X‐chromosome inactivation in female patients with Fabry disease

Fabry disease (FD) is an X‐linked genetic disorder caused by the deficient activity of lysosomal α‐galactosidase (α‐Gal). While males are usually severely affected, clinical presentation in female patients may be more variable ranging from asymptomatic to, occasionally, as severely affected as male patients. The aim of this study was to evaluate the existence of skewed X‐chromosome inactivation (XCI) in females with FD, its concordance between tissues, and its contribution to the phenotype. Fifty‐six females with FD were enrolled. Clinical and biological work‐up included two global scores [Mainz Severity Score Index (MSSI) and DS3], cardiac magnetic resonance imaging, measured glomerular filtration rate, and measurement of α‐Gal activity. XCI was analyzed in four tissues using DNA methylation studies. Skewed XCI was found in 29% of the study population. A correlation was found in XCI patterns between blood and the other analyzed tissues although some punctual variability was detected. Significant differences in residual α‐Gal levels, severity scores, progression of cardiomyopathy and deterioration of kidney function, depending on the direction and degree of skewing of XCI were evidenced. XCI significantly impacts the phenotype and natural history of FD in females.     

Comparison of X‐chromosome inactivation in Duchenne muscle/myocardium‐manifesting carriers,non‐manifesting carriers and related daughters

Female carriers of Duchenne muscular dystrophy (DMD) are usually asymptomatic. However, 2.5–7.8% of them may present muscle symptoms and cardiomyopathy, attributed to a reduced production of dystrophin, probably because of skewed patterns of X‐chromosome inactivation (XCI). To evaluate the role of XCI in symptomatic (at muscle or heart level) and asymptomatic DMD carriers, 44 subjects were selected from our database (12 manifesting, 21 non‐manifesting, 11 healthy females), and XCI pattern determined in the lymphocytes by the androgen receptor methylation‐based assay. The results showed that DMD‐manifesting carriers had a preferential inactivation of the X‐chromosome carrying the normal allele, while non‐manifesting carriers and healthy females showed a random XCI pattern. Moreover, when comparing muscle with heart manifesting carriers, the former group showed a higher degree of skewing. No concordance in XCI was found between mothers and daughters, when symptomatic/asymptomatic mother–daughter pairs were analyzed. The results confirm that DMD clinical manifestations in carriers are associated with non‐random patterns of X inactivation.     

一例Menkes病患儿的基因变异分析

目的对一例疑诊Menkes病患儿进行临床和遗传学分析。方法提取患儿及其父母基因组DNA,行家系全外显子测序及多重连接探针扩增技术检测ATP7A基因微重复、缺失,对疑似致病变异行生物信息学分析,并对患儿及其父母行一代测序验证变异位点。结果在受检者ATP7A基因发现c.1870-13T>G的核苷酸变异,为新发剪切变异。结论该患儿为ATP7A基因c.1870-13T>G变异而导致Menkes病,家系全外显子测序为表型异质性强的遗传性疾病提供了有力工具。     

X‐chromosomal inactivation directly influences the phenotypic manifestation of X‐linked protoporphyria

X‐linked protoporphyria (XLP), a rare erythropoietic porphyria, results from terminal exon gain‐of‐function mutations in the ALAS2 gene causing increased ALAS2 activity and markedly increased erythrocyte protoporphyrin levels. Patients present with severe cutaneous photosensitivity and may develop liver dysfunction. XLP was originally reported as X‐linked dominant with 100% penetrance in males and females. We characterized 11 heterozygous females from six unrelated XLP families and show markedly varying phenotypic and biochemical heterogeneity, reflecting the degree of X‐chromsomal inactivation of the mutant gene. ALAS2 sequencing identified the specific mutation and confirmed heterozygosity among the females. Clinical history, plasma and erythrocyte protoporphyrin levels were determined. Methylation assays of the androgen receptor and zinc‐finger MYM type 3 short tandem repeat polymorphisms estimated each heterozygotes X‐chromosomal inactivation pattern. Heterozygotes with equal or increased skewing, favoring expression of the wild‐type allele had no clinical symptoms and only slightly increased erythrocyte protoporphyrin concentrations and/or frequency of protoporphyrin‐containing peripheral blood fluorocytes. When the wild‐type allele was preferentially inactivated, heterozygous females manifested the disease phenotype and had both higher erythrocyte protoporphyrin levels and circulating fluorocytes. These findings confirm that the previous dominant classification of XLP is inappropriate and genetically misleading, as the disorder is more appropriately designated XLP.     

Screening of 383 unrelated patients affected with Menkes disease and finding of 57 gross deletions in ATP7A

Menkes disease (MD) is an X-linked multisystemic lethal disorder of copper metabolism dominated by neurodegenerative symptoms and connective tissue disturbances. MD results from mutations in the ATP7A gene, which encodes a membrane-bound copper transporting P-type ATPase located in the trans-Golgi network. In this study we describe screening of 383 unrelated patients affected with Menkes disease for gross deletions in ATP7A gene and finding of 57 patients. The present data suggests that gross deletion of ATP7A is the disease-causing mutation in 14.9% of the Menkes disease patients. Except for a few cases, gross gene deletions result in the classical form of Menkes disease with death in early childhood.     

