Heteromorphic X chromosomes in 46,XX males: evidence for the involvement of X-Y interchange

A comparative genomic hybridization study in a 46,XX male

OBJECTIVE

To identify Y chromosome material in an azoospermic male with an XX karyotype.

DESIGN

Case report.

SETTING

Faculty of medicine and Centro de Patologia Celular (CPC) medical center.

PATIENT(S)

A 33-year-old man with infertility.

INTERVENTION(S)

G-banding, fluorescence in situ hybridization (FISH), polymerase chain reaction (PCR), and comparative genomic hybridization (CGH).

MAIN OUTCOME MEASURE(S)

FISH for X and Y chromosomes, PCR for the SRYgene and amelogenin gene in the Xp (AMGX) and (AMGY), and losses or gains with CGH.

RESULT(S)

FISH analysis using X and Y chromosome-specific probes showed an X chromosome containing Y chromosome sequences on the top of the short arm; this Y chromosome region was not visible by conventional cytogenetic analysis. PCR amplification of DNA showed the presence of the sex-determining region of the Y chromosome (SRY) and the amelogenin gene in the pseudoautosomal boundary of the X chromosome (AMGX). CGH confirmed the presence of the chromosome region Yp11.2-pter and detected the presence of the two otherwise normal X chromosomes.

CONCLUSION(S)

The two Xpter (XPAR1) pseudoautosomal regions present in this XX male suggest the need to reevaluate XX males using CGH and PCR to characterize the clinical variability in XX males due to genes other than those located on the Y chromosome.

Steroid sulphatase levels in XX males, including observations on two affected cousins

Quantitative assays of steroid sulphatase in XX males have shown that some individuals have two functional loci, and others only one. Two affected cousins, who cannot share the same X-chromosome, nevertheless have male levels of steroid sulphatase, suggesting functional abnormality of the X chromosome. The hypothesis is advanced that these and other unusual features of X-chromosome function in some XX males, could be explained if such cases were due to an autosomal mutation, exercising its effect by causing abnormal inactivation of a subterminal area of Xp which normally escapes the inactivation process.

Molecular, cytogenetic, and clinical characterisation of six XX males including one prenatal diagnosis

Cytogenetic analysis, fluorescent in situ hybridisation (FISH), and molecular amplification have been used to characterise the transfer of Yp fragments to Xp22.3 in six XX males. PCR amplification of the genes SRY, RPS4Y, ZFY, AMELY, KALY, and DAZ and of several other markers along the Y chromosome short and long arms indicated the presence of two different breakpoints in the Y fragment. However, the clinical features were very similar in five of the cases, showing a male phenotype with small testes, testicular atrophy, and azoospermia. All these patients have normal intelligence and a stature within the normal male range. In the remaining case, the diagnosis was made prenatally in a fetus with male genitalia detected by ultrasound and a 46,XX karyotype in amniocytes and fetal blood. Molecular analysis of fetal DNA showed the presence of the SRY gene. FISH techniques also showed Y chromosomal DNA on Xp22.3 in metaphases of placental cells. To our knowledge, this is the second molecular prenatal diagnosis reported of an XX male.

A bird's eye view of human sex determination

In the beginning the dogma was that sex determination in man followed the Drosophila pattern in which XO is male, XXY female, and the Y chromosome has no direct influence on the determination of sex. On the grounds of specific anomalies with which they presented, females with Turner Syndrome were sex chromatin tested and found to be chromatin negative [1]. This result, confirmed in 1956 by the male frequency of red-green colour blindness in these subjects which indicated that they carried only one X chromosome in spite of their female phenotype, suggested that therefore they might be XO, and, so, hinted that sex determination in man might not follow the then accepted pattern [2]. In 1959 chromosome studies confirmed that XOs were female [3] and showed that subjects with the symmetrical XXY sex-chromosome anomaly were with Klinefelter syndrome [4]. In the same year, by showing that XOs were females also in mice [5] it became accepted that the Y chromosome was the determiner of the formation of the testis in the mammalian embryo, and so was the key element in primary sexual differentation. It would seem appropriate to call this formal model of chromosomal sex determination the Malandrium pattern [6].

