Identification of a case of maternal uniparental disomy of chromosome 10 associated with confined placental mosaicism

Causal variants screened by whole exome sequencing in a patient with maternal uniparental isodisomy of chromosome 10 and a complicated phenotype

Uniparental disomy (UPD), which is the abnormal situation in which both copies of a chromosomal pair have been inherited from one parent, may cause clinical abnormalities by affecting genomic imprinting or causing autosomal recessive variation. Whole Exome Sequencing (WES) and chromosomal microarray analysis (CMA) are powerful technologies used to search for underlying causal variants. In the present study, WES was used to screen for candidate causal variants in the genome of a Chinese pediatric patient, who had been shown by CMA to have maternal uniparental isodisomy of chromosome 10. This was associated with numerous severe medical problems, including bilateral deafness, binocular blindness, stunted growth and leukoderma. A total of 13 rare homozygous variants of these genes were identified on chromosome 10. These included a classical splice variant in the HPS1 gene (c.398+5G>A), which causes Hermansky-Pudlak syndrome type 1 and may explain the patient's ocular and dermal disorders. In addition, six likely pathogenic genes on other chromosomes were found to be associated with the subject's ocular and aural disorders by phenotypic analysis. The results of the present study demonstrated that WES and CMA may be successfully combined in order to identify candidate causal genes. Furthermore, a connection between phenotype and genotype was established in this patient.

Novel mutation of the perforin gene and maternal uniparental disomy 10 in a patient with familial hemophagocytic lymphohistiocytosis

Familial hemophagocytic lymphohistiocytosis is a rare disorder characterized by lethal primary immunodeficiency associated with hypercytokinemia and a concomitant defect in natural killer cell cytotoxicity. We report a fatal case of familial hemophagocytic lymphohistiocytosis homozygous caused by a novel nonsense mutation of the perforin gene. Homozygosity was established to be the result of uniparental disomy of the maternal chromosome 10. Uniparental disomy increases the risk of autosomal recessive disease.

Further evidence of a maternal parent-of-origin effect on chromosome 10 in late-onset Alzheimer's disease

The chromosome 10q region has recently received a great deal of attention in late-onset Alzheimer's disease (LOAD), given the growing evidence of linkage to LOAD, or to A-beta levels, reported by several groups. In a recent paper we reported evidence of linkage in this region in our subset of the NIMH AD genetics initiative pedigrees, approaching genome-wide significance (non-parametric LOD score = 3.27), when only families with maternal disease origin were analyzed [Bassett et al. (2002); Am J Med Genet 114:679-686]. We have now extended this work, using an independent subset of NIMH AD pedigrees from the University of Alabama at Birmingham (UAB), and show further evidence of linkage using parent-of-origin information. As in our Hopkins sample, maternal but not paternal pedigrees show significantly increased linkage in the chromosome 10q region compared to the unstratified sample. Combining data from our previous fine-mapping work on this region and five new markers genotyped in all pedigrees results in a non-parametric LOD score of 3.73 in the same region, a value that reaches genome wide significance for linkage, with an empirical P value = 0.003. These results support our earlier findings and narrow the region of interest. In combination with findings from other groups, these results provide further evidence that this chromosome 10 region harbors a gene implicated in LOAD, and our use of parent-of-origin information has been useful in further narrowing the region of interest.

A fascination with chromosome rescue in uniparental disomy: Mendelian recessive outlaws and imprinting copyrights infringements

