Molecular studies of chromosomal mosaicism: relative frequency of chromosome gain or loss and possible role of cell selection.

Studies of uniparental disomy and origin of nonmosaic trisomies indicate that both gain and loss of a chromosome can occur after fertilization. It is therefore of interest to determine both the relative frequency with which gain or loss can contribute to chromosomal mosaicism and whether these frequencies are influenced by selective factors. Thirty-two mosaic cases were examined with molecular markers, to try to determine which was the primary and which was the secondary cell line: 16 cases of disomy/trisomy mosaicism (5 trisomy 8, 2 trisomy 13, 1 trisomy 18, 4 trisomy 21, and 4 involving the X chromosome), 14 cases of 45,X/46,XX, and 2 cases of 45,X/47,XXX. Of the 14 cases of mosaic 45,X/46,XX, chromosome loss from a normal disomic fertilization predominated, supporting the hypothesis that 45,X might be compatible with survival only when the 45,X cell line arises relatively late in development. Most cases of disomy/trisomy mosaicism involving chromosomes 13, 18, 21, and X were also frequently associated with somatic loss of one (or more) chromosome, in these cases from a trisomic fertilization. By contrast, four of the five trisomy 8 cases were consistent with a somatic gain of a chromosome 8 during development from a normal zygote. It is possible that survival of trisomy 8 is also much more likely when the aneuploid cell line arises relatively late in development.     

Chromosomal mosaicism in amniotic fluid cell cultures.

Over the past 6 years, using in situ processing methods, we have identified 32 cases of mosaicism in amniotic fluid cell cultures prepared from 1,100 samples. Two of these (45,X/46,XX and 46,XX/47,XX, + 21) were called true mosaics because multiple colonies demonstrated the same abnormal chromosome complement, and on subsequent evaluation of the newborn blood or fetal tissues, mosaicism was confirmed. Of the remaining cases, 29 were designated as pseudomosaics because only single or partial colonies exhibited an aberrant chromosome complement, 12 having a trisomy 2 line. In the final case, a double trisomy was demonstrated in only one of eight colonies in the first culture, but in the culture from a repeat sample an additional two colonies showed the same double trisomy. Since no abnormal cells were observed in infant blood, it was postulated that the mosaicism may only have been present in the extraembryonic tissues. It is our conviction that the use of these cloning methods should diminish the danger of misdiagnosis in genetic amniocentesis.     

Incidence of major chromosome aberrations in 12,319 newborn infants in Tokyo

Summary In order to ascertain the frequency of chromosome aberrations among newborn infants in Japan, a chromosome survey of a large number of newborn infants is in progress. A consecutive series of 12,319 newborn babies, 6382 male and 5937 female, have been screened for clinical manifestations of autosomal aberrations and for sex chromatin and sex chromosome aberrations. Chromosome studies were carried out on 694 infants with suspected chromosome aberrations. The clinically abnormal infants were screened by conventional staining, and banding techniques have been used in the part of the study performed since 1974. Of the clincally abnormal infants, 25 had abnormal karyotypes, including two males with a 47,XXY complement, one female with a 45,X complement, three male infants with a 47,XYY complement, two with trisomy 13 syndrome, three with trisomy 18 (including one case of mosaicism), eleven with Down's syndrome (including one case of mosaicism), one with B5p partial trisomy, one with cri-du-chat syndrome, and one with Y/D translocation. The overall results are comparable to those of previous population cytogenetic studies only in the autosomal trisomies and sex chromosome abnormalities and in that the observed frequencies were comparable to those found in studies in Caucasians.To whom offprint requests should be sent     

Dynamic mosaicism manifesting as loss, gain and rearrangement of an isodicentric Y chromosome in a male child with growth retardation and abnormal external genitalia

