Spinal Cord Stimulation Research in the Restoration of Motor, Sensory, and Autonomic Function for Individuals Living With Spinal Cord Injuries: A Scoping Review.

OBJECTIVE: To describe the status of spinal cord stimulation (SCS) research for the improvement of motor, sensory, and autonomic function for individuals living with a spinal cord injury (SCI). DATA SOURCES: This scoping review identified original research published before March 31, 2021, via literature searches using MEDLINE, Embase, PubMed, Science Direct, Cumulative Index to Nursing and Allied Health, Sport Discus, and Web of Science, as well as a targeted search for well-known principal investigators. Search terms included permutations of "spinal cord stimulation," "epidural spinal cord stimulation," "transcutaneous spinal cord stimulation," "magnetic spinal cord stimulation," and "neuromodulation." STUDY SELECTION: Studies were included if they (1) were in English, (2) presented original research on humans living with a SCI, and (3) investigated at least 1 of the 3 forms of SCS. DATA EXTRACTION: Extracted data included authors, publication year, participant characteristics, purpose, study design, stimulation (device, location, parameters), primary outcomes, and adverse events. DATA SYNTHESIS: As a scoping review the extracted data were tabulated and presented descriptively. Themes and gaps in the literature were identified and reported. Of the 5754 articles screened, 103 articles were included (55 epidural, 36 transcutaneous, 12 magnetic). The primary research design was a case study or series with only a single randomized controlled trial. Motor recovery was the most common primary outcome for epidural and transcutaneous SCS studies, whereas bowel and bladder outcomes were most common for magnetic SCS studies. Seventy percent of the studies included 10 or fewer participants, and 18 articles documented at least 1 adverse event. Incomplete stimulation parameter descriptions were noted across many studies. No articles mentioned direct engagement of consumers or advocacy groups. CONCLUSIONS: This review identified a need for more robust study designs, larger sample sizes, comparative studies, improved reporting of stimulation parameters, adverse event data, and alignment of outcomes with the priorities of the community with SCI.

Incidence and prevalence of traumatic spinal cord injury in Canada using health administrative data.

Introduction: Incidence and prevalence data are needed for the planning, funding, delivery and evaluation of injury prevention and health care programs. The objective of this study was to estimate the Canadian traumatic spinal cord injury (TSCI) incidence, prevalence and trends over time using national-level health administrative data. Methods: ICD-10 CA codes were used to identify the cases for the hospital admission and discharge incidence rates of TSCI in Canada from 2005 to 2016. Provincial estimates were calculated using the location of the admitting facility. Age and sex-specific incidence rates were set to the 2015/2016 rates for the 2017 to 2019 estimates. Annual incidence rates were used as input for the prevalence model that applied annual survivorship rates derived from life expectancy data. Results: For 2019, it was estimated that there were 1,199 cases (32.0 per million) of TSCI admitted to hospitals, with 123 (10% of admissions) in-hospital deaths and 1,076 people with TSCI (28.7 per million) were discharged in Canada. The estimated number of people living with TSCI was 30,239 (804/million); 15,533 (52%) with paraplegia and 14,706 (48%) with tetraplegia. Trends included an increase in the number of people injured each year from 874 to 1,199 incident cases (37%), an older average age at injury rising from 46.6 years to 54.3 years and a larger proportion over the age of 65 changing from 22 to 38%, during the 15-year time frame. Conclusion: This study provides a standard method for calculating the incidence and prevalence of TSCI in Canada using national-level health administrative data. The estimates are conservative based on the limitations of the data but represent a large Canadian sample over 15 years, which highlight national trends. An increasing number of TSCI cases among the elderly population due to falls reported in this study can inform health care planning, prevention strategies, and future research.

Spinal Cord Injury Community Personal Opinions and Perspectives on Spinal Cord Stimulation.

