Detection of gonadal mosaicism in parents of offspring with down syndrome

A review of data of a standard clinical karyological analysis and study of QFQ chromosomal polymorphism in 151 families of children with Down syndrome is performed. A total of eight families with proved and predicted trisomy 21 gonadal mosaicism were identified; in all the cases the mothers were younger than 35 years of age. The prevalence of carriers of mosaicism in young families amounted to 6.5% (8/123). In a standard analysis, if the capabilities of the QFQ method had not been employed, this prevalence would have amounted to only 1%. A comparison of the results of a retrospective analysis of the data obtained by the QFQ method with the expected results of molecular testing of the same families leads to the conclusion that both methods should be used.     

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.     

High rate of mosaicism in tuberous sclerosis complex.

Six families with mosaicism are identified in a series of 62 unrelated families with a mutation in one of the two tuberous sclerosis complex (TSC) genes, TSC1 or TSC2. In five families, somatic mosaicism was present in a mildly affected parent of an index patient. In one family with clinically unaffected parents, gonadal mosaicism was detected after TSC was found in three children. The detection of mosaicism has consequences for genetic counseling of the families involved, as changed risks apply to individuals with mosaicism, both siblings and parents. Clinical investigation of parents of patients with seemingly sporadic mutations is essential to determine their residual chance of gonadal and/or somatic mosaicism, unless a mosaic pattern is detected in the index patient, proving a de novo event. In our data set, the exclusion of signs of TSC in the parents of a patient with TSC reduced the chance of one of the parents to be a (mosaic) mutation carrier from 10% to 2%. In the five families with somatic mosaicism, the parent was given the diagnosis after the diagnosis was made in the child.     

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.     

Sex chromosome analysis in Turner Syndrome by a pentaplex PCR assay

In this study, we describe a pentaplex PCR to determine the parental origin of the X chromosome and the presence of mosaicism, via amplification of four polymorphic markers located along the X chromosome (DXS10011, DXS6807, HUMARA, DXS101) and the X-Y amelogenin marker, in 41 families having a daughter with Turner Syndrome. Our results confirmed the cytogenetic findings and we found that the parental origin of the single X chromosome to be maternal in 84% of cases.     

Distribution of mosaicism in human placentae

Traditional first trimester chorionic villus sampling (CVS) for prenatal diagnosis can be performed by cytogenetic analysis of cytotrophoblast or chorionic villous stroma. Approximately 2% of pregnancies studied by CVS show confined placental mosaicism (CPM) involving either cytotrophoblast, stroma or both. We present the results of a cytogenetic study of nine term placentae from pregnancies with prenatally diagnosed CPM. The aneuploid cell lines involved trisomies for chromosomes 7,9,16, and X. The cytotrophoblast and villous stroma from multiple biopsies of these placentae were examined using a combination of interphase and metaphase cytogenetic analysis. CPM was detected in all nine of the term placentae and both tissue-specific and site-specific patterns of mosaicism could be discerned. These results indicate that the analysis of villous stroma and cytotrophoblast from multiple placental biopsies is necessary to improve our understanding of the evolution of CPM during pregnancy and its effect on the fetus. Received: 1 May 1995 / Revised: 11 August 1995     

Rapid confirmation of previously detected prenatal mosaicism by fluorescence in situ hybridization in interphase uncultured amniocytes

Fluorescence in situ hybridization (FISH) of chromosome-specific probes to interphase uncultured amniocytes was performed in cases in which follow-up amniocenteses were done for confirmation of previously detected mosaicism. FISH results were informative in all seven cases included in the study, and confirmed by subsequent cytogenetic analysis. FISH analysis provides rapid results for referral physicians and in most cases reassurance for patients within 24 hours of the follow-up aminocentesis. Although FISH studies are not considered accurate in determining a primary diagnosis of mosaicism in uncultured cells, the analysis is accurate and clinically useful when the diagnosis is known and mosaicism involving a specific chromosome needs to be confirmed in follow-up testing.     