Isodicentric chromosomes are considered the most common structural abnormality of the human Y chromosome. Because of their instability during cell division, loss of an isodicentric Y seems mainly to lie at the origin of mosaicism in previously reported patients with a 45,X cell line. Here, we report on a similar case, which, however, turned out to be an example of dynamic mosaicism involving isodicentric chromosome Y and isochromosome Y after FISH with a set of chromosome Y-specific probes and multicolor banding. Cytogenetic analyses (GTG-, C-, and Q-banding) have shown three different cell lines: 45,X/46, X,idic(Y)(q12)/46,X,+mar. The application of molecular cytogenetic techniques established the presence of four cell lines: 45,X (48%), 46,X,idic(Y)(q11.23) (42%), 46,X,i(Y)(p10) (6%) and 47,X,idic(Y)(q11.23),+idic(Y)(q11.23) (4%). According to the available literature, this is the first case of dynamic mosaicism with up to four different cell lines involving loss, gain, and rearrangement of an idic(Y)(q11.23). The present report indicates that cases of mosaicism involving isodicentric and isochromosome Ys can be more dynamic in terms of somatic intercellular variability that probably has an underappreciated effect on the phenotype.     

Parental trisomy 21 mosaicism.

A family with three children with trisomy 21 in which the mother is a phenotypically normal, trisomy 21/normal mosaic was studied. Chromosome 21 fluorescent heteromorphisms were used to document that two of the three number 21's in two of the Down syndrome offspring were of maternal origin. Five cytogenetic surveys in which both parents of a child with trisomy 21 were studied have been reviewed. From these data, it is estimated that 3% of couples producing a child with trisomy 21 can be explained by parental mosaicism. From 17 informative sibships, with one parent mosaic, the segregation ratio was estimated to be 0.43 +/- 0.11.     

The Origin of Abnormalities in Recurrent Aneuploidy/Polyploidy

Recurrent miscarriage due to sporadic chromosomal abnormalities may simply be a consequence of the dramatic increase of trisomic conceptions with increased maternal age. However, it is also possible that some couples are at increased risk of abnormalities as a result of gonadal mosaicism, factors affecting chromosome structure and segregation, increased sperm aneuploidy in the male partner, or accelerated "aging" of the ovaries. We report cytogenetic and molecular findings from 122 spontaneous abortions (SAs) from 54 couples who were ascertained as having two or more documented aneuploid or polyploid SAs. The distribution of abnormalities in this group was similar to those from 307 SAs that involved chromosome abnormalities and were diagnosed at the same center but did not involve documented recurrent aneuploidy/polyploidy. Although recurrence of the same abnormality was observed in eight families, this number was equal to that expected by chance, indicating that gonadal mosaicism is rarely the explanation for recurrence. The origin of the abnormality was determined in 37 SAs from 23 of the couples in the study. A maternal meiotic origin was involved in 30 trisomies and in 1 triploid SA; 3 additional maternal trisomies were of possible somatic origin. A paternal origin was found in the remaining two trisomies and in one triploid SA. In addition, one double trisomy was the consequence of both a maternal and a paternal meiotic error. These results confirm that the etiology of trisomy is predominantly a result of meiotic errors related to increased maternal age, regardless of whether the couple has experienced one or multiple aneuploid SAs. Furthermore, this is true even when a second SA involves the same abnormality. Nonetheless, these data do not exclude some population variability in risk for aneuploidy.     

PCR-based detection of mosaicism in Turner syndrome patients

To study the possible role of cryptic mosaicism in phenotypical variations of 45,X Turner syndrome, we analyzed low-level mosaicism by methods based on the polymerase chain reaction. For the detection of Y-chromosome-derived fragments, we used three Y-specific primer pairs representing the centromere, Yp11.3, and Yq12. None of the 18 patients with 45,X had Y-derived chromosomes. For the detection of X chromosome mosaicism, we employed a novel modified HUMARA (human androgen receptor) assay, which proved to be a sensitive method with a detection limit as low as 1 in 960 cells. Using this assay, we detected low frequency cryptic X chromosome mosaicism in 2 of 18 cytogenetically 45,X patients. Received: 24 April 1996 / Revised: 19 August 1996     

True hermaphroditism in 45,X/46,XY mosaicism.