Background: Spinal cord stimulation (SCS) clinical trials are evaluating its efficacy and safety for motor, sensory, and autonomic recovery following spinal cord injury (SCI). The perspectives of people living with SCI are not well known and can inform the planning, delivery, and translation of SCS. Objectives: To obtain input from people living with SCI on the top priorities for recovery, expected meaningful benefits, risk tolerance, clinical trial design, and overall interest in SCS. Methods: Data were collected anonymously from an online survey between February and May 2020. Results: A total of 223 respondents living with SCI completed the survey. The majority of respondents identified their gender as male (64%), were 10+ years post SCI (63%), and had a mean age of 50.8 years. Most individuals had a traumatic SCI (81%), and 45% classified themselves as having tetraplegia. Priorities for improved outcome for those with complete or incomplete tetraplegia included fine motor skills and upper body function, whereas priorities for complete or incomplete paraplegia included standing and walking, and bowel function. The meaningful benefits that are important to achieve are bowel and bladder care, less reliance on caregivers, and maintaining physical health. Perceived potential risks include further loss of function, neuropathic pain, and complications. Barriers to participation in clinical trials include inability to relocate, out-of-pocket expenses, and awareness of therapy. Respondents were more interested in transcutaneous SCS than epidural SCS (80% and 61%, respectively). Conclusion: SCS clinical trial design, participant recruitment, and translation of the technology can be improved by better reflecting the priorities and preferences of those living with SCI identified from this study.

Effect of body mass index on survival after spinal cord injury.

Introduction: Increased mortality after acute and chronic spinal cord injury (SCI) remains a challenge and mandates a better understanding of the factors contributing to survival in these patients. This study investigated whether body mass index (BMI) measured after acute traumatic SCI is associated with a change in mortality. Methods: A prospective longitudinal cohort study was conducted with 742 patients who were admitted to the Acute Spine Unit of the Vancouver General Hospital between 2004 and 2016 with a traumatic SCI. An investigation of the association between BMI on admission and long-term mortality was conducted using classification and regression tree (CART) and generalized additive models (spline curves) from acute care up to 7.7 years after SCI (chronic phase). Multivariable models were adjusted for (i) demographic factors (e.g., age, sex, and Charlson Comorbidity Index) and (ii) injury characteristics (e.g., neurological level and severity and Injury Severity Score). Results: After the exclusion of incomplete datasets (n = 602), 643 patients were analyzed, of whom 102 (18.5%) died during a period up to 7.7 years after SCI. CART identified three distinct mortality risk groups: (i) BMI: > 30.5 kg/m2, (ii) 17.5-30.5 kg/m2, and (iii) < 17.5 kg/m2. Mortality was lowest in the high BMI group (BMI > 30.5 kg/m2), followed by the middle-weight group (17.5-30.5 kg/m2), and was highest in the underweight group (BMI < 17.5 kg/m2). High BMI had a mild protective effect against mortality after SCI (hazard ratio 0.28, 95% CI: 0.09-0.88, p = 0.029), concordant with a modest "obesity paradox". Moreover, being underweight at admission was a significant risk factor for mortality up to 7.7 years after SCI (hazard ratio 5.5, 95% CI: 2.34-13.17, p < 0.001). Discussion: Mortality risk (1 month to 7.7 years after SCI) was associated with differences in BMI at admission. Further research is needed to better understand the underlying mechanisms. Given an established association of BMI with metabolic determinants, these results may suggest unknown neuro-metabolic pathways that are crucial for patient survival.

Nationwide emergency department visits for pediatric traumatic spinal cord injury in the United States, 2016-2020.