In 1966 Jacobs and Ross [7], from work on males with Y chromosome deletions narrowed down the testis determining function of the Y chromosome to its short arm. Then, in 1975, Wachtel and collaborators [8] were the first to formulate a hypothesis on the sex determining gene, or, more precisely on the nature of its product. They suggested that this developmental role might be played by the H-Y antigen, a weak histocompatibility antigen which had been known to be involved in the rejection of male skin grafted onto otherwise histocompatible female mice. The idea had run into technical difficulties and a major problem was related to the significance that should be attached to the results of two different ways for demonstrating the antigen, namely the cell-mediated cytotoxicity test or the serological test. Efforts were made to keep the H-Y hypothesis alive, largely because there was a certain elegance about it [9, 10]. However eventually XX male mice, lacking H-Y by either test, spelt the end of the candidature of H-Y as the testis determining mechanism [11, 12].

[Genetics of human sex determination and its disturbances]

The genetics of human sex determination is considered in view of the various disorders of gonad development. The Y chromosome plays an important role in the induction of sex determination by encoding the testis-determining factor (TDF). However, not all deviations in regular development can be explained by mutations of the TDF as unique factor. Therefore, it is necessary to postulate other mutations in still unknown genes of the cascade for male-specific determination as well as the requirement of an ovary-determining factor for regular female development.

Molecular cytogenetic analysis of XX males using Y-specific DNA sequences, including SRY

XX maleness is the most common condition in which testes develop in the absence of a cytogenetically detectable Y chromosome. Using molecular techniques, it is possible to detect Yp sequences in the majority of XX males. In this study, we could detect Y-specific sequences, including the sex-determining region of the Y chromosome (SRY), using fluorescence in situ hybridization. In 5 out of 6 previously unpublished XX males, SRY was translocated onto the terminal part of an X chromosome. This is the first report in which translocation of an SRY-bearing fragment to an X chromosome in XX males could be directly demonstrated.

Genotype-phenotype correlations in XX males and their bearing on current theories of sex determination

Clinical, chromosomal and molecular studies of a group of 15 XX males confirm the presence of two main groups. A Y + ve group of ten patients exhibit sex reversal as the result of transfer of the distal end of the short arm of the Y chromosome, including testis determining factors, to the short arm of one X-chromosome, presumably by accidental crossing-over in paternal meiosis. The ten patients have Klinefelter's syndrome but differ from XXY cases in that they are short and shown no impairment of intelligence. The four Y-ve XX males have no demonstrable Y sequences and differ from Y + ve cases in abnormality of the external genitalia and invariable gynaecomastia; in this, they more closely resemble XX true hermaphrodites than XY males. These observations on Y - ve XX males and an additional exceptional Y + patients suggest that the ZFY locus is not essential for male differentiation and is not the primary testis determining factor. Male sex determination in sporadic, and familial Y-ve XX males and true hermaphrodites is likely to be the result of mutation in an X-linked TDF gene and its consequent escape from the constraints of X-inactivation. It seems premature to abandon the dosage model of sex determination on the recent evidence that ZFX does not show dosage compensation.

Synaptic behaviour and recombination nodules in the human XY pair

A sample of 90 XY pairs from men with normal karyotypes has been analyzed by measuring their morphological features in electron micrographs of microspread spermatocytes. The classification of human XY types (Solari, 1980) has been given stricter definitions. Stepwise splitting of the axes is seen in types 1 and 2. The development of axial branches and lengthening of the X axis is seen in type 3. In the two subtypes a and b of type 4 the net-like filamentous array grows in length to a maximum (average = 59.7 microns) in subtype b. The location of the putative Y kinetochore defines a short arm that measures 22.34% of Y axis length, and the kinetochore of the X axis defines a short arm of 38.15% of the axial length. The average number of excrescences in the X axis is 19.9 and in the Y is 4.3. The frequency of a non-homologous, distal end-joining grows steadily from type 0 to type 3. The average length of the synaptonemal complex (SC) in 51 XY pairs of types 1 and 2 is 1.33 microns (SD = 0.65) and it corresponds to 25.54% of the Y axis length. Thus, the average SC covers the short arm of the Y and the pericentromeric region. Maximum lengths of this SC may reach up to 81.8% of the Y axis. 30 recombination nodules (RNs) were located in 26 XY pairs, and 90% of the nodules are located in the distal half of the short arm of the Y axis. Thus, RNs are restricted to a segment much shorter than the length of the average SC. A gradient of decreasing probability of recombination may reach up to the centromeric region of the Y chromosome. Some possible consequences of these facts are discussed.

