X-autosome translocation in normal mother and effectively 21-monosomic daughter

Detection of a novel unbalanced X;21 translocation in a girl with Turner syndrome phenotype

OBJECTIVE

To describe a novel unbalanced X;21 translocation resulting in a derivative pseudodicentric chromosome X;21 lacking the critical region for ovarian development and function, in a 16-year-old girl referred for cytogenetic analysis due to primary amenorrhea and Turner-like features.

METHODS

Cytogenetic analysis of the proband and her parents was performed on peripheral blood lymphocytes by GTG banding. Molecular cytogenetic FISH analysis was performed on metaphase preparations, using X chromosome centromeric probe and telomeric and pancentromeric peptide nucleic acid (PNA) analog probes. The HUMARA assay as well as methylation studies for PCSK1N and FMR-1 loci were performed.

RESULTS

Cytogenetic analysis revealed a de novo unbalanced X;21 translocation, described as 45,X,der(X)t(X;21)(q22.2;p11.2),-21. FISH analysis showed that the derivative X chromosome carried both the X and 21 centromeres, as well as, the Xp and 21q telomeres. The karyotype was thus reevaluated as 45,X,psu dic(21;X)(21qter→21p13::Xq22.2→Xpter),-21. X inactivation studies revealed that the derivative chromosome was of paternal origin and confirmed the selective inactivation of the derivative X segment of the pseudodicentric chromosome.

CONCLUSIONS

Primary amenorrhea and other Turner-like characteristics of the proband are apparently due to the loss of the Xq22.2→Xqter critical region which contains critical genes for the ovarian development and function. The chromosome X segment of the derivative pseudodicentric chromosome is selectively inactivated, but inactivation does not seem to spread onto the translocated chromosome 21, accounting probably for the lack of severe clinical consequences which would result from monosomy 21.

Spread of X-chromosome inactivation into autosomal sequences: role for DNA elements, chromatin features and chromosomal domains

X-chromosome inactivation results in dosage equivalence between the X chromosome in males and females; however, over 15% of human X-linked genes escape silencing and these genes are enriched on the evolutionarily younger short arm of the X chromosome. The spread of inactivation onto translocated autosomal material allows the study of inactivation without the confounding evolutionary history of the X chromosome. The heterogeneity and reduced extent of silencing on autosomes are evidence for the importance of DNA elements underlying the spread of silencing. We have assessed DNA methylation in six unbalanced X-autosome translocations using the Illumina Infinium HumanMethylation450 array. Two to 42% of translocated autosomal genes showed this mark of silencing, with the highest degree of inactivation observed for trisomic autosomal regions. Generally, the extent of silencing was greatest close to the translocation breakpoint; however, silencing was detected well over 100 kb into the autosomal DNA. Alu elements were found to be enriched at autosomal genes that escaped from inactivation while L1s were enriched at subject genes. In cells without the translocation, there was enrichment of heterochromatic features such as EZH2 and H3K27me3 for those genes that become silenced when translocated, suggesting that underlying chromatin structure predisposes genes towards silencing. Additionally, the analysis of topological domains indicated physical clustering of autosomal genes of common inactivation status. Overall, our analysis indicated a complex interaction between DNA sequence, chromatin features and the three-dimensional structure of the chromosome.

Chromosomal basis of X chromosome inactivation: identification of a multigene domain in Xp11.21-p11.22 that escapes X inactivation

A number of genes have been identified that escape mammalian X chromosome inactivation and are expressed from both active and inactive X chromosomes. The basis for escape from inactivation is unknown and, a priori, could be a result of local factors that act in a gene-specific manner or of chromosomal control elements that act regionally. Models invoking the latter predict that such genes should be clustered in specific domains on the X chromosome, rather than distributed at random along the length of the X. To distinguish between these possibilities, we have constructed a transcription map composed of at least 23 distinct expressed sequences in an approximately 5.5-megabase region on the human X chromosome spanning Xp11.21-p11.22. The inactivation status of these transcribed sequences has been determined in a somatic cell hybrid system and correlated with the position of the genes on the physical map. Although the majority of transcribed sequences in this region are subject to X inactivation, eight expressed sequences (representing at least six different genes) escape inactivation, and all are localized to within a region of less than 370 kb. Genes located both distal and proximal to this cluster are subject to inactivation, thereby defining a unique multigene domain on the proximal short arm that is transcriptionally active on the inactive X chromosome.