Introduction: Traumatic spinal cord injury (tSCI) is a debilitating neurological condition resulting in lifelong disability for many individuals. The primary objectives of our study were to describe national trends in incident emergency department (ED) visits for tSCI among children (less than 21 years) in the United States, and to determine the proportion of visits that resulted in immediate hospitalization each year, including stratified by age and sex. Secondary objectives were to examine associations between select characteristics and hospitalization following tSCI, as well as to assess sports-related tSCIs over time, including by individual sport and geographic region. Methods: We used the Healthcare Cost and Utilization Project Nationwide Emergency Department Sample to identify ED visits among children between January 2016 and December 2020 for incident tSCI. Diagnosis codes were used to identify tSCI and sports-related injury etiologies. Census Bureau data were used to approximate annual rates of pediatric ED visits for tSCI per 100,000 children. Unconditional logistic regression modeling assessed whether select factors were associated with hospital admission. Results: We found that the annual ED visit rate for tSCI remained relatively stable between 2016 and 2020, with approximately 2,200 new all-cause pediatric ED visits for tSCI annually. Roughly 70% of ED visits for tSCI resulted in hospitalization; most ED visits for tSCI were by older children (15-20 years) and males, who were also more often admitted to the hospital. Notable secondary findings included: (a) compared with older children (15-20 years), younger children (10-14 years) were less likely to be hospitalized immediately following an ED visit for tSCI; (b) patient sex and race were not associated with hospital admission; and (c) American tackle football was the leading cause of sports-related ED visits for tSCI among children. Our findings also suggest that the proportion of sports-related tSCI ED visits may have increased in recent years. Discussion: Future research should further examine trends in the underlying etiologies of pediatric tSCI, while assessing the effectiveness of new and existing interventions aimed at tSCI prevention.

Pediatric Traumatic Spinal Cord Injury in the United States: A National Inpatient Analysis.

BACKGROUND: Traumatic spinal cord injury (tSCI) is a debilitating neurological condition often associated with lifelong disability. Despite this, there are limited data on pediatric tSCI epidemiology in the United States. OBJECTIVES: Our primary objective was to estimate tSCI hospitalization rates among children, including by age, sex, and race. Secondary objectives were to characterize tSCI hospitalizations and examine associations between sociodemographic characteristics and tSCI etiology. METHODS: We used the 2016 Kids' Inpatient Database to examine tSCI hospitalizations among children (<21 years). Descriptive statistics were used to report individual and care setting characteristics for initial tSCI hospitalizations. We used Census Bureau data to estimate tSCI hospitalization rates (number of pediatric tSCI hospitalizations / number of US children) and logistic regression modeling to assess associations between documented sociodemographic characteristics and injury etiology. RESULTS: There were 1.48 tSCI admissions per 100,000 children; highest rates of hospitalization involved older (15-20 years), male, and Black children. Hospitalization involving male (adjusted odds ratio [AOR] 0.43; 95% CI, 0.33-0.58) or Black (AOR 0.37; 95% CI, 0.25-0.55) children were less likely to involve a motor traffic accident. Hospitalizations of Black children were significantly more likely to have a diagnosis of tSCI resulting from a firearm incident (AOR 18.97; 95% CI, 11.50-31.28) or assault (AOR 11.76; 95% CI, 6.75-20.50) compared with hospitalizations of White children. CONCLUSION: Older, male, and Black children are disproportionately burdened by tSCI. Implementation of broad health policies over time may be most effective in reducing pediatric tSCI hospitalizations and preventable injuries.

Development of a machine learning algorithm for predicting in-hospital and 1-year mortality after traumatic spinal cord injury.

BACKGROUND CONTEXT: Current prognostic tools such as the Injury Severity Score (ISS) that predict mortality following trauma do not adequately consider the unique characteristics of traumatic spinal cord injury (tSCI). PURPOSE: Our aim was to develop and validate a prognostic tool that can predict mortality following tSCI. STUDY DESIGN: Retrospective review of a prospective cohort study. PATIENT SAMPLE: Data was collected from 1245 persons with acute tSCI who were enrolled in the Rick Hansen Spinal Cord Injury Registry between 2004 and 2016. OUTCOME MEASURES: In-hospital and 1-year mortality following tSCI. METHODS: Machine learning techniques were used on patient-level data (n=849) to develop the Spinal Cord Injury Risk Score (SCIRS) that can predict mortality based on age, neurological level and completeness of injury, AOSpine classification of spinal column injury morphology, and Abbreviated Injury Scale scores. Validation of the SCIRS was performed by testing its accuracy in an independent validation cohort (n=396) and comparing its performance to the ISS, a measure which is used to predict mortality following general trauma. RESULTS: For 1-year mortality prediction, the values for the Area Under the Receiver Operating Characteristic Curve (AUC) for the development cohort were 0.84 (standard deviation=0.029) for the SCIRS and 0.55 (0.041) for the ISS. For the validation cohort, AUC values were 0.86 (0.051) for the SCIRS and 0.71 (0.074) for the ISS. For in-hospital mortality, AUC values for the development cohort were 0.87 (0.028) and 0.60 (0.050) for the SCIRS and ISS, respectively. For the validation cohort, AUC values were 0.85 (0.054) for the SCIRS and 0.70 (0.079) for the ISS. CONCLUSIONS: The SCIRS can predict in-hospital and 1-year mortality following tSCI more accurately than the ISS. The SCIRS can be used in research to reduce bias in estimating parameters and can help adjust for coefficients during model development. Further validation using larger sample sizes and independent datasets is needed to assess its reliability and to evaluate using it as an assessment tool to guide clinical decision-making and discussions with patients and families.