Hypothyroidism and sex chromosomes

The observation of Campbell and Price in 1979 that their Unit had diagnosed four subjects with both Klinefelter's syndrome and congenital hypothyroidism raised the suspicion of an association between the two conditions. This, and the published reports of an XX male, five XXY males, and one mosaic XY/XXY with congenital or acquired forms of hypothyroidism, together with the higher incidence in women and the absence of sex difference among inherited congenital cases, suggested a possible sex chromosome effect in the aetiology of sporadic hypothyroidism. Various hypotheses can be tested either by examining the frequency of hypothyroidism in sex chromatin positive males or by establishing a higher frequency of sex chromatin positive males among hypothyroid cases than in normal males. We examined 57 boys with hypothyroidism for the presence of sex chromatin and found all to be negative. From this relatively small sample we can only exclude the possibility of a very large (100 fold) difference in frequency between the two populations and therefore more data are needed.

Further cytologic evidence for Xp-Yp translocation in XX males using in situ hybridization with Y-derived probe

Chromosome preparations from seven subjects with aberrations of sex chromosomes were utilized for in situ hybridization studies with the tritium-labeled Y-derived probe p50f. Two subjects had a pseudodicentric chromosome consisting of two copies of Yp and a portion of Y long arm; two were XX males [46,XX,t(Xp;Yp)], one was missing part of the Y short arm, and another had t(5p;Yq); in addition cells from an XYY male as well as a normal 46,XY male, and a 46,XX female, were hybridized with the same probe. The hybridization technique of Harper and Saunders (1981) was used. There was excess labeling of the Yp/paracentromeric regions in the cases with the normal Y, the XYY, the pseudodicentric Y, and the 5/Y translocation. No significant label was seen on metaphases from the normal 46,XX female or the female with the partially missing Y short arm. Excess label was present on the X short arm in the cases of the XX males; there were 8% and 9.5% of cells with label. The combined cytogenetic and hybridization data indicate that one X short arm in these XX males has undergone a translocation with Yp, and that genes for sex determination probably reside on the distal half of the Y short arm.

The presence of intersexuality in patients with advanced hypospadias and undescended gonads

We studied 20 patients with advanced degrees of hypospadias and undescended testes for the presence of an intersex disorder. A comprehensive clinical, cytogenetic, endocrinological and surgical evaluation was performed. All patients were found to have an intersex disorder, including 10 with male pseudohermaphroditism and 10 with a gonadal/genetic intersex disorder. In the latter group 4 patients had mixed gonadal dysgenesis, 3 had dysgenetic male pseudohermaphroditism, 1 had the 46XX male syndrome, 1 had true hermaphroditism and 1 had Klinefelter's syndrome. Genetic and gonadal intersex disorders were more frequent in patients with a unilateral undescended testis and perineal hypospadias.

Deletion mapping of the testis determining locus with DNA probes in 46,XX males and in 46,XY and 46,X,dic(Y) females

Eleven Y-specific DNA probes hybridizing with DNA from one or more 46,XX males were isolated from a recombinant phage DNA library constructed from flow sorted human Y chromosomes. Two probes hybridized with DNA from nine out of eleven, i.e. greater than 80% of these 46,XX males. The relative frequency of hybridization of the probes in the 46,XX males and in a 46,X,dic(Y) female, together with in situ hybridization data, allowed mapping of the probes on Yp in relation to a putative testis determining locus. Several of those probes were also absent in a 46,XY female, further refining a model for ordering the probes on Yp. The DNA of one XX male hybridized both with probes from Yp and probes from proximal Yq (excluding the pericentral region). This suggests that complex translocations may occur into the DNA of 46,XX males that involve not only parts of Yp but also parts of Yq.

Moderately repeated DNA sequences specific for the short arm of the human Y chromosome are present in XX males and reduced in copy number in an XY female

Four DNA sequences specific for the Y chromosome were isolated from a recombinant phage library constructed from flow sorted human Y chromosomes. Two of these sequences were moderately repeated and assigned to the short arm of the Y chromosome by in situ hybridization. Both sequences were detected in five out of six [corrected] 46,XX males and were reduced in copy number in one out of two 46,XY gonadal dysgenesis patients tested. The findings suggest close proximity of these Y-specific moderately repeated DNA sequences to a testis determining locus.

