An abnormal large human chromosome identified as an end-to-end fusion of two X's by combined results of the new banding techniques and microdensitometry

Characterization of chromatin at structurally abnormal inactive X chromosomes reveals potential evidence of a rare hybrid active and inactive isodicentric X chromosome

X chromosome structural abnormalities are relatively common in Turner syndrome patients, in particular X isochromosomes. Reports over the last five decades examining asynchronous DNA replication between the normal X and isochromosome have clearly established that the structurally abnormal chromosome is the inactive X chromosome (Xi). Here the organization of chromatin at a deleted X chromosome, an Xq isochromosome, and two isodicentric chromosomes were examined. Consistent with previous differential staining methods, at interphase, the X isochromosome and isodicentric X chromosomes frequently formed bipartite Barr bodies, observed by fluorescence microscopy using numerous independent bona fide markers of Xi heterochromatin. At metaphase, with the exception of the pseudoautosomal region and the duplicated locus of the macrosatellite DXZ4 (if present on the abnormal X chromosome based on break points), euchromatin markers were absent from the Xi, whereas histone variant macroH2A formed reproducible banded mirror-image chromosomes. Unexpectedly, the isodicentric chromosome in 46,X,idic(X)(q28) cells, which carry a near full-length q-arm-to-q-arm fused chromosome, showed at interphase very rare instances of Xi chromatin bodies that were separated by large distances in the nucleus. Further examination using immunofluorescence and FISH support the possibility that these rare cells may represent ones in which one half of the isodicentric chromosome is active and the other half is inactive.

A combined banding method that allows the reliable identification of chromosomes as well as differentiation of AT- and GC-rich heterochromatin

Сonstitutive heterochromatin areas are revealed by differential staining as C-positive chromosomal regions. These C-positive bands may greatly vary by location, size, and nucleotide composition. CBG-banding is the most commonly used method to detect structural heterochromatin in animals. The difficulty in identification of individual chromosomes represents an unresolved problem of this method as the body of the chromosome is stained uniformly and does not have banding pattern beyond C-bands. Here, we present the method that we called CDAG for sequential heterochromatin staining after differential GTG-banding. The method uses G-banding followed by heat denaturation in the presence of formamide with consecutive fluorochrome staining. The new technique is valid for the concurrent revealing of heterochromatin position due to differential banding of chromosomes and heterochromatin composition (AT-/GC-rich) in animal karyotyping.

Genetic and epigenetic regulation of centromeres: a look at HAC formation

The centromere is a specialized chromosomal locus required for accurate chromosome segregation. A specific histone H3 variant, CENP-A, assembles at centromeres. CENP-A is required for kinetochore protein assembly and is an epigenetic marker for the maintenance of a functional centromere. Human CENP-A chromatin normally assembles on α-satellite DNA (alphoid DNA), a centromeric repetitive sequence. Using alphoid DNA arrays, human artificial chromosomes (HACs) have been constructed in human HT1080 cells and used to dissect the requirements for CENP-A assembly on DNA sequence. However, centromere formation is not a simple genetic event. In other commonly used human cell lines, such as HeLa and U2OS cells, no functional de novo centromere formation occurs efficiently with the same centromeric alphoid DNA sequences. Recent studies using protein tethering combined with the HAC system and/or genetic manipulation have revealed that epigenetic chromatin regulation mechanisms are also involved in the CENP-A chromatin assembly pathway and subsequent centromere/kinetochore formation. We summarize the DNA sequence requirements for CENP-A assembly and discuss the epigenetic regulation of human centromeres.

De novo X/X translocation in a patient with secondary amenorrhea

A 46,X,t(X;X) (qter-->p22;;p22-->qter) karyotype was found in the chromosome analysis of a 22 years old female patient with secondary amenorrhea. Further analysis with fluorescence in situ hybridization indicated that the marker chromosome had one active and one inactive centromere originating from the X chromosome. RBG-banding showed that the derivative X chromosome was preferentially inactivated in cultured lymphocytes.