Evaluation of a Clinical Protocol to Assess and Diagnose Neuropathic Pain During Acute Hospital Admission: Results From Traumatic Spinal Cord Injury.

OBJECTIVES: A clinical protocol was developed for clinicians to routinely assess and initiate treatment for patients with neuropathic pain (NP) in an acute care setting. The objectives of this study were to: (1) determine the incidence and onset of NP in patients with traumatic spinal cord injury during acute care and (2) describe how the implementation of a clinical protocol impacts the assessment and diagnosis of NP. MATERIALS AND METHODS: The study was a cohort analysis with a pre-post-test utilizing a historical control. Data were retrospectively collected from a patient registry and charts. Participants were randomly selected in cohort 1 (control) and cohort 2 (NP clinical protocol). RESULTS: The incidence of NP was 56% without significant difference between the cohorts (P=0.3). Onset of NP was 8 days (SD=14) across the study and >85% of the participants with NP were diagnosed within 2 weeks. Participants with incomplete injuries had a significant earlier onset than participants with complete injuries (6.2±12.8, 10.9±15.8 d; P=0.003). The mean number of days from hospital admission to initial assessment decreased with use of the NP clinical protocol (3.7±5.7 d; P=0.02). DISCUSSION: This study demonstrates a high incidence and early onset of NP in traumatic spinal cord injury during acute hospital care, with an earlier emergence in participants with incomplete injury. The NP clinical protocol ensured continuous assessment and documentation of NP while decreasing the time to an initial screen, but did not impact diagnosis.

Impact of bladder, bowel and sexual dysfunction on health status of people with thoracolumbar spinal cord injuries living in the community.

OBJECTIVE: The disruption of autonomic function following a spinal cord injury (SCI) is common and can negatively affect quality of life. The objective of this study was to describe the prevalence of bladder/bowel incontinence and sexual dysfunction in community-dwelling individuals with a thoracolumbar SCI and examine the impact on general physical and mental health status. METHODS: Participants who sustained a traumatic SCI to the thoracolumbar region of the spinal cord and classified as American Spinal Injury Association Impairment Scale (AIS) A to D were recruited. Demographic, injury data, MRI classification and neurological data were collected on admission. At follow-up, the neurological data, a questionnaire collecting participant-reported secondary health conditions (SHCs) (e.g. bladder incontinence, depression etc.) following SCI and health status measured by Short Form-36 were obtained. Regression models determined the association of health status with demographic/injury-related data, types and number of SHCs. RESULTS: Of the 51 participants, 58.8% reported bladder incontinence, 54.0% bowel incontinence, 60.8% sexual dysfunction and 29.4% had all three. The regression models demonstrated that age at injury, bowel incontinence, sexual dysfunction, presence of pain, motor score at follow-up and the number of SHCs were significant predictors of health status. The number of SHCs was more predictive than all other demographic and injury variables for health status. CONCLUSION: Results highlight the high prevalence of self-reported bowel/bladder incontinence and sexual dysfunction in the traumatic thoracolumbar SCI population and support the need for standardized assessments. Several demographic, injury-related and SHCs impacted health status and should be considered for the management of individuals living in the community.

