Inherited X-chromosome inverted tandem duplication in a male traced to a grandparental mitotic error.

A male infant was referred for cytogenetic evaluation because of dysmorphic features and developmental delay. In both lymphocytes and skin fibroblasts, a modal number of 46 chromosomes was obtained with an obvious elongation of the long arm of the X chromosome (Xq+). Studies of seven members in 3 generations of this family showed that the proband's mother, sister, and maternal grandmother were phenotypically normal carriers of this abnormal X chromosome. High resolution GTG- and RBG-banding defined the extra chromatin material as an inverted duplication of Xq21----Xq24. This was supported by an approximate twofold increase in alpha-galactosidase A activity, localized to Xq21----q24, observed in the proband's lymphocytes and fibroblasts. BrdU-incorporation studies of the mother's lymphocytes showed the abnormal X to be late replicating in all 100 cells studied and normal alpha-galactosidase A levels. Cytogenetic analysis of the maternal grandmother revealed cytogenetic mosaicism with one cell line containing the abnormal X (37%), and the other, a normal female karyotype (63%). This family is instructive since: (1) it represents only the second case of a dysmorphic male demonstrating a confirmed interstitial partial Xq duplication, and (2) the origin of this familial structural rearrangement has been traced to a grandparental mitotic error.     

Mapping of the Menkes locus to Xq13.3 distal to the X-inactivation center by an intrachromosomal insertion of the segment Xq13.3-q21.2

Summary During a systematic chromosomal survey of 167 unrelated boys with the X-linked recessive Menkes disease (MIM 309400), a unique rearrangement of the X chromosome was detected, involving an insertion of the long arm segment Xq13.3-q21.2 into the short arm at band Xp11.4, giving the karyotype 46,XY,ins(X) (p11.4q13.3q21.2). The same rearranged X chromosome was present de novo in the subject's phenotypically normal mother, where it was preferentially inactivated. The restriction fragment length polymorphism and methylation patterns at DXS255 indicated that the rearrangement originated from the maternal grandfather. Together with a previously described X;autosomal translocation in a female Menkes patient, the present finding supports the localization of the Menkes locus (MNK) to Xq13, with a suggested fine mapping to sub-band Xq13.3. This localization is compatible with linkage data in both man and mouse. The chromosomal bend associated with the X-inactivation center (XIC) was present on the proximal long arm of the rearranged X chromosome, in line with a location of XIC proximal to MNK. Combined data suggest the following order: Xcen-XIST(XIC), DXS128-DXS171, DXS56-MNK-PGK1-Xqter.     

Molecular studies of the parental origin and nature of human X isochromosomes

X-chromosome restriction fragment length polymorphisms were used to determine the parental origin of the isochromosome in nine individuals with an i(Xq) or idic(Xq). We were able to specify the parental source of eight of the nine isochromosomes, with six being maternal and two paternal in origin. In two cases, one i(Xq) and one idic(Xq), we used Xq markers to determine the level of heterozygosity in the isochromosome. Each was homozygous at all tested loci, suggesting that each originated from a single X chromosome and not from an exchange of material between two X's.     

A molecular study of X isochromosomes: parental origin, centromeric structure, and mechanisms of formation.

Fourteen individuals with an i(Xq) or idic(Xq) were studied using RFLP analysis in order to determine both parental origin and extent of heterozygosity of the isochromosome and to search for the presence of short-arm material. In five cases the isochromosome was paternally derived, while nine patients had a maternal i(Xq). The analysis of heterozygosity of the nine maternally derived isochromosomes by using Xq markers showed heterozygosity in two cases, suggesting an origin from two homologous X chromosomes. Homozygosity was found at all informative loci in seven cases, which therefore are probably the product of either centromere misdivision or sister-chromatid exchange. Presence of Xp markers was seen both in the three i(Xq) chromosomes which appeared dicentric by cytogenetic analysis and in three additional cytogenetically monocentric cases. Mean parental ages were greater for the maternally derived cases as compared with the paternally derived cases.     

Paternal or maternal origin of human i(Xq) isochromosomes

Summary We present a method to test if the proportion of 45,X cases resulting from loss of the maternal chromosome or of cases of 46,X,i(Xq) with the isochromosome of maternal origin is different from 1/2. The available data are consistent with the hypothesis that the normal X present in i(Xq) patients originates with equal probabilities in the fathers and mothers of the patientsThis paper was supported in part by grants from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)     

Tightly linked flanking markers for the Lowe oculocerebrorenal syndrome, with application to carrier assessment.