Monosomy 21q: two cases of del(21q) and review of the literature

We report on two cases of partial monosomy 21 and review cases with a partial or an apparently full monosomy 21. In situ hybridization and/or molecular studies appear to be necessary tools to study imbalance in such a small chromosome and to perform further genotype-phenotype correlations. The segregation mode in cases with a translocation is adjacent 1, adjacent 2, and 3:1 in about 1/4, 1/4 and 1/2 of the cases, respectively.

A complex rearrangement associated with sex reversal and the Wolf-Hirschhorn syndrome: a cytogenetic and molecular study

We report a male infant referred with multiple congenital abnormalities consistent with the Wolf-Hirschhorn syndrome. Cytogenetic analysis showed a chromosome complement of 46,XX with a deletion of 4p15.2----4pter and its replacement by material of unknown origin. The patient was positive for a number of Yp probes including SRY, the testis determining factor, and in situ hybridisation localised the Yp material to the tip of the short arm of one X chromosome. Using pDP230, a probe for the pseudoautosomal region, and M27 beta, which recognises a locus in proximal Xp, the material translocated on to 4p was identified as originating from the short arm of the paternal X chromosome. The most reasonable explanation for this complex rearrangement is two separate exchange events involving both chromatids of Xp during paternal meiosis. An aberrant X-Y interchange gave rise to the sex reversal and an X;4 translocation resulted in additional, apparently active Xp material and a deletion of 4p which produced the Wolf-Hirschhorn phenotype.

Molecular genetics of human chromosome 21

Chromosome 21 is the smallest autosome, comprising only about 1.9% of human DNA, but represents one of the most intensively studied regions of the genome. Much of the interest in chromosome 21 can be attributed to its association with Down's syndrome, a genetic disorder that afflicts one in every 700 to 1000 newborns. Although only 17 genes have been assigned to chromosome 21, a very large number of cloned DNA segments of unknown function have been isolated and regionally mapped. The majority of these segments detect restriction fragment length polymorphisms (RFLPs) and therefore represent useful genetic markers. Continued molecular genetic investigation of chromosome 21 will be central to elucidating molecular events leading to meiotic non-disjunction and consequent trisomy, the contribution of specific genes to the pathology of Down's syndrome, and the possible role of chromosome 21 in Alzheimer's disease and other as yet unmapped genetic defects.

DNA replication and inactivation patterns in structural abnormality of sex chromosomes. I.X-A translocations, rings, fragments, isochromosomes, and pseudo-isodicentrics

High resolution chromosome analysis and bromodeoxyuridine (BrdUrd) incorporation have been applied to study patterns of chromosomal replication (inactivation) in two cases of unbalanced X-autosome translocations, seven cases of X and Y chromosome rings or fragments, and five cases of dicentric isochromosomes (Xq). Our results indicate the following: (1) In (X-A) translocations, detailed replicational analysis of the translocated autosomal segment is informative. Absence of "spreading effect" and partial-incomplete spreading effect are the most common observations. (2) Sex chromosome derived fragments and rings can be differentiated based on their replicational features. (3) Dicentric isochromosomes (Xq) can be classified based on intercentromeric distances, replicational asynchrony, and centromere inactivation. (4) A correlation between intercentromeric distance and degree of 45,X mosaicism was observed in dicentric "i(Xq)" chromosomes. Evidence for spreading effect based on our results and on the review of the literature has been critically analyzed and general rules in evaluating spreading effects (SE) proposed. The cytologic detection of active regions on the late replicating X chromosome and the inactivation capacity of the juxtacentromeric region of Xp is evaluated. It is proposed that centromere suppression and underreplication are related phenomena. Finally, the analysis of informative replicational stages is emphasized and the application of their analysis in basic and clinical cytogenetics demonstrated.