Using Evidence To Inform Practice and Policy To Enhance the Quality of Care for Persons with Traumatic Spinal Cord Injury.

In today's economic climate, there is a need to demonstrate a return on investment for healthcare spending and for clinical practice and policy to be informed by evidence. Navigating this process is difficult for decision-makers, clinicians, and researchers alike. This article will describe how a knowledge translation framework and an evidence-based policy-making process were integrated to clarify the problem, frame options, and plan implementation, to impact clinical practice and policy in the area of traumatic spinal cord injury (tSCI). The Access to Care and Timing (ACT) project is focused on optimizing the access and timing of specialized healthcare delivery for persons sustaining a tSCI in Canada. A simulation model was developed that uses current patient data to address complex problems faced by the healthcare system. At a workshop, participants stressed the importance of linking interventions to short- and long-term outcomes to drive change. Presently, there are no national, system level indicators to monitor performance after tSCI. Although the ideal system of care after tSCI is unknown, indicator collection will establish a baseline to measure improvement. The workshop participants prioritized two indicators important from the clinician and patient perspective-timely admission to rehabilitation and meaningful community participation. The ACT simulation model for tSCI care will be used to promote the uptake of identified indicators and provide a predictive link between interventions on potential outcomes. The standardized collection of outcome-oriented indicators will help to evaluate the access and timing of care and to define the ideal system of care after SCI.

Creation of an Algorithm to Identify Non-traumatic Spinal Cord Dysfunction Patients in Canada Using Administrative Health Data.

Background: The lack of consensus on the best methodology for identifying cases of non-traumatic spinal cord dysfunction (NTSCD) in administrative health data limits the ability to determine the burden of disease and provide evidence-informed services. Objective: The purpose of this study is to develop an algorithm for identifying cases of NTSCD with Canadian health administrative databases using a case-based approach. Method: Data were provided by the Canadian Institute for Health Information that included all acute care hospital and day surgery (Discharge Abstract Database), ambulatory (National Ambulatory Care Reporting System), and inpatient rehabilitation records (National Rehabilitation Reporting System) of patients with neurological impairment (paraplegia, tetraplegia, and cauda equina syndrome) between April 1, 2004 and March 31, 2011. The approach to identify cases of NTSCD involved using a combination of diagnostic codes for neurological impairment and NTSCD etiology. Results: Of the initial cohort of 23,703 patients with neurological impairment, we classified 6,362 as the "most likely NTSCD" group (had a most responsible diagnosis or pre-existing diagnosis of NTSCD and diagnosis of neurological impairment); 2,777 as "probable NTSCD" defined as having a secondary diagnosis of NTSCD, and 11,179 as "possible NTSCD" who had no NTSCD etiology diagnoses but neurological impairment codes. Conclusion: The proposed algorithm identifies an inpatient NTSCD cohort that is limited to patients with significant paralysis. This feasibility study is the first in a series of 3 that has the potential to inform future research initiatives to accurately determine the incidence and prevalence of NTSCD.

Characteristics of Non-traumatic Spinal Cord Dysfunction in Canada Using Administrative Health Data.