The Lowe oculocerebrorenal syndrome (OCRL) is characterized by congenital cataract, mental retardation, and defective renal tubular function. A map assignment of OCRL to Xq24-q26 has been made previously by linkage analysis with DXS42 at Xq24-q26 (theta = 0, z = 5.09) and with DXS10 at Xq26 (theta = 0, z = 6.45). Two additional families were studied and three additional polymorphisms were identified at DXS42 by using a 35-kb sequence isolated with the probe detecting the original polymorphism at DXS42. With additional OCRL families made informative for DXS42, theta remained 0 with z = 6.63; and for DXS10 theta = 0.03 and z = 7.07. Evidence for placing OCRL at Xq25 also comes from a female with Lowe syndrome and an X;3 translocation. We have used the Xq25 breakpoint in this patient to determine the position of OCRL relative to the two linked markers. Each derivative chromosome was isolated away from its normal counterpart in somatic cell hybrids. DXS42 was mapped to the derivative chromosome X containing Xpterq25, and DXS10 was mapped to the derivative chromosome 3 containing Xq25-qter. The markers DXS10 and DXS42 therefore show tight linkage with OCRL in six families and flank the Xq25 breakpoint in a female patient with an X;3 translocation. Linkage analysis with flanking markers was used to assess OCRL carrier status in women at risk. Results, when compared with carrier determination by ophthalmologic examination, indicated that the slit-lamp exam can be a sensitive and specific method of carrier determination in many cases.     

Identification of a duplication of Xq28 associated with bilateral periventricular nodular heterotopia.

Bilateral periventricular nodular heterotopia (BPNH) is a malformation of neuronal migration and is characterized by nodules of heterotopic gray matter lining the lateral ventricles of the brain. The majority of BPNH patients are female and have epilepsy as a sole clinical manifestation of their disease. Familial BPNH has been mapped to Xq28 by linkage analysis. A multiple congenital anomaly-mental retardation syndrome (BPNH/MR) was recently delineated in three unrelated boys with BPNH, cerebellar hypoplasia, severe mental retardation, epilepsy, and syndactyly. High-resolution chromosome analysis revealed a subtle abnormality of Xq28 in one of the boys with BPNH/MR syndrome. FISH with cosmids and YACs from Xq28 further characterized this abnormality as a 2.25-3.25-Mb inverted duplication. No abnormality of Xq28 was detected by G-banding or FISH in the other two boys. These data support the linkage assignment of BPNH to band Xq28 and narrow the critical region to the distal 2.25-3.25 Mb of Xq28.     

Folate-sensitive fragile sites on the X-chromosome heterochromatin of the Indian mole rat, Nesokia indica

Folate-sensitive fragile sites have been demonstrated on the X chromosome of the Indian mole rat, Nesokia indica (subfamily Murinae), utilizing peripheral blood lymphocyte cultures. All normal female individuals expressed fragile sites on the constitutive heterochromatic long arm of one of their two X chromosomes (heterozygous expression); in contrast, no fragile sites were found on the single X chromosome of normal males. Preferential transmission of the maternal fragile X to the daughters is therefore suggested. Four sites have been detected so far: fra Xq1, fra Xq2, fra Xq3, and fra Xc (centromeric). It is significant that their location corresponds to the regions where constitutive heterochromatic deletions occur that result in a variety of polymorphic X chromosomes in natural populations of Nesokia. Thus there is a correlation between fragile sites, deletion sites, and karyotypic changes. In individuals that did not reproduce in the laboratory, there were more fragile sites on both X chromosomes of the females (homozygous/double heterozygous expression) and also on the X of the males (hemizygous expression). This difference in fragile site expression from the normal situation could be attributed to one or more new mutations. However, the mechanism by which fragile sites influence reproductive performance is unclear.     

Ovarian dysgenesis due to an idic(X)(q2803)

A 17-year-old female patient with gonadal dysgenesis but no other turnerian features was found to have a 46,X,idic(X)(pter----q2803:q2803----pter) karyotype in her lymphocytes. Replication of the rearranged X was consistently late and symmetrical. It is postulated that the ovarian dysgenesis usually seen in nonmosaic carriers of Xq;Xq terminal rearrangements may be secondary to a nonreactivation of the abnormal chromosome before meiosis.     

A genomewide scan of male sexual orientation

This is the first report of a full genome scan of sexual orientation in men. A sample of 456 individuals from 146 families with two or more gay brothers was genotyped with 403 microsatellite markers at 10-cM intervals. Given that previously reported evidence of maternal loading of transmission of sexual orientation could indicate epigenetic factors acting on autosomal genes, maximum likelihood estimations (mlod) scores were calculated separated for maternal, paternal, and combined transmission. The highest mlod score was 3.45 at a position near D7S798 in 7q36 with approximately equivalent maternal and paternal contributions. The second highest mlod score of 1.96 was located near D8S505 in 8p12, again with equal maternal and paternal contributions. A maternal origin effect was found near marker D10S217 in 10q26, with a mlod score of 1.81 for maternal meioses and no paternal contribution. We did not find linkage to Xq28 in the full sample, but given the previously reported evidence of linkage in this region, we conducted supplemental analyses to clarify these findings. First, we re-analyzed our previously reported data and found a mlod of 6.47. We then re-analyzed our current data, after limiting the sample to those families previously reported, and found a mlod of 1.99. These Xq28 findings are discussed in detail. The results of this first genome screen for normal variation in the behavioral trait of sexual orientation in males should encourage efforts to replicate these findings in new samples with denser linkage maps in the suggested regions.Brian S. Mustanski and Michael G. DuPree contributed equally to this work.     