This report discusses the clinical findings on two patients with 45,X/46,XY mosaicism, two boys presented with penile hypospadias and cryptorchidism. A dysgenetic ovary and a testis were found in one boy, and a dysgenetic ovary in the other. Both patients can be considered to be true hermaphrodites on the basis of histology and clinical and hormonal observations. 45,X/46,XY mosaics have a wide range of phenotypic appearances and their gonadal morphology can also show great differences. However, the incidence of true hermaphroditism in individuals with 45,X/46,XY mosaicism is low and the reports in the literature rare. It is likely that males with 45,X/46,XY who suffer only mild maldevelopment of the external genitalia will not be recognized. In all patients with penoscrotal hypospadias and cryptorchidism with 45,X/46,XY mosaicism, the possibility of true hermaphroditism should be considered.     

Size of the exon 1-CAG repeats of the androgen receptor gene employed as a molecular marker in the diagnosis of Turner syndrome in girls with short stature

Turner syndrome (TS) is one of the most common chromosomal abnormalities among girls. Complete monosomy of X chromosome is responsible for almost 50% of all cases of TS, and mosaicism and X anomaly are detected in the other half. It has already been demonstrated that early diagnosis of these children allows appropriate growth hormone treatment with better final height prognosis and introduction of estrogen at an ideal chronological age. Sixty-four short-stature girls were selected and the clinical data obtained were birth weight and height, weight and height at the first medical visit and target height. Other clinical data including cardiac and renal abnormalities, otitis, Hashimoto thyroiditis, cubitus valgus, short neck, widely separated nipples, and pigmented nevi were obtained from the patients' medical records. The aim of the present study was to evaluate the screening of a group of short-stature girls for TS based on the number of CAG repeats of the androgen receptor gene analyzed by GeneScan software. Patient samples with two alleles (heterozygous) were 49/64 (76.5%) and with one allele (homozygous) were 15/64 (23.5%). A karyotype was determined in 30 patients, 9 homozygous and 21 heterozygous. In the homozygous group, 6/9 were 45,X and 3/9 were 46,XX. In the heterozygous group, 17/21 were 46,XX, and 4/21 were TS patients with mosaicism (45,X/46,XX; 45,X/46XiXq; 46XdelXp). The pattern obtained by GeneScan in two patients with mosaicism in the karyotype was an imbalance between the peak heights of the two alleles, suggesting that this imbalance could be present when there is a mosaicism. The frequency of TS abnormalities (18.7%) did not differ between TS and 46,XX girls. Thus, it is important to accurately assess the incidence of TS in growth-retarded girls, even in the absence of other dysmorphisms. In this study, we diagnosed 6 cases of TS 45,X (9.4%) by molecular analysis, with a 100% sensitivity and 85% specificity. This molecular analysis was able to detect all cases of TS 45,X and the majority of mosaicisms, without the need for more X chromosome markers. In conclusion, determining the number of CAG repeats of the androgen receptor gene analyzed by GeneScan was a fast method with high sensitivity for the detection of TS 45,X, suggesting that it could be interesting as a method for screening a population of growth-retarded girls.     

Inherited aneuploidy: germline mosaicism

Germline mosaicism has been thought to be a rare cause of aneuploidy in the human population. Recent evidence from cytological and population studies suggests otherwise. Approximately 5% of young couples with a Down syndrome child show evidence of germinal mosaicism. Molecular cytogenetic analysis of oocytes has proved germinal or gonadal mosaicism for trisomies of chromosomes 13 and 21 in several studies involving both oocytes and first polar bodies. Most recently direct analysis of fetal ovarian pre-meiotic, meiotic, and stromal cells proved low level trisomy 21 mosaicism in every sample tested. Based upon this evidence, germinal or gonadal mosaicism is likely to make a significant contribution to aneuploidy in the human population, particularly where younger women are concerned.     