BrdU-Hoechst-Giemsa analysis of DNA replication in synchronized lymphocyte cultures. Study of human X and Y chromosomes

The combination of a technique of reversible methotrexate (MTX) imposed G1/S block in cultures of human lymphocytes with the BrdU-Hoechst-Giemsa technique permitted the study of DNA replication patterns in individual chromosomes at different intervals of the S phase in a cell cohort with uniform S + G2 duration. The procedure did not increase either the frequency of chromosomal breakage or SCE frequency. The technique applied permitted visualization of the banding pattern in over 90% of mitoses. Examination of mitosis following different times of exposure to BrdU revealed a high degree of synchrony in the progression of the cell cohort examined through the S phase. The presence of two distinct late replication patterns of the allocyclic X chromosome was confirmed in studies on lymphocytes from normal human females by this technique. Interindividual and intercellular differences of the replication pattern have been demonstrated. The replicating patterns from one individual were relatively constant. The analysis of the Y chromosome has revealed marked differences of the termination of replication in individual cells. Euchromatic regions have been shown to complete DNA synthesis first, followed by the distal part of the long arm and, finally, by the region of Yq11/Yq12 junction. Lateral asymmetry was localised at this region.

Structural anomalies of the X chromosome: personal observation and review of non-mosaic cases

We describe a new case of partial deletion of the long arm of the X chromosome, found in a 24-year-old female with secondary amenorrhea; the karyotype of the proposita is 46,X,del(X)(q22). We take this opportunity to review the previously published descriptions of non-mosaic structural anomalies of the X chromosome (X isochromosomes excepted) with the goal of "testing" the recent hypothesis formulated about: (a) the existence of an X inactivation center (Therman et al. 1974b); (b) the presence of a "b" segment remaining active on Xp (Therman et al. 1976); (c) the potential importance of a critical area on Xq linked to gonadal function (Sarto et al. 1973); and (d) the presence of normal gonadal function despite and Xp terminal deletion (Fraccaro et al. 1977). We conclude that the above-mentioned theories, as well as those concerning phylogenetic evolution of sex chromosome morphology presented by Lyon (1974) and Hoo (1975), receive support from practically all of the 149 cases we compared. Regarding the features of the Turner syndrome, we propose "mapping" of the X chromosome as follows: the genes involved in gonadal function seem to be located on the proximal part of Xp and on the distal part of Xq, whereas the genes whose absence is responsible for somatic features of the syndrome may be distributed along the length of Xp and the middle section of Xq(q21-q26). Furthermore, we note some interesting analogies between the evolutional model proposed by Hoo (1975) and the map we visualize.

Translocation (X;6) in a female with Duchenne muscular dystrophy: implications for the localisation of the DMD locus

A female with Duchenne muscular dystrophy who was a carrier of a balanced translocation t(X;6)(p21;q21) is reported. Four other previously described (X;A) translocations associated with DMD share with the present case a breakpoint at Xp21. The extremely low probability of five independent (X;A) translocations having a breakpoint at Xp21 points to a non-rand association of this site with the DMD phenotype. A DMD locus at Xp21 could be damaged by the translocation, giving rise to Duchenne muscular dystrophy. Alternatively, a pre-existing DMD gene could weaken the chromosome, favouring breaks at Xp21.

X-autosome translocation with a breakpoint in Xq22 in a fertile woman and her 47,XXX infertile daughter

An unusual case is presented of a fertile woman heterozygous for a balanced X-autosome translocation t(X;12)(q22;p12) with a break-point (Xq22) in the critical region of the X chromosome. The karyotypes of her daughter, who is infertile, and one of her two sons are 47,XXX,t(X;12)(q22;p12) and 46,XY,t(X;12)(q22;p12) respectively. The literature on balanced X-autosome translocations in males and females involving both arms of the X chromosome is reviewed. All 23 of the 36 cases of females with balanced Xq-autosome translocation, that exhibited gonadal failure have a break-point between bands Xq13 and Xq26.U

Structural variation of chromosome 21 and symptoms of Down's syndrome

The analysis of the fine structure of the chromatids permits the identification of different regions on the long arm of chromosome 21. The preponderant role of the distal third of the long arm in the syndrome of trisomy 21 is now well established. Thus, trisomy of only band 21q22 results in a state identical to that caused by complete trisomy 21. If the trisomy involves only a part of band 21q22, the intensity of the symptoms is diminished, but the appearance of the patient is still reminiscent of Down's syndrome. Monosomy for band 21q22 results in a pathologic condition in which the morphological anomalies are the inverse of those observed in trisomic patients. This syndrome, as a "contre-type" to trisomy 21, is lethal. Trisomy of the proximal long arm region of chromosome 21 (21q21 leads to 21q22) is not associated with malformations but is accompanied by mental retardation. Monosomy of the same region results in a pathologic condition, which does not have features of the contre-type of trisomy 21.