Determining the origin of human X isochromosomes by use of DNA sequence polymorphisms and detection of an apparent i(Xq) with Xp sequences

Summary The parental origin of five X isochromosomes were determined using 11 DnA markers. The isochromosome was derived from a maternal X chromosome in three cases and from a paternal X chromosome in two. Unexpected heterozygosity was detected for the proximal Xp region in one individual in whom the i(Xq) chromosome was paternally derived. This was confirmed by in situ hybridisation. A mode of formation of isochromosomes by breakage and reunion between the sister chromatids of the arms of an X chromosome is proposed to account for this. Sister chromatid breakage and reunion can be considered as a significant mechanism for the origin of i(Xq) chromosomes.     

Identification and characterization of an Xq26-q27 duplication in a family with spina bifida and panhypopituitarism suggests the involvement of two distinct genes

We investigated a family with a duplication, dup(X)q26-q27, that was present in two brothers, their mother, and their maternal grandmother. The brothers carrying the duplication displayed spina bifida and panhypopituitarism, whereas a third healthy brother inherited the normal X chromosome. Preferential inactivation of the X chromosome containing the duplication was evident in healthy carrier females. We determined the boundaries of the Xq26-q27 duplication. Via interphase FISH analysis we narrowed down each of the two breakpoint regions to approximately 300-kb intervals. The proximal breakpoint is located in Xq26.1 between DXS1114 and HPRT and is contained in YAC yWXD599, while the distal breakpoint is located in Xq27.3 between DXS369 and DXS1200 and contained in YAC yWXD758. The duplication comprises about 13 Mb. Evidence from the literature points to a predisposing gene for spina bifida in Xq27. We hypothesize that the spina bifida in the two brothers may be due to interruption of a critical gene in the Xq27 breakpoint region. Several candidate genes were mapped to the Xq27 critical region but none was shown to be disrupted by the duplication event. Recently, M. Lagerstr?m-Fermér et al. (1997, Am. J. Hum. Genet. 60, 910-916) reported on a family with X-linked recessive panhypopituitarism associated with a duplication in Xq26; however, no details were reported on the extent of the duplication. Our study corroborates their hypothesis that X-linked recessive panhypopituitarism is likely to be caused by a gene encoding a dosage-sensitive protein involved in pituitary development. We place the putative gene between DXS1114 and DXS1200, corresponding to the interval defined by the duplication in the present family.     

46,X,i(Xq)/47,XX,+13 mosaicism

A 10-year-old girl with short stature and other features of Turner's syndrome was found to be a mosaic consisting of 46,X,i(Xq) and 47,XX,+13 cell lines, a hitherto undescribed situation. She had none of the clinical features of trisomy 13 syndrome, with a possible exception of postaxial polydactyly of the left foot. Her PHA-stimulated blood lymphocytes and EB virus-transformed B lymphocytes both revealed the Xi(Xq)/XX,+13 mosaicism, while her skin fibroblasts showed an exclusively 46,X,i(Xq) karyotype. Studies using Q-and R-banding heteromorphisms as markers indicated that the patient started as a 13 trisomic zygote resulting from a maternal meiotic error, followed by the loss of chromosome 13 at an early mitotic division. C-banding analysis revealed two C banding blocks in the iso X chromosome, an indication that the chromosome was dicentric. BrdU-Hoechst-Giemsa analysis revealed that the iso X chromosome was late-replicating with both its arms either synchronously or asynchronously replicating. The iso X chromosome was thus designated as idic (Xq)(p11:p11). In view of the presence of the XX cell line, it was concluded that the patient started as an XX,+13 zygote, followed by two mitotic events, the loss of a chromosome 13 and the formation of the iso X chromosome, occurring either simultaneously or in succession.     

A novel X-linked dominant condition: X-linked congenital isolated ptosis

We present a large family with a previously undescribed condition: X-linked dominant congenital bilateral isolated ptosis. Linkage analysis defined a critical region between Xq24 and Xq27.1, with a maximum single-point LOD score of 2.88 at DXS1047 and DXS984. Male and female family members are equally affected, providing an example of an X-linked, truly dominant condition.     