Alternate centromere inactivation in a pseudodicentric (15;20)(pter;pter) associated with a progressive neurological disorder

A 13 year old male with a severe progressive neurological disorder was found to have a pseudodicentric chromosome resulting from a telomeric fusion 15p;20p. In lymphocytes, the centromeric constriction of the abnormal chromosome was always that of the chromosome 20, while in fibroblasts both centromeres were alternately constricted. Cd staining was positive only at the active centromere, but a weak anticentromere immunofluorescence was present at the inactive one. We suggest that centromere inactivation results from a modified conformation of the functional DNA sequences preventing normal binding to centromere specific proteins. We also postulate that the patient's disorder, reminiscent of a spongy glioneuronal dystrophy as seen in Alper's and Creutzfeldt-Jakob diseases, may be secondary to the presence of the pathogenic isoform of the prion protein encoded by a gene mapped to 20p12----pter.

Simultaneous cell type identification and premature chromosome condensation analysis in a case of multiple myeloma

The technique of premature chromosome condensation was combined with immunocytochemical techniques to determine the karyotype of the plasma cells in a patient with multiple myeloma. Although the patient's myeloma cells had a diploid DNA content, the mean chromosome number was 39. Multiple chromosome rearrangements were documented in the G- and C-banded G1 and G2 prematurely condensed chromosomes, and several of these involved telomeric regions resulting in dicentric chromosomes. That the aberrant karyotype was present in the kappa light chain positive plasma cells was proved by simultaneous chromosome analysis and immunocytochemical examination of the fused cells. Thus the combination of premature chromosome condensation and immunocytochemistry proved a powerful tool for cytogenetic analysis of a slow-growing, heterogeneous cell population.

Visualization of centromere proteins CENP-B and CENP-C on a stable dicentric chromosome in cytological spreads

We have screened for the presence of two centromere autoantigens, CENP-B (80 kDa) and CENP-C (140 kDa) at the inactive centromere of a naturally occurring stable dicentric chromosome using specific antibodies that do not cross-react with any other chromosomal proteins. In order to discriminate between the active and inactive centromeres on this chromosome we have developed a modification of the standard methanol/acetic acid fixation procedure that allows us to obtain high-quality cytological spreads that retain antigenicity with the anti-centromere antibodies. We have noted three differences in the immunostaining patterns with specific anti-CENP-B and CENP-C antibodies. (1) The amount of detectable CENP-B varies from chromosome to chromosome. The amount of CENP-C appears to be more or less the same on all chromosomes. (2) CENP-B is present at both active and inactive centromeres of stable dicentric autosomes. CENP-C is not detectable at the inactive centromeres. (3) While immunofluorescence with anti-CENP-C antibodies typically gives two discrete spots, staining with anti-CENP-B often appears as a single bright bar connecting both sister centromeres. This suggests that while CENP-C may be confined to the outer centromere in the kinetochore region, CENP-B may be distributed throughout the entire centromere. Our data suggest that CENP-C is likely to be a component of some invariant chromosomal substructure, such as the kinetochore. CENP-B may be involved in some other aspect of centromere function, such as chromosome movement or DNA packaging.

Dicentric chromosomes and the inactivation of the centromere

The origin and behavior of human dicentric chromosomes are reviewed. Most dicentrics between two nonhomologous or two homologous chromosomes (isodicentrics), which are permanent members of a chromosome complement, probably originate from segregation of an adjacent quadriradial; such configurations are the result of a chromatid translocation between two nonhomologous chromosomes, or they represent an adjacent counterpart of a mitotic chiasma. The segregation of such a quadriradial may also give rise to a cell line monosomic for the chromosome concerned (e.g., a 45, X line). Contrary to the generally held opinion, isodicentrics rarely result from an isolocal break in two chromatids followed by rejoining of sister chromatids. In this case the daughter centromeres go to opposite poles in the next anaphase, and the resulting bridge breaks at a random point. This mechanism, therefore, leads to the formation of an isodicentric chromosome only if the two centromeres are close together, or if one centromere is immediately inactivated. Observations on the origin of dicentrics in Bloom syndrome support these conclusions. One centromere is permanently inactivated in most dicentric chromosomes, and even when the dicentric breaks into two chromosomes, the centromere is not reactivated. The appearance and behavior of the "acentric" X chromosomes show that their centromeres are similarly inactivated and not prematurely divided. Two Bloom syndrome lymphocytes, one with an extra chromosome 2 and the other with an extra chromosome 7, each having an inactivated centromere, show that this can also happen in monocentric autosomes.

