by skewed XCI that favors expression of the wild-type X. This study shows the utility of integrated sequencing approach in XCI ratio estimation.

Identification of novel informative loci for DNA-based X-inactivation analysis

The HUMARA assay, the most common method for evaluation of X-inactivation skewing in blood cells, has been reported to be usable in only about 80% of females, emphasizing the need for alternative methods for testing of HUMARA-uninformative individuals. We conducted an in silico search for potentially polymorphic tri-to-hexanucleotide repeats in the proximity of CpG islands located in 5' regions of X-chromosome genes to design five candidate assays (numbered I, II, III, IV, and V) combining methylation-specific restriction digest with PCR amplification in a manner similar to the HUMARA assay. The results obtained by these assays in 100 healthy females were compared to X-inactivation skewing measured by the AR-MSP method which is based on methylation-specific PCR amplification of the first exon of the AR gene. On the basis of statistical evidence, three of the novel assays (II, IV, and V), which were informative in 18%, 61%, and 55% of females in the cohort, respectively, may be used as alternatives or conjointly with the HUMARA assay to improve its reliability. The three new assays were combined with the HUMARA assay into a novel X-inactivation test leading to the increase of informative females in the cohort from 67% to 96%.

A genetic mechanism for Tibetan high-altitude adaptation

Tibetans do not exhibit increased hemoglobin concentration at high altitude. We describe a high-frequency missense mutation in the EGLN1 gene, which encodes prolyl hydroxylase 2 (PHD2), that contributes to this adaptive response. We show that a variant in EGLN1, c.[12C>G; 380G>C], contributes functionally to the Tibetan high-altitude phenotype. PHD2 triggers the degradation of hypoxia-inducible factors (HIFs), which mediate many physiological responses to hypoxia, including erythropoiesis. The PHD2 p.[Asp4Glu; Cys127Ser] variant exhibits a lower K(m) value for oxygen, suggesting that it promotes increased HIF degradation under hypoxic conditions. Whereas hypoxia stimulates the proliferation of wild-type erythroid progenitors, the proliferation of progenitors with the c.[12C>G; 380G>C] mutation in EGLN1 is significantly impaired under hypoxic culture conditions. We show that the c.[12C>G; 380G>C] mutation originated ∼8,000 years ago on the same haplotype previously associated with adaptation to high altitude. The c.[12C>G; 380G>C] mutation abrogates hypoxia-induced and HIF-mediated augmentation of erythropoiesis, which provides a molecular mechanism for the observed protection of Tibetans from polycythemia at high altitude.

Juvenile myelomonocytic leukaemia and Noonan syndrome

BACKGROUND

Infants with Noonan syndrome (NS) are predisposed to developing juvenile myelomonocytic leukaemia (JMML) or JMML-like myeloproliferative disorders (MPD). Whereas sporadic JMML is known to be aggressive, JMML occurring in patients with NS is often considered as benign and transitory. However, little information is available regarding the occurrence and characteristics of JMML in NS.

METHODS AND RESULTS

Within a large prospective cohort of 641 patients with a germline PTPN11 mutation, we identified MPD features in 36 (5.6%) patients, including 20 patients (3%) who fully met the consensus diagnostic criteria for JMML. Sixty percent of the latter (12/20) had severe neonatal manifestations, and 10/20 died in the first month of life. Almost all (11/12) patients with severe neonatal JMML were males. Two females who survived MPD/JMML subsequently developed another malignancy during childhood. Although the risk of developing MPD/JMML could not be fully predicted by the underlying PTPN11 mutation, some germline PTPN11 mutations were preferentially associated with myeloproliferation: 10/48 patients with NS (20.8%) with a mutation in codon Asp61 developed MPD/JMML in infancy. Patients with a p.Thr73Ile mutation also had more chances of developing MPD/JMML but with a milder clinical course. SNP array and whole exome sequencing in paired tumoral and constitutional samples identified no second acquired somatic mutation to explain the occurrence of myeloproliferation.

CONCLUSIONS

JMML represents the first cause of death in PTPN11-associated NS. Few patients have been reported so far, suggesting that JMML may sometimes be overlooked due to early death, comorbidities or lack of confirmatory tests.

Demonstration of the Cell Clonality in Canine Hematopoietic Tumors by X-chromosome Inactivation Pattern Analysis