With uniparental disomy (UPD), the presence in a diploid genome of a chromosome pair derived from one genitor carries two main types of developmental risk: the inheritance of a recessive trait or the occurrence of an imprinting disorder. When the uniparentally derived pair carries two homozygous sequences (isodisomy) with a duplicated mutant, this 'reduction to homozygosity' determines a recessive phenotype solely inherited from one heterozygote. Thus far, some 40 examples of such recessive trait transmission have been reported in the medical literature and, among the current 32 known types of UPDs, UPD of chromosomes 1, 2, and 7 have contributed to the larger contingent of these conditions. Being at variance with the traditional mode of transmission, they constitute a group of 'Mendelian outlaws'. Several imprinted chromosome domains and loci have been, for a large part, identified through different UPDs. Thus, disomies for paternal 6, maternal 7, paternal 11, paternal and maternal 14 and 15, maternal 20 (and paternal 20q) and possibly maternal 16 cause as many syndromes, as at the biological level the loss or duplication of monoparentally expressed allele sequences constitutes 'imprinting rights infringements'. The above pitfalls represent the price to pay when, instead of a Mendelian even segregation and independent assortment of the chromosomes, the fertilized product with a nondisjunctional meiotic error undergoes correction (for unknown or fortuitous reasons) through a mitotic adjustment as a means to restore euploidy, thereby resulting in UPD. Happily enough, UPDs leading to the healthy rescue from some chromosomal mishaps also exist.

Risk estimates for uniparental disomy following prenatal detection of a nonhomologous Robertsonian translocation

Carriers of nonhomologous Robertsonian translocations (ROB) are at risk for having offspring with uniparental disomy (UPD). Although risk estimates have been calculated in several independent studies, the estimates have not been optimal because most studies are not of sufficient size and UPD events are rare. However, these collective data have provided the opportunity to derive an overall risk estimate for UPD in the fetus after the prenatal identification of a ROB.

Trisomy 10 mosaicism and maternal uniparental disomy 10 in a liveborn infant with severe congenital malformations

We report on a liveborn infant with trisomy 10 mosaicism combined with maternal uniparental heterodisomy for chromosome 10. The mosaicism 47,XY,+10/46,XY was found in five different tissues, including one blood sample, while cultured lymphocytes from two other blood samples showed a normal karyotype, 46,XY. DNA analysis with six PCR-based microsatellite markers demonstrated the trisomic cell line to be a result of maternal meiotic nondisjunction, and revealed maternal uniparental heterodisomy in the diploid cell line, suggesting that the formation of the diploid cell line was due to trisomy rescue. The boy had severe growth retardation, major dysmorphism, and malformations, and died at 37 days. We reviewed the previous nine reports of infants and fetuses with trisomy 10 mosaicism reported in the literature. We suggest that a common clinical syndrome can be defined comprising skull, jaw and ear abnormalities, cleft lip/palate, malformations of eyes, heart and kidneys, deformity of hands and feet, and most often death neonatally or in early infancy. The cytogenetic findings in the present patient demonstrate the importance of karyotyping more than one tissue, and not only lymphocytes, when a chromosomal aberration is strongly suspected.

Sex-specific chromosome instability in early human development

The predominance of females segregating chromosome aberrations to their offspring has been explained mostly by selection disadvantage of unbalanced products of spermatogenesis. However, analysis of data from the literature supports the idea that somatic cells of early female embryos are similar to female germ cells in that they are prone to malsegregation. The goal of this study was to compare the sex ratio (male to female ratio) of carriers of presumably mitotic-occurring chromosome abnormalities to identify any sex biases. In examining the literature, we found a female prevalence in cases of mosaicism associated with uniparental disomy (UPD) (26 male individuals/conceptions and 45 female individuals/conceptions, sex ratio is 0.58, significantly different from 1.06 in newborn population, P = 0.0292). This predominance was highest at gestational age <16 week (8 male and 22 female conceptuses, sex ratio is 0.36, significantly different from expected figure of 1.28, P = 0.0025), which diminished at later stages of fetal development indicating potential correction of trisomies predominantly in females. There is a threefold prevalence of 46,XX/45,X mosaics over 46,XY/45,X mosaics in prenatally diagnosed cases, which also suggests a gender-specific postzygotic chromosome loss. The male prevalence in Prader-Willi syndrome with maternal UPD of chromosome 15 also can be explained by sex-specific trisomy correction, with predominant loss of a maternal chromosome causing biparental inheritance and therefore, complete correction of trisomy in females (without UPD). Finally, there is a female predominance in carriers of chromosome rearrangement with pericentromere break (mosaicism for Robertsonian translocation/isochromosome, centric fission, nonacrocentric isochromosome, and whole arm rearrangement), in both prenatal (21 males and 36 females, sex ratio is 0.58, P < 0.0184) and postnatal ill-defined cases (14 males and 35 females, sex ratio is 0.40, P = 0.001). Thus, the findings presented in this paper suggest that, in addition to reduction in male fertility, and to probable selection against abnormal cell line(s), there are two mechanisms that contribute to female preponderance among carriers of mosaicism: sex-specific chromosome loss and sex-specific centromere instability. The data obtained suggest that females may have gonadal mosaicism for aneuploidies and structural rearrangements more often than males. This may lead to the maternal origin bias in offspring with trisomies or structural rearrangements.