Three related centromere proteins are absent from the inactive centromere of a stable isodicentric chromosome

We developed an aqueous spreading procedure that permits simultaneous analysis of human chromosomes by Q-banding and indirect immunofluorescence. Using this methodology and anticentromere antibodies from an autoimmune patient we compared the active and inactive centromeres of an isodicentric X chromosome. We show that a family of structurally related human centromere proteins (CENP-A, CENP-B, and CENP-C) is detectable only at the active centromere. These antigens therefore may be regarded both as morphological and functional markers for active centromeres.

Telomeric association of chromosomes in B-cell lymphoid leukemia

About 20% of leukemic bone marrow cells from each of two patients with B-cell lymphoid leukemias showed apparent translocations which appeared to be the result of telomeric association. In one patient, whole chromosomes were associated telomere to telomere in pairs; in the other patient, telomeres of whole chromosomes were associated with breakpoints located close to the proximal or distal ends of the heterochromatic band 1q12. Repeated base sequences, particularly (CA)n sequences, are believed to be the basis of telomere pairing, and likewise repeated base sequences of heterochromatin may explain the association of 1qh and telomeres. Telomeric association may be considered as a potential origin of new stable cytogenetic combinations that have a role in oncogene transposition and tumor etiology.

Two cases of ring chromosome 11

Two cases of ring chromosome 11 are reported. Both had mental retardation, microcephaly, and short stature. High resolution G banding in case 1 showed no visible loss of chromatin, the karyotype being assessed as 46,XX,r(11) (p15 X 4q2 X 5). In case 2, a Wilm's tumour developed at 8 months and the child died at 18 months. Cytogenetic analysis by Q banding demonstrated minimal chromosome deletion and the karyotype was considered to be 46,XY,r(11) (p15q25).

Dicentric chromosome 13 and centromere inactivation

The karyotype of a child with dysmorphic findings suggestive of both trisomy 13 and the 13q--syndrome was found to have cells with one of two different dicentric chromosomes: one bearing a duplication of chromosome 13q [46,XX,-13, + psu dic (13)t(13;13)(pter leads to cen leads to q34::q34 leads to pter)] and the other a deletion of 13q [46,XX,-13, + psu dic (13)t(13;13)(pter leads to cen leads to q22::q11 leads to pter]. Longitudinal cytogenetic studies in leukocytes demonstrated a loss of those cells possessing the small dicentric [psu dic(13)(q22;q11)], whereas fibroblasts from two separate skin biopsies contained only this marker. Q-band polymorphisms indicated that both dicentrics were of paternal origin, with the smaller dicentric derived from the larger via the bridge-breakage-fusion cycle. The presence of two active centromeres could not be confirmed in either dicentric.

DNA replication sequence in a dicentric (functionally monocentric) X chromosome formed by the joining of two X chromosomes at region p22

In this study we used densitometry to evaluate DNA replication kinetics in a rearranged chromosome formed by the joining of two X chromosomes at region p22. No 45X mosaicism is present in peripheral blood or fibroblast cultures. The patient has primary amenorrhea, short stature, and gonadal dysgenesis. The sequence of replication in the majority of cells is p11, q11, q13, q22-24, q12, p22, q26, q28, q27, q25, and p21, q21. Thus p11 is the earliest region to replicate, and q21 is the last. In 66% of 127 cells analyzed, the replication pattern is asymmetric, and bands q12, q26, and q28 are most likely to be out of phase on the two sides of the breakpoint. We find that band p22 has a delay of replication compared to an abnormal X derived from two X chromosomes joined at the q23 region previously reported by us. Structural rearrangement may therefore delay replication in the region of the break.

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.