X‐linked spinal muscular atrophy in mice caused by autonomous loss of ATP7A in the motor neuron

ATP7A is a copper‐transporting P‐type ATPase that is essential for cellular copper homeostasis. Loss‐of‐function mutations in the ATP7A gene result in Menkes disease, a fatal neurodegenerative disorder resulting in seizures, hypotonia and failure to thrive, due to systemic copper deficiency. Most recently, rare missense mutations in ATP7A that do not impact systemic copper homeostasis have been shown to cause X‐linked spinal muscular atrophy type 3 (SMAX3), a distal hereditary motor neuropathy. An understanding of the mechanistic and pathophysiological basis of SMAX3 is currently lacking, in part because the disease‐causing mutations have been shown to confer both loss‐ and gain‐of‐function properties to ATP7A, and because there is currently no animal model of the disease. In this study, the Atp7a gene was specifically deleted in the motor neurons of mice, resulting in a degenerative phenotype consistent with the clinical features in affected patients with SMAX3, including the progressive deterioration of gait, age‐dependent muscle atrophy, denervation of neuromuscular junctions and a loss of motor neuron cell bodies. Taken together, these data reveal autonomous requirements for ATP7A that reveal essential roles for copper in the maintenance and function of the motor neuron, and suggest that SMAX3 is caused by a loss of ATP7A function that specifically impacts the spinal motor neuron. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

三例Menkes病患儿的家系及基因变异分析

目的:对3例Menkes病患儿家系的 ATP7A基因进行变异分析,明确其致病原因,为临床诊断提供依据。 方法:应用二代测序(next-generation sequencing,NGS)对3个Menkes病家系的先证者进行Menkes病相关致病基因 ATP7A基因外显子检测,发现可疑致...     

Evidence of skewed X‐chromosome inactivation in 47,XXY and 48,XXYY Klinefelter patients

Klinefelter (47,XXY) syndrome occurs in approximately 1:800 male births and accounts for about 10‐20% of males attending infertility clinics. Recent studies have shown no obvious phenotypic differences between subjects in which the extra X‐chromosome is of paternal or maternal origin; however, a minority of Klinefelter patients are adversely affected clinically and intellectually to an exceptional level, and the underlying basis of this phenotypic variation is not known. We hypothesize that skewed X‐inactivation and possibly parental origin of the X‐chromosomes is involved. In this study, we determined parental origin and inactivation status of the X‐chromosomes in 17 cytogenetically confirmed 47,XXY cases, two 48,XXYY cases and one mosaic 46,XY/47,XXY case. Eight highly polymorphic markers specific to the X‐chromosome and the polymorphic human androgen‐receptor (HUMARA) methylation assay were used to determine the parental origin and X‐inactivation status of the X‐chromosomes, respectively. Overall, 17 cases were fully informative, enabling parental origin to be assigned. In 59% of cases, both X‐chromosomes were of maternal origin (X^m); in the remaining 41%, one X was of maternal (X^m) and one was of paternal origin (X^p). In 5 of 16 (31%) cases informative at the HUMARA locus, skewed X‐inactivation was observed as defined by greater than 80% preferential inactivation involving one of the two X‐chromosomes. The two 48,X^mX^pYY cases both showed preferential paternal X‐chromosome (X^p) inactivation. Three 47,X^mX^mY cases also showed preferential inactivation in one of the two maternal X‐chromosomes. These results suggest that skewed X‐inactivation in Klinefelter (47,XXY and 48,XXYY) patients may be common and could explain the wide range of mental deficiency and phenotypic abnormalities observed in this disorder. Further studies are warranted to examine the role of X‐inactivation and genetic imprinting in Klinefelter patients. © 2001 Wiley‐Liss, Inc.     

A 37‐year‐old Menkes disease patient—Residual ATP7A activity and early copper administration as key factors in beneficial treatment

Menkes disease (MD) is a lethal disorder characterized by severe neurological symptoms and connective tissue abnormalities; and results from malfunctioning of cuproenzymes, which cannot receive copper due to a defective intracellular copper transporting protein, ATP7A. Early parenteral copper‐histidine supplementation may modify disease progression substantially but beneficial effects of long‐term treatment have been recorded in only a few patients. Here we report on the eldest surviving MD patient (37 years) receiving early‐onset and long‐term copper treatment. He has few neurological symptoms without connective tissue disturbances; and a missense ATP7A variant, p.(Pro852Leu), which results in impaired protein trafficking while the copper transport function is spared. These findings suggest that some cuproenzymes maintain their function when sufficient copper is provided to the cells; and underline the importance of early initiated copper treatment, efficiency of which is likely to be dependent on the mutant ATP7A function.     