Background: There is a paucity of studies using administrative health data to examine the epidemiology, health care utilization, and outcomes for non-traumatic spinal cord dysfunction (NTSCD). Objective: The purpose of this study is to characterize discrete NTSCD cohorts using decision algorithms with Canadian health administrative databases. Method: Data were provided by the Canadian Institute for Health Information that included all acute care hospital, day surgery, ambulatory, and inpatient rehabilitation records of patients with neurological impairment between April 1, 2004 and March 31, 2011. Diagnostic codes for neurological impairment and NTSCD etiology were used to identify cases and classify 3 NTSCD groups (most likely, probable, and possible). Logistic regression identified factors related to inpatient rehabilitation admission within 7 days of discharge among the preferred group. Results: The most likely NTSCD group (n = 6,362) was significantly older and had a greater proportion of women and individuals with cauda equina lesions compared to the other 2 NTSCD groups (probable [n = 2,777] and possible [n = 11,179]; ps < .001). Factors associated with the likelihood of an inpatient rehabilitation admission included being older (odds ratio [OR], 1.01; 95% CI, 1.00-1.01), being female (OR, 1.18; 95% CI, 1.06-1.32), having paraplegia diagnosis compared to cauda equina (OR, 1.24; 95% CI, 1.09-1.41), residing in an urban area compared to a rural area (OR, 1.34; 95% CI, 1.13-1.58), having degenerative etiology compared to other (OR, 1.59; 95% CI, 1.41-1.80), and having an MRI on record compared to not (OR = 1.57; 95% CI, 1.39-1.76). Conclusion: Administrative data allow for ongoing surveillance of a population in a relatively cost-effective manner. Advancing our knowledge of NTSCD epidemiology, health outcomes, and system performance can inform policy and system planning.

Mechanisms of X-chromosome inactivation.

Mammalian X-chromosome inactivation is an impressive example of epigenetic gene regulation, whereby the majority of genes on the approximately 160 Mb X chromosome are silenced in a strictly cis-limited fashion. In this review we will discuss the important players involved in the silencing process. The process is initiated by transcription and cis-localization of the non-coding XIST RNA, which then recruits many of the epigenetic features generally associated with heterochromatin, including histone modifications, histone variants and DNA methylation.

Onset, risk factors, and impact of delirium in patients with traumatic spinal cord injury.

Delirium is a commonly reported acute care adverse event in patients with traumatic spinal cord injury (TSCI), but studies specifically investigating it in this population are lacking. The purpose of this study was to characterize the onset, risk factors, and impact of delirium in patients with TSCI. Patients discharged between 2008 and 2010 were identified from a prospective registry in an acute SCI center. Controls were matched to delirium cases based on date of discharge from acute care. Patient characteristics, risk factors, and the hospital unit (intensive care, spine step-down, spine ward) in which delirium occurred were collected retrospectively. Length of stay (LOS) was calculated and compared between cases and controls. A predictive model was built for patient characteristics and risk factors associated with delirium using logistical regression. There were 192 patients identified from the study group; 34 (17.7%) were delirium cases and 34 were selected as controls. Most delirious episodes were reported during high acuity care (76.5%). The median time interval between injury and delirium identification was 8.5 days (interquartile range=5-31). Age at injury (p<0.01) and initial motor score (p<0.05) were significantly associated with delirium. Patients with delirium had significantly greater LOS than controls (median LOS=46.9 vs. 15.3 days respectively, p<0.0001). Elderly patients who sustain a TSCI and have a low motor score on admission are at increased risk of delirium. These results could contribute to the development of a screening program to address the problem of delirium in the TSCI population.

The validity of administrative data to classify patients with spinal column and cord injuries.

International Classification of Diseases (ICD) codes are used to document patient morbidity in administrative databases. Although administrative data are used for research purposes, the validity of the data to accurately describe clinical diagnostic information is uncertain. We compared the clinical diagnoses for spinal cord and column injuries from a longitudinal patient registry, the Rick Hansen Spinal Cord Injury Registry (RHSCIR), to the ICD-10 spinal injury codes from the Discharge Abstract Database (DAD) at one institution. There were 603 RHSCIR participants with data describing the spinal cord injury, and 341 had data on the spinal column injury. The validity of DAD data to describe spinal injuries was evaluated using the sensitivity and positive predictive values of specific ICD-10 codes; 5.3% of the spinal column injuries and 10.9% of the spinal cord injuries documented in RHSCIR were missed in data from the DAD using ICD-10 codes. The most problematic spinal column ICD-10 code was the dislocation of the cervical vertebra (S13.1); only 14.0% of the dislocations of the cervical vertebrae in RHSCIR were correctly coded in the DAD. The most problematic spinal cord injury ICD-10 code was the incomplete lesion of the lumbar spinal cord (S34.1X); 66.7% of incomplete lesions of the lumbar spinal cord in RHSCIR were correctly coded in the DAD. The validity of DAD data to code spinal injuries is variable, and cannot be reliably used to classify all types of spinal injuries. Patient registries, such as RHSCIR, should be used if accurate detailed diagnostic data are required.