An interstitial duplication of the X chromosome in a male allows physical fine mapping of probes from the Xq13-q22 region

Summary An insertional translocation into the proximal long arm of the X chromosome in a boy showing muscular hypotony, growth retardation, psychomotor retardation, cryptorchidism, and Pelizaeus-Merzbacher disease (PMD) was identified as a duplication of the Xq21–q22 segment by employing DNA probes. With densitometric scanning for quantitation of hybridization signals, 15 Xq probes were assigned to the duplicated region. Analysis of the duplication allowed us to dissect the X-Y homologous region physically at Xq21 and to refine the assignments of the loci for DXYS5, DXYS12, DXYS13, DXS94, DXS95, DXS96, DXS111, and DXS211. Furthermore, we demonstrated the presence of two different DXYS13, and DXS17 alleles in genomic DNA of our patient, suggesting that the duplication resulted from a meiotic recombination event involving the two maternal X chromosomes.     

Del Xq23 in a mosaic Turner female: molecular and cytogenetic studies.

We report a Turner patient aged 22 years with a 45,X/46,X,del(X)(q23) karyotype. Late replication studies showed preferential inactivation of the deleted X chromosome; FISH studies with a probe for total human telomeres showed hybridisation signal in the telomeres on both the normal and the deleted X chromosomes. Microsatellite analysis in the proposita and her family permitted us to conclude to the maternal origin of the deleted X chromosome, and to detect using the marker DXS1106 (Xq22) a probable meiotic recombination event above the breakage point suggesting that the deletion occurred underneath this point.The mild Turner stigmata may be explained by the 45,X cell line, and the gonadal dysgenesis probably by a partial deletion of the gonadal dysgenesis region Xq13-q23 (excluding Xq22).     

Isolation and characterization of a highly polymorphic human locus (DXS455) in proximal Xq28

Human Xq28 is highly gene dense with over 27 loci. Because most of these genes have been mapped by linkage to polymorphic loci, only one of which (DXS52) is informative in most families, a search was conducted for new, highly polymorphic Xq28 markers. From a cosmid library constructed using a somatic cell hybrid containing human Xq27.3----qter as the sole human DNA, a human-insert cosmid (c346) was identified and found to reveal variation on Southern blot analyses with female DNA digested with any of several different restriction endonucleases. Two subclones of c346, p346.8 and p346.T, that respectively identify a multiallelic VNTR locus and a frequent two-allele TaqI polymorphism were isolated. Examination of 21 unrelated females showed heterozygosity of 76 and 57%, respectively. These two markers appeared to be in linkage equilibrium, and a combined analysis revealed heterozygosity in 91% of unrelated females. Families segregating the fragile X syndrome with key Xq28 crossovers position this locus (designated DXS455) between the proximal Xq28 locus DXS296 (VK21) and the more distal locus DXS374 (1A1), which is proximal to DXS52. DXS455 is therefore the most polymorphic locus identified in Xq28 and will be useful in the genetic analysis of this gene dense region, including the diagnosis of nearby genetic disease loci by linkage.     

RFLPs研究三例X染色体结构异常的起源和形成机理

本文对三例X染色体结构异常46,X,dup(X)(p21);46,X,del(X)(p11);46,X,i(Xq)患者及其父母,用X染色体短臂或长臂上的限制性片段长度多态性(RFLPs)作为遗传标记,研究了异常X染色体的起源和形成机理。结果表明,dup(X)(p21)和del(X)(p11)起源于父方,而i(Xq)起源于母方。dup(X)(p21)是由X染色体姊妹染色单体不均等的互换所引起的,del(X)(p11)是由于X染色体断裂后丢失所致,i(Xq)的发生是由于卵母细胞X染色体着丝粒错分裂。     

A submicroscopic deletion in Xq26 associated with familial situs ambiguus.

Abnormal left-right-axis formation results in heterotaxy, a multiple-malformation syndrome often characterized by severe heart defects, splenic abnormalities, and gastrointestinal malrotation. Previously we had studied a large family in which a gene for heterotaxy, HTX1, was mapped to a 19-cM region in Xq24-q27.1. Further analysis of this family has revealed two recombinations that place HTX1 between DXS300 and DXS1062, an interval spanning approximately 1.3 Mb in Xq26.2. In order to provide independent confirmation of HTX1 localization, a PCR-based search for submicroscopic deletions in this region was performed in unrelated males with sporadic or familial heterotaxy. A cluster of sequence-tagged sites failed to amplify in an individual who also had a deceased, affected brother. FISH identified the mother as a carrier of the deletion, which arose as a new mutation from the maternal grandfather. The deletion interval spans 600-1,100 kb and lies wholly within the 1.3-Mb region identified by recombination. Discovery of this deletion supports localization of HTX1 to Xq26.2 and reveals the first molecular-genetic abnormality associated with human left-right-asymmetry defects.