Molecular analysis of the parental origin of trisomy in two families with two children having regular trisomy 21

In order to investigate the parental origin of the trisomy in two families with two free trisomy 21 affected siblings, cytogenetic and molecular analyses were performed. In each case the parental origin was the same for both patients. In one of the families an association between the concordance of the paternal origin of the trisomy and the existence of mosaicism in the blood was established. The various etiologies which may account for recurrence are discussed.     

Exclusively paternal X chromosomes in a girl with short stature

A 13 1/2 year-old girl with short stature and very few Turner stigmata revealed 45,X/46,XX mosaicism with 90%–100% 46,XX cells in three sequential blood lymphocyte cultures. Molecular investigation of the parental origin of her X chromosomes revealed homozygosity for paternal X markers and an absence of maternal markers. Luteinizing hormone response to growth hormone releasing hormone was increased. Impaired gonadal function and shortness of stature in this case could be a result of the mild mosaicism with a 45,X cell line and/or is a consequence of the paternal-only origin of her X chromosomes.     

Clinical outcome and follow-up of the first reported case of Russell-Silver syndrome with the unique combination of maternal uniparental heterodisomy 7 and mosaic trisomy 7

BACKGROUND: Russell-Silver syndrome (RSS) has been associated with maternal uniparental disomy (UPD) for chromosome 7 although the etiology of the syndrome is still unknown. Cases of RSS associated with maternal UPD7 have involved isodisomies, heterodisomies, and mixed isodisomy with heterodisomy simultaneously. This publication is a follow-up report of the postnatal clinical outcome of the first prenatally suspected case of combined mosaic trisomy 7 with maternal uniparental disomy of chromosome 7 (UPD7). CASE: The diagnosis of RSS in the proband was suspected prenatally because trisomy 7 mosaicism (47,XX,+7[13]/46,XX[19]) and maternal uniparental heterodisomy 7 were both found in amniotic fluid cells. Cord blood karyotype analysis showed only disomic cells (46,XX[50]), whereas postpartum chorionic villus analysis was completely trisomic for chromosome 7 (47,XX,+7[19]). Postnatally, the diagnosis of RSS was confirmed by physical findings, her trisomy 7 mosaicism was confirmed by cytogenetic analysis of her skin biopsy (47,XX,+7[9]/46,XX[20]) and her UPD7 was confirmed on both peripheral blood and skin biopsy using microsatellite markers. During infancy, the proband experienced growth deficiency, persistent hypoglycemia, and psychomotor developmental delay. CONCLUSIONS: Trisomic rescue as a life-saving mechanism, with subsequent chromosomal mosaicism in combination with UPD may occur more frequently in RSS than has been reported. Systematic testing of cases suspected prenatally or postnatally would be informative regarding the individual contribution of each factor. Imprinting, loss of heterozygosity for recessive genes, and mosaicism may explain the short stature, asymmetry, and the variable expression of the phenotype. The contribution of these mechanisms to the syndrome should be evaluated in these cases.     

Cytogenetics and fluorescence in situ hybridization assessment of sex-chromosome mosaicism in Klinefelter's syndrome

A retrospective study was carried out in 152 infertile men to determine the prevalence of sex chromosome abnormalities among non-obstructive azoospermic and severe oligospermic men (n = 51) and to evaluate the feasibility of fluorescence in situ hybridization (FISH) techniques to assess mosaicism in Klinefelter's patients in comparison with conventional cytogenetics. Cytogenetic analysis were performed for 51 infertile men and among 14 chromosomal abnormalities found, nine were compatible with Klinefelter's syndrome. FISH staining with a CEP X/CEP Y probes were performed for Klinefelter's patients and for five of them; testes were biopsied for histopathologic examination. Six Klinefelter's patients showed a non-mosaic 47,XXY and three showed a 47,XXY/46,XY mosaic by G or R banding analysis of 20 cells with a ratio of 17%, 20% and 33%, respectively. FISH analysis confirmed mosaicism in only one patient (the first) in whom a third cells population was found. There was no relationship between the ratios of mosaicism by banding and FISH analysis. Conventional histopathologic findings in five non-mosaic Klinefelter's patients confirm the diagnosis of Sertoli Only Cells syndrome. FISH is recommended in Klinefelter's syndrome to define exactly the cytogenetic statute as mosaic or non-mosaic and then discussing prognosis and decision regarding fertility counseling.     