The human Y chromosome

Despite its central role in sex determination, genetic analysis of the Y chromosome has been slow. This poor progress has been due to the paucity of available genetic markers. Whereas the X chromosome is known to include at least 100 functional genetic loci, only three or four loci have been ascribed to the Y chromosome and even the existence of several of these loci is controversial. Other factors limiting genetic analysis are the small size of the Y chromosome, which makes cytogenetic definition difficult, and the absence of extensive recombination. Based on cytogenetic observation and speculation, a working model of the Y chromosome has been proposed. In this classical model the Y chromosome is defined into subregions; an X-Y homologous meiotic pairing region encompassing most of the Y chromosome short arm and, perhaps, including a pseudoautosomal region of sex chromosome exchange; a pericentric region containing the sex determining gene or genes; and a long arm heterochromatic genetically inert region. The classical model has been supported by studies on the MIC2 loci, which encode a cell surface antigen defined by the monoclonal antibody 12E7. The X linked locus MIC2X, which escapes X inactivation, maps to the tip of the X chromosome short arm and the homologous locus MIC2Y maps to the Y chromosome short arm; in both cases, these loci are within the proposed meiotic pairing region. MIC2Y is the first biochemically defined, expressed locus to be found on the human Y chromosome. The proposed simplicity of the classical model has been challenged by recent molecular analysis of the Y chromosome. Using cloned probes, several groups have shown that a major part of the Y chromosome short arm is unlikely to be homologous to the X chromosome short arm. A substantial block of sequences of the short arm are homologous to sequences of the X chromosome long arm but well outside the pairing region. In addition, the short arm contains sequences shared with the Y chromosome long arm and sequences shared with autosomes. About two-thirds of XX males contain detectable Y derived sequences. As the amount of Y sequences present varies in different XX males, DNA from these subjects can be used to construct a map of the region around the sex determining gene. Assuming that XX males are usually caused by simple translocation, the sex determining genes cannot be located in the pericentric region. Although conventional genetic analysis of the Y chromosome is difficult, this chromosome is particularly suited to molecular analysis. Paradoxically, the Y chromosome may soon become the best defined human chromosome at the molecular level and may become the model for other chromosomes.

Genetic homology and crossing over in the X and Y chromosomes of Mammals

The "X-Y crossover model" described in this paper postulates the (1) the pairing observed between the X and the Y chromosome at zygotene is a consequence of genetic homology, (2) there is a single obligatory crossover between the X and Y pacing segments, and (3) the segment of the X which pairs with the Y is protected from subsequent inactivation. Genes distal to the proposed crossover ("pseudoautosomal genes") will appear to be autosomally inherited because they will be transmitted to both male and female offspring. Some criteria for identifying pseudoautosomal genes are outlined. The existence of a single obligatory crossover between the X and Y of the mouse is strongly supported by a recent demonstration that the sex-reversing mutation Sxr, which is passed equally to XX and XY offspring by male carriers, is transmitted on the sex chromosomes. Pseudoautosomally inherited genes may also be responsible for XX sex reversal in goats and familial XX sex reversal in man.

Pairing of X and Y chromosomes, non-inactivation of X-linked genes, and the maleness factor

In this paper observations are summarized and speculations discussed, and it is suggested that some loci on the distal short arm of the X chromosome (Xp) are not randomly inactivated in the female, because they are within the proximal part of the pairing segment between Xp and Yp. This peculiarity of gene expression may be a remnant of the evolutionary history of the sex chromosomes, the pairing segment of which may involve at least 27% of Xp and 95% of Yp. Crossing over seems to occur mostly in the terminal third of the X/Y pairing segment. However, crossing-over inhibition control may lapse, or may be somewhat variable, within the pairing segment, so that some loci on the X and Y (e.g. Xg. H-Y, STS, and perhaps others) might cross over with a variable frequency which is proportional to their distances from the telomeres of the short arms. It is postulated that the DNA of the pairing segment is composed in a way which may also permit unequal crossing over to occur between the X and the Y, thereby giving rise to exceptions to X-or Y-linked inheritance. The peculiarities of behaviour and the position of other loci on the sex chromosomes are also discussed briefly.