Evidence for the repeated primary non-disjunction of chromosome 21 as a result of premature centromere division (PCD)

Summary A clinically normal 28-year-old woman had three conceptuses with trisomy 21 and one normal child. She showed minimal cytogenetic evidence of mosaicism: 4% of her blood cells and 6% of skin fibroblasts had trisomy 21. Also, 7% of her blood cells showed aneuploidy of the X chromosome which was associated with premature centromere division (PCD, X); 6% of fibroblasts showed trisomy 18, 10% of fibroblasts showed PCD,21, and 1% PCD, 18. It is unlikely that this woman is a constitutional mosaic for trisomies X, 18, and 21, all at low levels. We suggest that she has a predisposition to irregular centromere separation and that chromosomes X, 18, and 21 are most susceptible to its action.     

Trisomy 21 Down syndrome

Summary The lymphocyte chromosomes of trisomy 21 Down syndrome patients and their parents in a random series of 374 families were analyzed, the objective being the identification of parental mosaicism. The numbers of parents in whom at least two trisomy 21 cells were detected were seven mothers and three fathers, a frequency of 2.7% of families. Confirmation of mosaicism was by identification of parental transmission of the extra chromosome to the progeny, by repeat chromosome analysis, and/or by the presence of more than one affected child. If to these are added six others in whom only one trisomic cell was detected, but with no other supporting evidence, the frequency could be as high as 4.3%. Differences in parental age at the birth of Down syndrome progeny may be accounted for by differences in frequencies of mosaicism in germ cells and somatic tissue. Mosaicism was found more frequently in the mothers than in the fathers, but more data are required for confirmation of a real difference.     

Dermatoglyphic studies in the parents of trisomy 21 children I. Distribution of dermatoglyphic discriminants

A sample of 312 parents of a child with complete trisomy 21 (168 mothers and 144 fathers) has been compared with 295 parents of non-mongol children (61 mothers and 134 fathers) with respect to distribution of individual dermatoglyphic discriminant scores. Selection of dermatoglyphic traits as well a weightings have been based on the discriminant function, constructed for normal controls against cytogenetically diagnosed trisomy 21 mosaics. The results indicate that the proportion of individuals with an increased chance of mosaicism is appreciably greater in a sample of both the mothers and the fathers of mongol children, as compared with the parents of non-mongol children. For D greater than + 3.00, including also the overlap range values, it is, on the average, twice as high as in the control parents, while for the D values greater than + 4.00, strongly indicative of mosaicism, it is about five times higher than in control parents. This is so in spite of the fact that all parents, who had previously been cytogenetically tested and diagnosed as mosaics, were not included in this sample. Although the meaning of these results cannot yet be completely understood, they justify the extension of the use of dermatoglyphic discriminants in studies on parental mosaicism in trisomy 21.     

45, X/47, XY, +13 mosaicism in a 19-year-old girl]

The 45, X/47, XY, +13 mosaicism was observed in a 19-year-old mentally deficient girl who was examined because of primary amenorrhea. Certain clinical features were reminiscent of Turner's syndrome, while no features of trisomy 13 were present. The study of blood groups, HLA genotypes, and cytogenetic markers provided no evidence of chimerism.     

Distribution of α1-(PI) phenotypes in chromosome abnormalities

Summary PI phenotypes (including subtypes) were determined for 168 individuals with chromosomal abnormalities ascertained in Adelaide. These included patients with mosaicism, trisomy 21, trisomy 13, trisomy 18, and various sex chromosome aberrations (45,X, 47,XXX, 47,XXY, 47,XYY, and 48,XXXY). Data did not support an existing proposition that mildly deficient PI phenotypes predispose to abnormal chromosome segregation during mitosis or meiosis. Phenotypic distributions of each group were statistically similar to control populations of cord bloods and bloods donors.     

Interphase cytogenetics on paraffin-embedded sections of ovary for detection of genomic constitution in a patient with Turner's syndrome and chromosomal mosaicism