Molecular analysis in true hermaphroditism: demonstration of low-level hidden mosaicism for Y-derived sequences in 46,XX cases

True hermaphroditism (TH) is an unusual form of sex reversal, characterized by the development of testicular and ovarian tissue in the same subject. Approximately 60% of the patients have a 46,XX karyotype, 33% are mosaics with a second cell line containing a Y chromosome, while the remaining 7% are 46,XY. Molecular analyses have demonstrated that SRY is present in only 10% of TH with a 46,XX karyotype; therefore, in the remaining 90%, mutations at unknown X-linked or autosomal sex determining loci have been proposed as factors responsible for testicular development. True hermaphroditism presents considerable genetic heterogeneity with several molecular anomalies leading to the dual gonadal development as SRY point mutations or SRY hidden gonadal mosaicism. In order to identify genetic defects associated with subjects with the disease, we performed molecular analyses of the SRY gene in DNA from blood leukocytes and gonadal tissue in 12 true hermaphrodites with different karyotypes. Our results using PCR and FISH analyses reveal the presence of hidden mosaicism for SRY or other Y sequences in some patients with XX true hermaphroditism and confirms that mosaicism for SRY limited to the gonads is an alternative mechanism for testicular development in 46,XX true hermaphrodites.     

Incidence and significance of supernumerary marker chromosomes in prenatal diagnosis

The finding of a supernumerary marker chromosome in amniotic fluid cells poses a considerable counseling dilemma. In 6,500 cases referred to our laboratory over a 4 1/2-year period, eight such cases were identified (0.123% of all cases). In five of the eight cases, a diagnosis of true mosaicism between cells with 46 and 47 chromosomes was made. In the remaining three cases, the marker was present in 100% of the cells. In three cases, the marker was determined to be familial in nature with mosaicism present in the parents of two of these cases. Detailed cytogenetic findings for each case are provided. In no cases were abnormalities noted in either abortuses or live borns. The high incidence of mosaicism in these cases seems to indicate a propensity for supernumerary chromosomes to be lost. Familial markers may not be passed on for many generations, and they may arise as new mutations relatively frequently. There is an urgent need for more information on the risks associated with the prenatal detection of supernumerary chromosomes. We recommend that in considering the implications of the prenatal detection of marker chromosomes cases be considered in at least four distinct groups: type 1--familial and nonmosaic; type 2--familial with mosaicism in either the amniotic fluid cells, a parent, or both; type 3--de novo markers and nonmosaic; and type 4--de novo with mosaicism present in the amniotic fluid cells.     

X-linked Menkes disease: first documented report of germ-line mosaicism

This work investigated a three-generation Menkes disease family, where germ-line mosaicism was suspected in the maternal grandmother of the index patient. She had given birth to 2 boys who died of suspected Menkes disease on the basis of clinical and photographic evidence. Biochemical analysis of the index patient confirmed the diagnosis of Menkes disease, and DNA analysis established a partial gene deletion (EX11_EX23del), involving exons 11-23 and the 3'-untranslated region (UTR) of ATP7A. A junction fragment was detectable by Southern blot analysis, which enabled carrier analysis. The mother was demonstrated to be a carrier, whereas analysis of lymphoblasts and skin fibroblasts from the maternal grandmother gave no indication of a partial gene deletion. No materials were available from the possibly affected maternal uncles. Further genetic analyses, including biochemical testing of the grandmother and haplotype analysis using four intragenic markers on DNA from selected members of the family, corroborated this finding. The combined results from DNA analyses showed that the grandmother had transmitted three different ATP7A haplotypes to her offspring: (1) the at-risk allele (CA(B))-1 and the deletion; (2) the at-risk allele (CA(B))-1 without deletion; and (3) the second allele (CAB)-2 without deletion. In conclusion, our study demonstrated segregation of Menkes disease within the family investigated that can best be explained by extensive germ-line mosaicism in the maternal grandmother. The finding of germ-line mosaicism has obvious implications for genetic counseling of Menkes disease families.     