Phenotypic diversity of Menkes disease in mottled mice is associated with defects in localisation and trafficking of the ATP7A protein

Owing to mutations in the copper-transporting P-type ATPase, ATP7A (or MNK), patients with Menkes disease (MD) have an inadequate supply of copper to various copper-dependent enzymes. The ATP7A protein is located in the trans-Golgi network, where it transports copper via secretory compartments to copper-dependent enzymes. Raised copper concentrations result in the trafficking of ATP7A to the plasma membrane, where it functions in copper export. An important model of MD is the Mottled mouse, which possesses mutations in Atp7A. The Mottled mouse displays three distinct phenotypic severities: embryonic lethal, perinatal lethal and a longer-lived viable phenotype. However, the effects of mutations from these phenotypic classes on the ATP7A protein are unknown. In this study, we found that these classes of mutation differentially affect the copper transport and trafficking functions of the ATP7A protein. The embryonic lethal mutation, Atp7a(mo11H) (11H), caused mislocalisation of the protein to the endoplasmic reticulum, impaired glycosylation, and abolished copper delivery to the secretory pathway. In contrast, the perinatal lethal and viable mutations, Atp7a(moMac) (Macular) and Atp7a(moVbr) (Viable brindle) both resulted in a reduction in copper delivery to the secretory pathway and constitutive trafficking of the ATP7A protein to the plasma membrane in the absence of additional copper. In the case of Viable brindle, this hypertrafficking response was dependent on the catalytic phosphorylation site of ATP7A, whereas no such requirement was found for the Macular mutation. These findings provide evidence that the degree of MD severity in mice is associated with both copper transport and trafficking defects in the ATP7A protein.     

Identification of Intellectual Disability Genes in Female Patients with a Skewed X‐Inactivation Pattern

Intellectual disability (ID) is a heterogeneous disorder with an unknown molecular etiology in many cases. Previously, X‐linked ID (XLID) studies focused on males because of the hemizygous state of their X chromosome. Carrier females are generally unaffected because of the presence of a second normal allele, or inactivation of the mutant X chromosome in most of their cells (skewing). However, in female ID patients, we hypothesized that the presence of skewing of X‐inactivation would be an indicator for an X chromosomal ID cause. We analyzed the X‐inactivation patterns of 288 females with ID, and found that 22 (7.6%) had extreme skewing (>90%), which is significantly higher than observed in the general population (3.6%; P = 0.029). Whole‐exome sequencing of 19 females with extreme skewing revealed causal variants in six females in the XLID genes DDX3X, NHS, WDR45, MECP2, and SMC1A. Interestingly, variants in genes escaping X‐inactivation presumably cause both XLID and skewing of X‐inactivation in three of these patients. Moreover, variants likely accounting for skewing only were detected in MED12, HDAC8, and TAF9B. All tested candidate causative variants were de novo events. Hence, extreme skewing is a good indicator for the presence of X‐linked variants in female patients.     

Absence of correlation between skewed X inactivation in blood and serum creatine-kinase levels in Duchenne/Becker female carriers

The pattern of X inactivation in lymphocyte DNA was investigated in 107 Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) carriers (102 asymptomatic and 5 manifesting carriers) and 117 normal female controls of different ages, with the aim: a) to analyze the pattern of X inactivation in blood DNA of a large number of DMD/BMD carriers as compared to normal female controls; b) to determine if there is a decrease in serum creatine kinase (CK) levels with age in obligate DMD/BMD carriers; c) to determine if there is a correlation between X-chromosome inactivation and serum CK among asymptomatic DMD/BMD carriers of different ages or with different clinical manifestations in symptomatic carriers. A high proportion of females showed extremely skewed X inactivation (>90 of one X preferentially inactivated), which was almost the same among carriers and normal controls (19 and 24, respectively). The mean serum CK was significantly greater among young (<20 years old) than adult (>20 years old) DMD/BMD carriers and it decreased significantly until age 20 with an apparent stabilization afterwards. No statistically significant correlation was found between the proportion of active X^DMD in blood and serum CK activity in DMD/BMD carriers although it was higher among those less than 20 years old. Our observations suggest that highly skewed X-chromosome pattern in blood (with preferential inactivation of the X^N chromosome) is not enough to predict that a young DMD carrier will develop muscular weakness. Am. J. Med. Genet. 80:356–361, 1998.     

Female factor IX deficiency due to maternally inherited X‐inactivation

Esquilin JM, Takemoto CM, Green, NS. Female factor IX deficiency due to maternally inherited X‐inactivation. X‐chromosome inactivation is normally a random event that is regulated by the X chromosome itself. Rarely, females are affected by X‐linked disorders from extremely skewed X‐chromosome inactivation. Here, we report a family with hemophilia B with female expression through inherited X skewing that appears to be independent of either X chromosome. This finding suggests the possibility of a dominant autosomal contribution to inherited skewed X inactivation.