Genetic and cytological characterisation of fusion chromosomes of Dictyostelium discoideum

Abnormally large chromosomes which appear to result from the fusion of 2 chromosomes of the normal karyotype have been found in diploids of Dictyostelium discoideum formed by parasexual fusion of haploid strains HU483 (N = 7) and HU245 (n = 7). These fusion chromosomes appear to be the products of the tandem translocation of most, if not all, of one acrocentric chromosome to the telomere of a second acrocentric. Thus the chromosome number of the diploids is reduced from the normal 2n = 14 to 2n = 13 with the formation of an abnormally large acrocentric fusion chromosome. Experimental haploidisation of such diploids result in two types of products, those with a normal 7 chromosome karyotype and those with an abnormal 6 chromosome karyotype which contains the fusion chromosome. Genetic analysis of haploid segregants indicates that linkage groups II and VII are involved in this fusion. Phenotypes of recombinant diploids obtained following mitotic crossing-over establishes that linkage group II is proximal to linkage group VII. Cytological examination of the karyotypes of haploid strains bearing the fusion chromosome suggest that chromosome 2 may correspond to linkage group II and chromosome 3 to linkage group VII. Haploid strains bearing the fusion chromosome grow and develop normally so little or no genetic information can have been lost in the fusion event. While the nature of this event is unknown it may have involved aberrant recombinational DNA repair since the parental haploid strain HU483 bears the radB13 DNA repair mutation.

Cytogenetic and clinical studies in gonadal dysgenesis with 46,X,Xt(qter leads to p221::p223 leads to qter) karyotype: review and phenotype/karyotype correlations

Chromosome analysis by Q, R, and C banding was performed in a case diagnosed clinically as gonadal dysgenesis and the karyotype was shown to be 46,X,Xt(qter leads to p221::p223 leads to qter). Localisation of the breakpoints in the fused X chromosomes and replication studies have led to a hypothesis on the origin of the translocation. A comparison of clinical and cytogenetical findings in this and other published cases has also been made in an attempt to detect some phenotype/karyotype correlations.

Tandem duplication dup(X)(q13q22) in a male proband inherited from the mother showing mosaicism of X-inactivation

An aberrant X chromosome containing extra material in the long arm was observed in a psychomotoric retarded boy and his healthy, short-statured mother. The proband showed generalized muscular hypotony, growth retardation, and somatic anomalies including hypoplastic genitalia and cryptorchism. Chromosomal banding techniques suggested a tandem duplication of the segment Xq13 leads to Xq22. In the mother the vast majority of lymphocytes showed late replication of the aberrant X chromosome. Some of her cells, however, contained an apparently active aberrant X. Both the early- and late-replicating aberrant X exhibited late replication patterns very similar to those described for normal X chromosomes in lymphocytes. Asynchrony of DNA replication among the two segments Xq13 leads to Xq22 in the dup(X) was never observed. We consider that the clinical picture of the proband is caused by an excess of active X material.

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).

A duplication-deficiency X chromosome in a girl with severe mental retardation

Clinical and cytogenetic findings in a 14-year-old girl with severe mental retardation and partial Turner's syndrome are reported. An anomalous X was observed in the patient's karyotype. On the basis of QFQ, GAG and RBA banding studies, an Xq duplication (q12 leads to qter) and an Xp deficiency (p22 leads to pter) were apparent in this chromosome.

A girl with an end-to-end fusion of two X'S

An abnormal large chromosome was seen in the karyotype of a 3-year-old girl with features of Turner's syndrome: i.e., short stature, cubitus valgus, coarctation of aorta. With the banding technics this abnormal chromosome appears to be the result of a fusion of two X chromosomes, short arm-to-short arm. This chromosome has two regions with C-heterochromatin and is late replicating.

Single Cd band in dicentric translocations with one suppressed centromere

Six human dicentric translocations involving the presence of a suppressed centromere were studied: one case of t dic(9;22), one of t dic(13;18), one of t dic(14;15), and three of t dic(13;14). All exhibited chromatid separation at an acrocentric centromere and were demonstrated to have two regions of centromeric constitutive heterochromatin. The four that were available for retrospective study showed a single Cd band, with absence of the Cd band at the suppressed centromere.