Paternally inherited alleles in male body parts of an ant (<Emphasis Type="Italic">Diacamma</Emphasis> sp.) sex mosaic: implication for androgenetic male production in the Hymenoptera

Sex mosaicism, also called gynandromorphism, refers to an accidental phenomenon in dioecious organisms (mainly animals) in which an individual phenotype includes both female and male characteristics. Despite the rarity of this phenomenon, elucidating the mechanisms of naturally occurring sex mosaicism should deepen our understanding of diverse sex determination and differentiation systems in nature. We report the results of a genetic study of a sex mosaic individual of the ant Diacamma sp. from Japan’s Okinawa Island. Parentage analysis using microsatellite markers revealed that the female and male parts of the sex mosaic showed different inheritance patterns: female parts had alleles consistent with their biparental inheritance, whereas most of the male parts had alleles consistent with their paternal inheritance (i.e., androgenesis). We discuss plausible cytogenetic mechanisms that gave rise to the male parts of this individual: polyspermy and the subsequent independent cleavage by a surplus sperm pronucleus, and maternal genome elimination after fertilization of an ovule. Moreover, we hypothesize that the androgenetically produced males found in some Hymenoptera might share the same underlying cytogenetic mechanism with hymenopteran sex mosaicism.     

Y chromosome--specific DNA sequences in Turner-syndrome mosaicism.

Phenotypic females with Y-chromosomal material in their genome have an increased risk for development of gonadal malignancy. The detection and identification of Y-chromosomal material in these cases can be of critical importance for medical management. Chromosome analysis in four patients with Turner syndrome revealed the characteristic 45,X chromosome complement together with a second cell population containing a small marker chromosome (46,X, + mar). Molecular-hybridization analyses utilizing cloned, Y chromosome-specific DNA sequences were performed to determine whether Y-chromosomal material was present in each patient. Three cases contained some Y chromosome-specific sequences, whereas one case was negative with all four probes that we used. These results were compared with detailed cytogenetic studies--including G-, Q-, and G-11-banding--of the marker chromosomes. In one case in which Y chromosome-specific DNA sequences were demonstrated, the marker chromosome was G-11 negative. These results demonstrate that cytogenetic analysis alone can lead to misidentification of some Y chromosome-derived markers. The combination of cytogenetic and molecular analyses permits a more accurate characterization of anomalous Y chromosomes and in turn provides additional information that can be crucial to the correct medical management of Turner-syndrome patients.     

Germ-line mosaicism in tuberous sclerosis: how common?

Two-thirds of cases of tuberous sclerosis complex (TSC) are sporadic and usually are attributed to new mutations, but unaffected parents sometimes have more than one affected child. We sought to determine how many of these cases represent germ-line mosaicism, as has been reported for other genetic diseases. In our sample of 120 families with TSC, 7 families had two affected children and clinically unaffected parents. These families were tested for mutations in the TSC1 and TSC2 genes, by Southern blotting and by single-strand conformational analysis. Unique variants were detected in six families. Each variant was present and identical in both affected children of a family but was absent in both parents and the unaffected siblings. Sequencing of the variants yielded two frameshift mutations, one missense mutation, and two nonsense mutations in TSC2 and one nonsense mutation in TSC1. To determine which parent contributed the affected gametes, the families were analyzed for linkage to TSC1 and TSC2, by construction of haplotypes with markers flanking the two genes. Linkage analysis and loss-of-heterozygosity studies indicated maternal origin in three families, paternal origin in one family, and either being possible in two families. To evaluate the possibility of low-level somatic mosaicism for TSC, DNA from lymphocytes of members of the six families were tested by allele-specific PCR. In all the families, the mutant allele was detected only in the known affected individuals. We conclude that germ-line mosaicism was present in five families with mutations in the TSC2 gene and in one family with the causative mutation in the TSC1 gene. The results have implications for genetic counseling of families with seemingly sporadic TSC.     