Complex chromosome rearrangements. Report of a new case and literature review

A complex and unique, apparently balanced translocation involving three autosomes and an X in a phenotypically abnormal child is described. Family studies using glucose 6 phosphate dehydrogenase as a marker provided biochemical evidence of non-random expression of this Xq locus and suggested that this de novo abnormality in the proband could be paternal in origin--the first such instance to be recorded.

Gonadal dysgenesis in a patient with an X;3 translocation: case report and review

A patient with primary amenorrhoea and absence of secondary sex characteristics was found to have a balanced X;3 translocation. This phenotype is reported in approximately one-third of the balanced X;autosome translocation cases. The normal X chromosome is inactive in the present case which is in agreement with most of the similar cases. A review of the 66 balanced X;autosome translocations reported to date is presented.

X chromosome constitution and the human female phenotype

The correlations of abnormal X chromosome constitutions and the resulting phenotypes in the human female are reviewed. The following hypotheses put forward to explain these correlations are discussed in detail: (1) The damage is done before X inactivation; (2) An effect is exerted between reactivation of the X chromosome(s) and meiosis in oocytes; (3) A recessive gene(s) in hemizygous condition might be expressed in the cases in which the same X is active in all cells; (4) A change in the number of presumed active regions on the inactive X chromosomes might have an effect; (5) A position effect, in that the region Xq13-q27 has to be intact in both X chromosomes to allow normal development, may be responsible; (6) An effect during the period when cells with different inactivation patterns compete is a probability; (7) The original X inactivation may be neither regular nor random. The conclusion reached is that the phenotypic effects of a specific X chromosome aberration may be simultaneously exerted through different pathways (Tables 1 and 2). Hypotheses (2), (4), (5), and (6) are considered probable. Hypothesis (3) has been discarded, and there is very little evidence for hypotheses (1) and (7).

X;15 translocation in a retarded girl: X inactivation pattern and attempt to localise the hexosaminidase A and other loci

Cytogenetic studies on a retarded girl showed a complex S;15 translocation, karyotype 45,X,-15,+t(X15). The translocation X chromosome was non-randomly partially inactivated, the inactivation being mainly confined to the X segment and in some cells only to the X long arm. Gene marker studies failed to show anomalous segregation of the hexosaminidase A gene or any other gene markers tested.

Position of the human X inactivation center on Xq

In three women with a 46,XXq- chromosome constitution, the length of the deletion was expressed as the ratio of the remaining part of Xq to Xp c' over a + b. In one of them (KH) this ratio was 0.33, in another (GE) 0.59, and in the third (AP) the ratio fell between these values. The break in KH is more or less on the border of the Q-dark proximal region. A comparison with relevant X-autosomal translocations indicates that the X inactivation center lies near, but not at the border of, the Q-dark and the adjoining bright region (c and d).

Evidence for a correlation between late replication and autosomal gene inactivation in a familial translocation t(X;21)

A familial translocation t(X;21)(q2700;q11) is studied. A girl, trisomic for almost all the chromosome 21, has a mildly abnormal phenotype. A second girl, phenotypically abnormal, is monosomic for the juxtacentromeric region of chromosome 21 only. A comparison of the replication pattern and of the activity of superoxide dismutase (gene located on chromosome 21) shows a clear correlation between late replication, gene inactivation and phenotype expression of chromsome 21.

Spreading of inactivation in an (X;14) translocation

In the KOP translocation, t(X;14)(q13;q32), virtually the entire long arm of the X has been translocated to the end of the long arm of chromosome 14. Meiotic secondary nondisjunction in a female balanced carrier of the translocation has led to a son with two der(14) or 14-X chromosomes. The normal X chromosome is late replicating in the mother. One of the two 14-X chromosomes is late replicating in the son, with heavy terminal labeling of all but the centromeric end of the chromosome. This suggests that genetic inactivation has spread from the Xq segment of the translocation chromosome to at least two thirds of the segment derived from chromosome 14, and that the remaining proximal segment of chromosome 14 is possibly still genetically active. These findings provide an explanation for the phenotype: Klinefelter syndrome plus a few mild malformations that are sometimes seen in this syndrome but are also seen in duplication of the proximal portion of chromosome 14. Although the proband has a duplication of virtually an entire chromosome 14, 14(pter leads to q32), the phenotypic effect of the autosomal duplication has been mostly nullified by the spread of inactivation.