Uniparental disomy (UPD) other than 15: Phenotypes and bibliography updated

Uniparental disomy (UPD) describes the inheritance of a pair of chromosomes from only one parent. The concept was introduced in Medical Genetics by Engel (1980); Am J Med Genet 6:137-143. Aside UPD 15, which is the most frequent one, up to now (February 2005) 197 cases with whole chromosome maternal UPD other than 15 (124 X heterodisomy, 59 X isodisomy, and 14 cases without information of the mode of UPD) and 68 cases with whole chromosome paternal UPD other than 15 (13 X heterdisomy, 53 X isodisomy, and 2 cases without information of the mode of UPD) have been reported. In this review we discuss briefly the problems associated with UPD and provide a comprehensive clinical summary with a bibliography for each UPD other than 15 as a guide for genetic counseling.

Uniparental disomy of chromosome 2 resulting in lethal trifunctional protein deficiency due to homozygous alpha-subunit mutations

The mitochondrial trifunctional protein (TFP) is an enzyme complex of the fatty acid beta-oxidation cycle composed of an alpha- and a beta-subunit. The two encoding genes are located in the same region on chromosome 2 (2p23). TFP deficiency due to either alpha- or beta-subunit mutations is characterized by mutational and phenotypic heterogeneity with severe, early-onset, cardiac forms and milder, later-onset, myopathic phenotypes. In two unrelated patients with lethal TFP deficiency, we delineated apparently homozygous alpha-subunit mutations that were present in heterozygous form in both mothers, but not in either biological father. We performed a microsatellite repeat analysis of both patients and their parents using seven chromosome 2-specific polymorphic DNA markers and four nonchromosome 2 markers. In both patients, two chromosome 2-specific markers demonstrated maternal isodisomy of chromosome 2. The other five chromosome 2-specific markers were noninformative in each patient. Inheritance of alleles from chromosomes 4, 5, and 7 was consistent with paternity. These results explain the apparently anomalous pattern of transmission. Six of our 12 known TFP-deficient patients with alpha-subunit mutations have disease due to homozygous changes and two of them via the mechanism of uniparental disomy (UPD) (16.7%). For very rare autosomal recessive diseases, UPD may represent a common mechanism. This study emphasizes the need to confirm mutations in parents whenever possible. TFP deficiency is another disorder that has become manifest due to isodisomy of chromosome 2. This information will impact genetic counseling for these families, reducing greatly the 25% risk normally used for recessive disorders.

Maternal uniparental isodisomy 10 and mosaicism for an additional marker chromosome derived from the paternal chromosome 10 in a fetus

We report a case of maternal isodisomy 10 combined with mosaic partial trisomy 10 (p12.31-q11.1). Chromosome examinations from a CVS sample showed a karyotype 47,XX,+mar/46,XX [corrected]. The additional marker chromosome which was present in 6/25 interphase nuclei was shown by fluorescence in situ hybridization (FISH) to have been derived from a pericentromeric segment of chromosome 10. DNA analysis was performed from umbilical cord blood from the fetus after termination of the pregnancy at 18 weeks. The results showed that the two structurally normal chromosomes 10 were both of maternal origin, whereas the marker chromosome derived from the father. Autopsy of the fetus revealed hypoplasia of heart, liver, kidneys and suprarenal glands, but, apart from a right bifid ureter, no structural organ abnormalities. This fetus represents the second reported instance of a maternal uniparental disomy (UPD) 10.