Meeting the privacy requirements for the development of a multi-centre patient registry in Canada: the Rick Hansen Spinal Cord Injury Registry.

Privacy legislation addresses concerns regarding the privacy of personal information; however, its interpretation by research ethics boards has resulted in significant challenges to the collection, management, use and disclosure of personal health information for multi-centre research studies. This paper describes the strategy used to develop the national Rick Hansen Spinal Cord Injury Registry (RHSCIR) in accordance with privacy statutes and benchmarked against best practices. An analysis of the regional and national privacy legislation was conducted to determine the requirements for each of the 31 local RHSCIR sites and the national RHSCIR office. A national privacy and security framework was created for RHSCIR that includes a governance structure, standard operating procedures, training processes, physical and technical security and privacy impact assessments. The framework meets a high-water mark in ensuring privacy and security of personal health information nationally and may assist in the development of other national or international research initiatives.

Active chromatin marks are retained on X chromosomes lacking gene or repeat silencing despite XIST/Xist expression in somatic cell hybrids.

BACKGROUND: X-chromosome inactivation occurs early in mammalian development and results in the inactive X chromosome acquiring numerous hallmarks of heterochromatin. While XIST is a key player in the inactivation process, the method of action of this ncRNA is yet to be determined. METHODOLOGY/PRINCIPAL FINDINGS: To assess which features of heterochromatin may be directly recruited by the expression and localization of the XIST RNA we have analyzed a mouse/human somatic cell hybrid in which expression of human and mouse XIST/Xist has been induced from the active X by demethylation. Such hybrids had previously been demonstrated to disconnect XIST/Xist expression from gene silencing and we confirm maintenance of X-linked gene expression, even close to the Xist locus, despite the localized expression of mouse Xist. CONCLUSIONS/SIGNIFICANCE: Loss of the active chromatin marks H3 acetylation and H3 lysine 4 methylation was not observed upon XIST/Xist expression, nor was there a gain of DNA methylation; thus these marks of facultative heterochromatin are not solely dependent upon Xist expression. Cot-1 holes, regions of depleted RNA hybridization with a Cot-1 probe, were observed upon Xist expression; however, these were at reduced frequency and intensity in these somatic cells. Domains of human Cot-1 transcription were observed corresponding to the human chromosomes in the somatic cell hybrids. The Cot-1 domain of the X was not reduced with the expression of XIST, which fails to localize to the human X chromosome in a mouse somatic cell background. The human inactive X in a mouse/human hybrid cell also shows delocalized XIST expression and an ongoing Cot-1 domain, despite X-linked gene silencing. These results are consistent with recent reports separating Cot-1 silencing from genic silencing, but also demonstrate repetitive element expression from an otherwise silent X chromosome in these hybrid cells.

Inducible XIST-dependent X-chromosome inactivation in human somatic cells is reversible.

During embryogenesis, the XIST RNA is expressed from and localizes to one X chromosome in females and induces chromosome-wide silencing. Although many changes to inactive X heterochromatin are known, the functional relationships between different modifications are not well understood, and studies of the initiation of X-inactivation have been largely confined to mouse. We now present a model system for human XIST RNA function in which induction of an XIST cDNA in somatic cells results in localized XIST RNA and transcriptional silencing. Chromatin immunoprecipitation and immunohistochemistry shows that this silencing need only be accompanied by a subset of heterochromatic marks and that these can differ between integration sites. Surprisingly, silencing is XIST-dependent, remaining reversible over extended periods. Deletion analysis demonstrates that the first exon of human XIST is sufficient for both transcript localization and the induction of silencing and that, unlike the situation in mice, the conserved repeat region is essential for both functions. In addition to providing mechanistic insights into chromosome regulation and formation of facultative heterochromatin, this work provides a tractable model system for the study of chromosome silencing and suggests key differences from mouse embryonic X-inactivation.