X-chromosome inactivation pattern (XCIP) analysis has been widely used to assess cell clonality in various types of human neoplasms. In this study, a polymerase chain reaction–based canine XCIP analysis of the androgen receptor ( AR) gene was applied for the assessment of cell clonality in canine hematopoietic tumors. This XCIP analysis is based on the polymorphic CAG repeats in the AR gene and the difference of methylation status between active and inactive X chromosomes. We first examined the polymorphisms of 2 CAG tandem repeats in the AR gene in 52 male and 150 female dogs of various breeds. The 2 polymorphic CAG repeats contained 9 to 12 and 10 to 14 CAGs in the first and second CAG repeats, respectively. Of the 150 female dogs, 74 (49.3%) were heterozygous for the first and/or second polymorphic CAG tandem repeats, indicating the utility of XCIP analysis in these dogs. Canine XCIP analysis was then applied to clinical samples from female dogs with canine high-grade lymphoma, chronic myelogenous leukemia, acute myelogenous leukemia, and benign lymph node hyperplasia. Of 10 lymphoma cell samples, 9 (90%) showed skewed XCIPs, indicating their clonal origins, whereas all the nonneoplastic lymph node samples showed balanced XCIPs. Moreover, bone marrow specimen from a dog with acute myelogenous leukemia and peripheral leukocyte specimens from 2 dogs with chronic myelogenous leukemia showed skewed XCIPs. XCIP analysis was successfully employed to demonstrate the cell clonality of canine hematopoietic tumors in this study and will be applicable to evaluate the clonality in various proliferative disorders in dogs.

Clonality of smooth muscle and fibroblast cell populations isolated from human fibroid and myometrial tissues

Uterine fibroids are conventionally defined as clonally derived benign tumours from the proliferation of a single smooth muscle cell (SMC). We have previously identified fibroblast-like cells in fibroids, the presence of which raises the question as to whether all cells within the fibroid have the same clonal origin. The first aim of this study was to develop a fluorescence-activated cell sorting (FACS)-based method to isolate different cell types from human myometrium and fibroid tissues. Secondly, we aimed to use X chromosome inactivation analysis to determine the clonality of cell subpopulations isolated from myometrial and fibroid tissues. Human myometrium and fibroid tissues were collected from women undergoing hysterectomy. Immunohistochemistry (IHC) and flow cytometry confirmed that in addition to SMCs, fibroblasts constitute a significant proportion of cells in human myometrium and fibroid tissues. FACS based on CD90 and ALDH1 reliably separated cells into three myometrial and four fibroid subpopulations: SMCs, vascular smooth muscle cells and two fibroblast subsets. Clonality was first determined by X chromosome inactivation using the classic DNA methylation-sensitive HUMARA assay. Data from this assay were highly variable, with only a quarter of samples meeting the definition of clonal fibroid and non-clonal myometrium. However, using an RNA-based X chromosome inactivation HUMARA assay, we were able to demonstrate clonality of all cellular constituents of most fibroids. Our results confirm that most fibroids are derived from a single cell, and for the first time demonstrates that these clonal cells differentiate into fibroblast and SMC subpopulation as the fibroid grows.

Analysis of expressed SNPs identifies variable extents of expression from the human inactive X chromosome

BACKGROUND

X-chromosome inactivation (XCI) results in the silencing of most genes on one X chromosome, yielding mono-allelic expression in individual cells. However, random XCI results in expression of both alleles in most females. Allelic imbalances have been used genome-wide to detect mono-allelically expressed genes. Analysis of X-linked allelic imbalance in females with skewed XCI offers the opportunity to identify genes that escape XCI with bi-allelic expression in contrast to those with mono-allelic expression and which are therefore subject to XCI.

RESULTS

We determine XCI status for 409 genes, all of which have at least five informative females in our dataset. The majority of genes are subject to XCI and genes that escape from XCI show a continuum of expression from the inactive X. Inactive X expression corresponds to differences in the level of histone modification detected by allelic imbalance after chromatin immunoprecipitation. Differences in XCI between populations and between cell lines derived from different tissues are observed.

CONCLUSIONS

We demonstrate that allelic imbalance can be used to determine an inactivation status for X-linked genes, even without completely non-random XCI. There is a range of expression from the inactive X. Genes escaping XCI, including those that do so in only a subset of females, cluster together, demonstrating that XCI and location on the X chromosome are related. In addition to revealing mechanisms involved in cis-gene regulation, determining which genes escape XCI can expand our understanding of the contributions of X-linked genes to sexual dimorphism.

Evaluation of X-Chromosome Inactivation Patterns in Patients with Acute Myeloid Leukemia during Remission

The aim of this study was to evaluate the patterns of X-chromosome inactivation during the remission in acute myeloid leukemia (AML) at the RNA level. Two hundred normal females and 45 female patients with AML entered the study. The frequency of heterozygosity was 48.5% (119/245) for P55, 40% (93/245) for IDS, and only 28.9% (71/245) for G6PD. Some individuals were heterozygous for more than one gene polymorphism. Overall, one hundred normal individuals proved showed to be heterozygous for at least one of the above polymorphisms. 92/100 (92%) normal females showed a polyclonal pattern. Clonal patterns were observed in 44/45 (98%) AML patients at presentation. Of 27 patients who were followed after remission, 23 (85.2%) patients showed a clonal pattern. Ten patients were available for a longer followup (up to 12 months) and the clonal pattern was observed in seven patients. It can be concluded that clonality at remission is a frequent event in AML and does not necessarily mean relapse of the disease. There is also a possibility of conversion of clonality to polyclonality over time.