The difficulty of detecting sex chromosome mosaicism cytogenetically hinders the finding of an acceptable explanation for phenotypic-genotypic discrepancy amongst those patients. Fluorescence in situ hybridization (FISH) permits the genomic identification of patients with mosaic karyotypes in interphase nuclei by utilising an X chromosome-specific DNA probe (interphase cytogenetics). We evaluated the efficiency of interphase cytogenetics in the detection of the genomic constitution of the ovary from a patient with Turner's syndrome having mosaicism (46,XX/45,X0) previously established by blood lymphocyte karyotyping. We used a biotin-labelled alphoid repetitive sequence, pBAMX5, specific for the centromeric region of the human X chromosome. Although examination of ovarian sections and blood lymphocytes by FISH showed the presence of both 46,XX and 45,X0 cell lines, the genomic constitution of the germ cells/oocytes in ovarian primordial follicles was shown to be normal (46,XX). Our results (1) show the high applicability of interphase cytogenetics on paraffin sections, (2) indicate the possibility of genomic screening of different tissues that are otherwise not amenable to routine cytogenetic investigation and (3) offer a reliable methodological approach to defining accurate by the percentage of abnormal karyotypes in mosaicism of different organs and non-dividing tissues.     

45,X/46,XY qh? Karyotype and Aspermia. A case report

A 41-years old male with short stature, abnormal male sex differentiation, aspermia and schizoid character disorder is described. The patient was studied from clinical, endocrinological and genetic perspectives. Cytogenetical analysis revealed a chromosomic mosaicism formed by two normal lines 45X and 46,XY qh^?. Molecular studies on AZF region evidenced that it was conserved. The correlation of the symptoms with the cytogenetic finding is discussed.     

Isodicentric Y chromosome: cytogenetic,molecular and clinical studies and review of the literature

Dicentrics are among the most common structural abnormalities of the human Y chromosome. Predicting the phenotypic consequences of different duplications and deletions of dicentric Y chromosomes is usually complicated by varying degrees of mosaicism (45,X cell lines), which may, in some cases, remain undetected. Molecular studies in patients with dicentric Y chromosomes have been few, and only two studies have attempted to determine the presence of SRY (the putative testis-determining factor gene). We report an 18-year-old female with short stature, amenorrhea, hirsutism, hypoplastic labia minora, and clitoromegaly who has a 45,X/46,X,idic(Y)(p11.32)/47,X,idic(Y)(p11.32),idic(Y) (p11.32) karyotype. Southern analysis using Y-specific probes (Y97, 2D6, 1F5, pY3.4) and polymerase chain reaction (PCR) analysis using primers for ZFY and SRY were positive for all loci tested, indicating that almost all of the Y chromosome was present. Our findings and an extensive review of the literature emphasize the importance of molecular analyses of abnormal Y chromosomes before any general conclusions can be reached concerning the relative effects of the Y-chromosome abnormality and mosaicism on sexual differentiation.     

多重荧光定量PCR技术快速诊断21三体及18三体方法的建立及临床应用

利用收集的20例21三体、3例18三体DNA样本及40例正常人DNA样本,选择多对21、18号染色体短串联重复序列分子标记,建立多重荧光定量PCR检测技术用于21三体、18三体的快速产前诊断;利用建立的方法对165例产前诊断病例及4例消化道畸形新生儿进行检测,并与核型分析结果相比较。169例病例中共诊断21三体4例,18三体1例,所有病例均在1～3d得到结果,无漏诊和误诊,并利用建立的多重荧光定量PCR技术为5例核型分析失败的病例提供了明确的产前诊断。对于同期B超检查胎儿结构异常的22例胎儿,则采用传统的核型分析,检出1例(45,X),1例(47,XXY)。结果表明建立的多重荧光定量PCR技术可快速、准确地诊断21三体和18三体,减轻核型分析需时过长给孕妇带来的焦虑,可用于血清学筛查21三体、18三体高风险者及高龄孕妇,也适用于对其他遗传性疾病如遗传性耳聋进行产前诊断时并行检测以排除21三体和18三体。多重荧光定量PCR技术结合传统核型分析可更好的满足产前诊断的临床需求。     

由21三体综合征的核型引起的思考

不同的医学遗传学教材上对21三体综合征的概念及核型说法不一, 作者在查阅了大量教材的基础上, 对21三体综合征的概念及核型等问题进行了认真的思考, 并提出了自己的观点, 认为21三体综合征的核型应该为两种类型: 即三体型和嵌合型, 而先天愚型的核型应该为3种类型: 即三体型、嵌合型和易位型。