X-inactivation pattern in three cases of X/autosome translocation

We describe an X/15 translocation which was balanced in a phenotypically normal mother [46,X,t(X;15)(p22;q15)] and unbalanced in her phenotypically abnormal daughter [46,X,der(X),t(X;15)(p22;q15)mat]. A third case involves a balanced X/21 translocation in a girl with a multiple congenital anomaly-retardation syndrome [46,X,t(X;21)(p11;p11?)]. 5-BrdU acridine orange banding on lymphocytes revealed late replication of the normal X chromosome in the mother and of the normal or abnormal X chromosome in the two other cases. Our findings are only partially consistent with previous observations. All X-inactivation patterns can be explained by random inactivation and subsequent selection against specific cell lines. Furthermore, the findings in our patient with X/21 translocation support the hypothesis of the existence of one inactivation center on Xq.

Balanced reciprocal X-4 translocation in a female patient with early secondary amenorrhea

Balanced translocations involving an X chromosome and an autosome have been infrequently reported. A patient with a balanced X-autosome translocation 46,X,rcp(X;4)(q26;q21) who exhibited early secondary amenorrhea and gonadal dysgenesis is described. In an effort to explain the varied phenotypic expressions encountered in female cases of balanced X-autosome translocations, evidence will be provided to suggest an extension of the minimal limits of the "critical region" in the long arm of the X chromosome from A-B, as described by Sarto and associates, to new minimal limits C-D.

Familial t(X;2) (p223;q323) with partial trisomy 2q and male and female balanced carriers

A familial t(X;2) (p223;q323) is responsible for partial trisomy 2q in the proposita, a 3-year-old girl with severe mental retardation and hypotrophia. It is present in the balanced state in the mother, two daughters, and one son. X-replication was studied after BUDR incroporation and acridine orange staining. The reproductive impairment of balanced X/autosome translocations is discussed.

Primary amenorrhea with a new mosaic 46,XXqi/47,XXqi Xp-. Consideration on the X isochromosome formation and X chromosome inactivation

A case of Turner's syndrome was found to be 46,XXqi/47XX,qi Xp-, a new mosaic. The origin of such a mosaic, the formation of the Xq isochromosome using the C-banding technique, and the X chromosome inactivation are discussed. The Xq isochromosome was apparently monocentric, but probably with two strictly close centromeres. The inactivated X seemed to be the Xqi or the normal X alternatively.

X-short arm deletion gonadal dysgenesis in two siblings due to unique translocation (Xp-;16p+)

A family demonstrating short arm deletion of the X chromosome as a consequence of X-16 balanced translocation in the mother is reported. The two Xp- sisters exhibit clinical signs of gonadal dysgenesis, while the balanced carriers are phenotypically normal. To our knowledge this represents the only example of both the balanced carrier state for an X translocation and its genetic consequence occurring in the offspring, as well as the involvement of X-16 interchange. Literature data of 37 additional cases of verified X translocations are discussed.

BrdU-33258 Hoechst analysis of DNA replication in human lymphocytes with supernumerary or structurally abnormal X chromosomes

BrdU-33258 Hoechst techniques have been used to characterize DNA replication patterns in lymphocytes from hunam females with supernumerary or structurally abnormal X chromosomes. Fluorescence analysis permits identification of late replicating X chromosomes in a very high proportion of cells and affords a high resolution method for determining the interchange points of X-X and X-autosome translocations. Asynchrony among terminal replication patterns of multiple late replicating X chromosomes within an individual cell can occasionally be demonstrated. The arms of isochromosomes usually exhibit symmetrical fluorescence patterns, with replication terminating in bands Xq21 and Xq23 (predominant pattern) or in bands Xq25 and Xq27 (alternative pattern) in both arms. In the vast majority of lymphocytes containing a balanced X-13 or X-19 translocation, the normal X is late replicating. However, DNA synthesis in the translocation products occasionally appears somewhat delayed relative to that expected for an early replicating X, consistent with possible position effects on replication kinetics.