Mosaic down syndrome with a marker: molecular cytogenetic characterization of the marker chromosome

Down syndrome is a complex disorder characterized by well defined and distinctive phenotypic features. Approximately 2-3% of all live-born Down individuals are mosaics. Here we report a boy with suspected Down syndrome showing mosaicism for two different cell lines where one cell line is unexpected. The cytogenetic analysis by G-banding revealed a karyotype of 47 XY+21 [20]/46,X+marker [30]. Further, molecular cytogenetic analysis with spectral karyotyping identified the marker as a derivative of Y chromosome. The delineation of Y chromosomal DNA was done by quantitative real-time PCR and aneuploidy detection by quantitative fluorescence PCR. The Y-short tandem repeats typing was performed to estimate the variation in quantity as well as to find out the extent of deletion on Y chromosome using STR markers. Fluorescence in situ hybridization using Y centromeric probe was also performed to confirm the origin of the Y marker. Further fine mapping of the marker was carried out with three bacterial artificial chromosome clones RP11-20H21, RP11-375P13, RP11-71M14, which defined the hypothetical position of the deletion. In our study we defined the extent of deletion of the marker chromosome and also discussed it in relation with mosaicism. This is the first report of mosaic Down syndrome combined with a second de novo mosaic marker derived from the Y chromosome.     

The phenotype of 45,X/46,XY mosaicism: an analysis of 92 prenatally diagnosed cases.

We undertook an international survey of prenatally diagnosed 45,X/46,XY mosaicism to ascertain the phenotypic spectrum of this condition. Ninety-two cases were obtained by means of a questionnaire sent to over 730 cytogenetic laboratories. Seventy-six cases (75 males and 1 female) had physical examinations after delivery or termination of pregnancy. Among these, there were four significant genital anomalies: three hypospadias and one female with clitoromegaly. Gonadal histology was abnormal in three (27%) of 11 cases, all of whom had normal male external genitalia. Other anomalies were noted in five cases: one cystic hygroma in a male, two cardiac anomalies, one spina bifida with multiple other defects, and one intrauterine growth retardation. There was no relationship between the percent mosaicism and the presence or degree of abnormalities. We conclude that 95% of 45,X/46,XY fetuses will have normal male genitalia, although there will also be a significant risk (27%) for abnormal gonadal histology. Long-term follow-up studies of prenatally diagnosed cases of 45,X/46,XY mosaicism are needed to study, without ascertainment bias, stature, pubertal development, tumor risk, and fertility.     

DNA polymorphism analysis in families with recurrence of free trisomy 21

We used DNA polymorphic markers on the long arm of human chromosome 21 in order to determine the parental and meiotic origin of the extra chromosome 21 in families with recurrent free trisomy 21. A total of 22 families were studied, 13 in which the individuals with trisomy 21 were siblings (category 1), four families in which the individuals with trisomy 21 were second-degree relatives (category 2), and five families in which the individuals with trisomy 21 were third-degree relatives, that is, their parents were siblings (category 3). In five category 1 families, parental mosaicism was detected, while in the remaining eight families, the origin of nondisjunction was maternal. In two of the four families of category 2 the nondisjunctions originated in individuals who were related. In only one of five category 3 families, the nondisjunctions originated in related individuals. These results suggest that parental mosaicism is an important etiologic factor in recurrent free trisomy 21 (5 of 22 families) and that chance alone can explain the recurrent trisomy 21 in many of the remaining families (14 of 22 families). However, in a small number of families (3 of 22), a familial predisposing factor or undetected mosaicism cannot be excluded.