Hoechst 33258 induced uncondensed sites in marsupial chromosomes

The fluorochrome Hoechst 33258 induces pronounced uncondensed regions at mitosis at one or more specific sites on the X chromosomes of all eighteen species of marsupials belonging to the family Macropodidae which have been examined. The Y chromosomes of nearly all of these species also show sensitive sites. Autosomal regions which respond to this chemical were observed in only five species and there is evidence of polymorphism for two of these. The regions which respond usually show C-banding, but not all C-banding regions are affected. No specific effect was found in the chromosomes of eleven other species examined which are representative of 5 different Australian marsupial families. The implications of the apparent restriction of sex chromosome sensitive sites to macropods are discussed.

Localisation of male determining factors in man: a thorough review of structural anomalies of the Y chromosome

It is widely accepted that male determination in man depends on the presence of a factor or factors on the Y chromosome. These factors may be localised within the Y chromosome through the study of structural anomalies of the Y. A thorough review of seven different structural anomalies of the Y is presented: dicentric Y chromosomes, Y isochromosomes, ring Y chromosomes, Y; autosome, Y;X, and Y;Y translocations, and Y deletions. The evidence from these studies indicates that a gene or genes on the short arm or the Y near the centromere play a crucial role in the development of the testes. A few studies indicate that one or more factors on the long arm of the Y may also influence testicular development. If such a factor is present on the long arm, then it too must be very near the centromere. The theory that separate genes independently control the initial development and maturation of the tests (on the long and short arms of the Y, respectively) may be premature. Recently proposed arguments in its favour are examined. Some evidence also indicates the presence of a fertility factor on the non-fluorescent segment of the long arm. Relevant information on the H-Y antigen is discussed.

X;Y translocation in an adolescent mentally normal phenotypic male with features of hypogonadism

Cytogenetic studies on a 17-year-old phenotypic male, with short stature and clinical and hormonal features of hypogonadism similar to those of an XX male, revealed an X;Y translocation, karyotype, 46,Xt(X;Y)(p22;?p11?q11). He was H-Y antigen positive. X inactivation studies showed inactivation of the abnormal X in the majority of cells (60 to 70%) and inactivation of the normal X in the remaining cells. Gene marker studies, including Xg blood grouping, showed no anomalous segregation. This patient is the second reported male showing a positively identified X;Y tanslocation with no detectable free Y chromosome and provides further indirect evidence for an X-Y interchange in the aetiology of XX male sex reversal.

A synopsis of the human Y chromosome

Phenotypic features and functions known to depend on the presence of the Y chromosome or the H-Y antigen are discussed in relation to structural anomalies of the Y chromosome and other abnormalities of sexual and somatic development. Recent knowledge about molecular organization of constitutive heterochromatin in relation to the human Y is presented. An attempt is made at assigning different functions, genes and DNA sequences to different regions of the Y chromosome.

A 45,X male with translocation of euchromatic Y chromosome material

A phenotypically normal 32-year-old male with azoospermia was found to have a 45,X karyotype with presence of excess euchromatic material on 14p. The parent's karyotypes are normal. This observation is interpreted as a Y/14 translocation with loss of the heterochromatic Y chromosome material.

Congenital hypothyroidism in Klinefelter's syndrome

Congenital hypothyroidism has been found in four patients with Klinefelter's syndrome. It is likely that this reflects more than chance concurrence of these conditions.

Heteromorphic X chromosomes in 46,XX males?

This paper reports an attempt to determine whether the short arm of one of the X chromosomes in XX males is longer than normal. In a blind study comparing coded photomicrographs of 15 G-banded mitoses from each of five XX males and five control females, the results were ambiguous and somewhat contradictory, but gave the impression of, or were compatible with, an XXp+ phenomenon in at least two of the five XX males. Measurements of the X chromosomes from the above cells and, in addition, from 15 mitoses from each of six XXY males, failed to disclose any XXp+ phenomenon. Statistical analysis indicated that in the five XX males there was no difference in the lengths of the two Xp arms. The reasons for the apparent discrepancy between the results of ocular inspection and measurement are discussed. The putative heteromorphism might be an alteration in shape, staining intensity, or position of bands, neither of which necessarily leads to an increase in length. We conclude that our results do not indicate any XXp+ phenomenon in the five XX males tested. However, the presence or absence of XXp+ is not in itself evidence for or against interchange betweenthe X and Y in the paternal meiosis. Our results emphasize that the etiology of XX males is likely to be heterogeneous.