X-X translocation in a patient with gonadal dysgenesis and the problems of phenotype-karyotype correlations

A sex-chromatin-positive woman without stunted growth, but with primary amenorrhea, and some stigmas of pure gonadal dysgenesis had the chromosome constitution 45,X/46,Xt(X;X)(q27;q27). The abnormal chromosome formed a large Barr body and was late-labeling. The chromosome consisted of two X chromosomes attached by their long arms (end-to-end), both apparently having the partial distal deletion. Both centromeric regions showed C-staining but only one constriction. The chromosome is interpreted as an isodicentric with only one centromere functioning. Some problems of phenotype-karyotype correlations are discussed.

Replication patterns of three isodicentric X chromosomes and an X isochromosome in human lymphocytes

Chromosomes from four patients with variants of the Turner syndrome were investigated by G- and C-bandind and DNA replication techniques. Their karyotypes were: 1) 46,X,idic(X)(q28), 2) 45,X/46,X,idic(X)(q24), 3) 45,X/46,X,idic(X)(p11), and 4) 46,X,i(Xq). In patients 1, 2, and 3, the abnormal X was isodicentric, with different break-and-fusion points in each case. In each, the G-band pattern on one side of the breakpoint was a mirror image of that on the other side. Each had two distinct C-bands, only one of which was associated with a primary constriction. The fourth patient had an isochromosome of the long arm of an X in which only one C-band could be discerned. Replication studies were done on lymphocyte cultures by incorporating a thymidine analogue and staining with acridine orange. In addition, replication patterns of normal early- and late-replicating X chromosomes were studied in two normal females. In the four patients, all the normal X chromosomes had normal early-replication patterns. The two idic(X) chromosomes with break-and-fusion points on their long arms almost always had symmetric replication patterns, which demonstrates that the corresponding bands replicated synchronously. In contrast, many of the idic(X)(p11) and i(Xq) chromosomes showed asymmetric or asynchronous replication. In each, the replication pattern of the abnormal X was similar to the equivalent portions of a normal late-replicating X.

Duplication deficiency of an X-chromosome with and without 45,X mosaicism in three girls. Cytogenetic, clinical, and hormonal findings

In three girls, aged 14, 15 and 16 years, the chromosome analysis revealed a morphologically abnormal, enlarged X-chromosome resembling in size and centromere position the chromosome no. 2. The translocation points were different in all three cases. The Barr-bodies were enlarged. In two girls a 45,X mosaicism (25% and 10%) was found in lymphocyte cultures. The length at birth was 43, 47 and 48 cm, and none of the girls was born before term. The main clinical abnormalities in all three cases were a marked growth retardation, slight morphological dysplasias, lack of sexual development and social immaturity. GH and cortisol secretion during an insulin tolerance test were normal. LH and FSH were elevated and showed an exaggerated reaction on LH-RH. Oestrogens were low normal and androgens within the normal range. At laparatomy the gonads were found to be streak gonads. For two girls cell cultures of gonadal tissue were set up, the chromosome findings of which corresponded to those of the lymphocyte cultures. The abnormality of the gonosomes reported here seems to represent a special form of gonadal dysgenesis. Although the translocation points were different in the three patients and one had no mosaic, while the other two showed 45,X/46,XX mosaicism, the clinical and hormonal findings were nearly the same for all three girls.

A girl with mosaicism for a dicentric X chromosome (45,X/46,X,dic(X) (Xqter to p22::p22 to qter))

A 12-year-old girl was examined for growth retardation and a few very discrete dysmorphologic stigmata of Turner's syndrome; the genitalia were infantile yet both ovaries possessed functioning follicles. R- and C-banding techniques and Brdu treatment demonstrated a 45,X formula in 95% of lymphocytes, with 5% presenting a 46,X,dic(X) formula. Cytogenetic and clinical problems raised by this observation are discussed in relation to data from the literature.

Partial trisomy 21. Further evidence that trisomy of band 21q22 is essential for Down's phenotype

Cytogenetic analysis of a 6-year-old girl with moderate mental retardation revealed 46 chromosomes with a tandem translocation (21;21) resulting in a partial trisomy 21. Only the terminal band 21q22 was not in triplicate. G-, Q-, R-, and C-banding techniques and silver nitrate staining of the nucleolus organizer regions (NORs) were used to identify this chromosome fully. The phenotype of the patient was not typical for Down's syndrome, providing additional evidence that trisomy of band 21q22 is pathogenetic for the phenotype of Down's syndrome. This is also a new example in human pathology of a stable 'dicentric' chromosome in which one of the centromeric constrictions appears to be nonfunctional.

46, X,X-X terminal rearrangement/45, X mosaicism in a child with short stature

A phenotypically female child, investigated because of short stature, had abnormally large, often bipartite Barr bodies and a mosaicism of 45, X cells and cells with 46 chromosomes which included an exceptionally large metacentric chromosome (Xp+). G- and C-banding established that the chromosome was derived from two substantially entire X chromosomes joined short arm-to-short arm, and was likely to be an isodicentric X with functional inactivation of one centromere.

Stable dicentric autosome, tdic (8:22)(p23:p13), in a mentally retarded girl

A dicentric autosome, tdic(8:22)(p23:p13), was found in all metaphase cells examined from the peripheral blood of a mentally retarded girl. It is suggested that the centromere of chromosome 22 was inactive, allowing the dicentric to behave as a monocentric element. The involvement of acrocentric chromosomes in the stable dicentric autosomes of man is discussed.

A possible active segment on the inactive human X chromosome

An idic(Xp--) in which the two X chromosomes are attached short arm to short arm, and which thus has two b regions (the Q-dark segment next to the centromere on Xp) between the inactivation centers, assumed to be situated on the Q-dark region next to the centromere on Xq, showed 63.8% bipartite Barr bodies as compared with 22.2% formed by idic(Xq--). In addition, the mean distance of the two parts of the Barr bodies in the fibroblasts of a patient with idic(Xp--) is significantly greater than in the cases with one or no b region. Contrary to the other patients with abnormal X chromosomes, the buccal cells of a woman idic(Xp--) showed a number of bipartite Barr bodies. -- To explain these observations we have put forward the hypothesis that the b region on the Xp always remains active and thus, when the rest of the chromosome forms a Barr body, this segment is extended, allowing the two parts of the X chromatin to get farther apart and at the same time increasing the percentage of bipartite bodies.

Dicentric X isochromosomes in man

Four cases of Turner's syndrome are presented in which an apparent X isochromosome i(Xq) has been found to possess two regions of centromeric heterochromatin. It is suggested that these chromosomes were isodicentric structures capable of functioning as monocentric elements as a result of the inactivation of one centromere. The prevalence of mosaicism is believed to be a consequence of the dicentric nature of these chromosomes, and it is considered possible that a high proportion of X isochromosmes are structurally dicentric. Banding patterns showed that the exchange site involved in the formation of the dicentric chromosome was different in at least three of the cases.

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.

Structure and inheritance of some heterozygous Robertsonian translocation in man

Banding studies in 25 Robertsonian translocations showed that all could be interpreted as stable dicentrics. The mechanism for their stability is likely to be the proximity of their centromeres but centromeric suppression could also have a role. In many of these dicentric translocations, discontinuous centromeric suppression, as indicated by chromatid separation at one of the centromeric regions, was observed in C-banded preparations. A further observation of undefined relation to the first was that the ratio of the two constitutive centromeric heterochromatin (CCH) regions from the component chromosomes of the translocations was variable in the same translocation type, e.g. t(13;14). It is proposed that this ratio may influence the segregation ratio. Abnormal spermatogenesis is suggested as the likely mechanism for the difference in the proportion of aneuploid offspring in the progeny of maternal and paternal heterozygotes. Neither of the t dic(21;21)s could be interpreted as isochromosomes. It is proposed that Robertsonian fusion translocations be defined as stable, dicentric, whole-arm translocations, with both centromeres in a median position and resulting in the loss of a small acentric fragment during this formation. It is suggested that they occur at high frequency between telocentric or, as in man, certain acrocentric chromosomes because of some intrinsic property of those chromosomes not possessed by metacentric chromosomes and mediated by interphase association of centromeres.

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.

Fusion of two apparently intact human X chromosomes

Cytological studies have been presented from a 15-year-old girl with short stature and failure of puberty. Buccal mucosa preparations revealed X-chromatin mass approximately double in size of that of a normal female. Leukocyte metaphases suggested a two cell line composition of the patient. One population of cells conformed with 45,X chromosome distribution. The chromosome complement of her other cell line had a modal number of 46. In this cell line a "C" chromosome was replaced by an exceptionally large submetacentric chromosome. This abnormal element exhibited late DNA replicating pattern. G-banding study revealed that the abnormal chromosome was produced as a result of fusion involving telomeric ends of long arms of 2 intact X chromosomes. This translocation X was bearing 2 C-banded areas; one around the centromere and the other at the distal end of the long arm. The distal C-band area did not show any evidence for centromeric function. It appears that a centromere becomes latent in the presence of another centromere in a translocation bearing 2 total chromosomes. Such a change of state in the additional centromere is vital for the stability of the translocation chromosome.

Method for the determination of mean densitometric profiles of chromosomes: application to human chromosomes stained by quinacrine mustard, ethidium bromide or by the Feulgen reaction

When comparing the densitometric profiles of corresponding chromosomes registered from different metaphases or homologous pairs, one is always faced with the variability of their length and overall height. This makes difficult the quantitative comparison of a given chromosome treated by various staining procedures.--A simple and rapid method has been developed for normalizing the densitometric profiles and averaging them in order to obtain a "mean density pattern" of each chromosome. The analysis involves: photographic images, digitalization of the densitometric profiles and processing of the data by a mini-computer.--The method, based on a linear relationship between the area of the densitometric profiles and their length, has been applied to five human chromosomes (1, 2, 6, 12 and 16) stained by ethidium bromide, quinacrine mustard (with or without acidic hydrolysis), pararosaniline and bisaminophenyl-oxadiazole (Feulgen reaction).

Differences in human X isochromosomes

In this paper we describe two types of i(Xq), in three patients. A classification is proposed for at least seven different types of human i(Xq)s or X long-arm duplications described by banding in the literature. Type 1 reported here and also in the literature may be the most common. It consists of a single visible centromere, metacentric, length similar to number 3, G-banding interpreted as i(X)(qter leads to cen leads to qter), one C-band like a normal X. Type 2 reported here may not have a counterpart in the literature; it exhibits a single visible centromere, submetacentric, length similiar to number 3, extra G- and C-bands on region ql. The classification summarized in this paper implies that different breakpoints are involved in the production of human X long-arm isochromosomes or duplications. Some include duplications of short arm. Morphological differences in i(Xq)s will complicate their use for studying the effect of X chromosome structure on phenotype, unless differences are defined clearly. It seems important to resolve the question of whether these reported abnormal X chromosomes involve rearrangements between the same or two X chromosomes. We also report X chromosome defects in three generations of a family; both the mother and maternal grandmother of one 45,X,i(Xq)/45,X patient are themselves mosaics for 45,X/46,XX/46,X,r(X). This family suggests that familial predisposition to X chromosome abnormality included isochromosome formation, as well as ring formation and mosaicism.

Dicentric X-isochromosome (Xqi dic) and pericentric inversion of No. 2 [inv(2) (p15 q21)] in a patient with gonadal dysgenesis

A patient with the clinical stigmata of gonadal dysgenesis is presented. Cytogenetic investigations revealed two distinct structural chromosome rearrangements. One of these, an isochromosome for the long arm of the X, proved to be a dicentric element following C-banding. The second abnormality, an inherited familial marker, was a pericentric inversion of No. 2 [(inv 2) (p15q21)].

[Four new cases of X-autosome translocation in man (author's transl)]

Four new cases of translocations involving the X chromosome are reported. The first is a balanced t(1;X) observed in an abnormal girl. In most of the cells, the normal X is inactivated. The second case is a woman with oligomenorrhea, who has had two spontaneous abortions. She carries an umbalanced t(X-autosome). In all cells, the abnormal X, including the autosomal segment, is inactivated. The third and fourth cases are a mother, and her son. The mother has oligomenorrhea, carries a t(X,X), and has a karyotype of 46,X,+der t(X;X) and that of her son is 47,XY,+der t(X;X) and shows behavioral abnormalities. The abnormal X chromosome is inactivated in all the cells analyzed.