Severe intra-uterine growth retardation in a patient with maternal uniparental disomy 22 and a 22-trisomic placenta

We report on a maternal uniparental disomy of chromosome 22 in a patient with severe intra-uterine growth retardation. Karyotyping of a placental tissue revealed non-mosaic trisomy 22, whereas lymphocyte chromosomes from the newborn were normal 46,XY. Microsatellite analysis using DNA extracted from white blood cells showed maternal uniparental heterodisomy for chromosome 22. Thus, the conceptus started as maternal trisomy due to meiotic non-disjunction, and trisomy rescue occurred subsequently through loss of the paternal homologue resulting in maternal uniparental disomy. Normal phenotypes in previous reports have suggested that maternal UPD 22 has no impact on the phenotype. Thus, growth retardation in this patient was probably caused by dysfunction of the trisomic placenta.

Identification and characterization of a de novo partial trisomy 10p by comparative genomic hybridization (CGH)

We report the characterization of a de novo unbalanced chromosome rearrangement by comparative genomic hybridization (CGH) in a 15-day-old child with hypotonia and dysmorphia. We describe the combined use of CGH and fluorescence in situ hybridization (FISH) to identify the origin of the additional chromosomal material on the short arm of chromosome 6. Investigation with FISH revealed that the excess material was not derived from chromosome 6. Identification of unknown unbalanced aberrations that could not be identified by traditional cytogenetics procedures is possible by CGH analysis. Visual analysis of digital images from CGH-metaphase spreads revealed a predominantly green signal on the telomeric region of chromosome 10p. After quantitative digital ratio imaging of 10 CGH-metaphase spreads, a region of gain was found in the chromosome band 10p14-pter. The CGH finding was confirmed by FISH analysis, using a whole chromosome 10 paint probe. These results show the usefulness of CGH for a rapid characterization of de novo unbalanced translocation, unidentifiable by karyotype alone.

Trisomic zygote rescue revealed by DNA polymorphism analysis in confined placental mosaicism

Uniparental disomy can be caused by different genetic mechanisms such as gamete complementation, chromosome duplication in monosomic zygote, or post-zygotic aneuploidy correction. This last mechanism is well documented in human reproduction and is related to placental mosaicism. In the case of a trisomic zygote which has originated by paternal or maternal non-disjunction at the first or second meiotic cell division, mosaicism will result from chromosome loss and restoration of a 'normalized' diploid fetal karyotype. In order to enrich the literature with new observations on this subject, we studied by DNA polymorphism analysis ten cases of confined placental mosaicism (CPM). The finding in placental DNA of three different alleles at polymorphic loci of chromosomes 13, 16, and 20 demonstrated the trisomic status of the zygote in three cases. On the basis of these results, we believe that systematic DNA polymorphism analysis could give useful additional information to improve knowledge on aneuploidy correction in human reproduction.

Growth effects of uniparental disomies and the conflict theory of genomic imprinting

While numerous theories have been proposed for the evolution of genomic imprinting, few have been tested. The conflict theory proposes that imprinting is an intra-individual manifestation of classical parent-offspring conflict. This theory is unique in predicting that imprinted genes expressed from the paternally derived genome should be enhancers of pre- and post-natal growth, while those expressed from the maternally derived genome should be growth suppressors. We examine this prediction by reviewing the literature on growth of human and mouse progeny that have inherited both copies (or part thereof) of a particular chromosome from only one parent. Perhaps surprisingly, we find that much of the data do not support the hypothesis.

Maternal uniparental disomy of chromosome 2 and confined placental mosaicism for trisomy 2 in a fetus with intrauterine growth restriction, hypospadias, and oligohydramnios

We present a case of maternal uniparental heterodisomy for chromosome 2 (UPD 2) detected after trisomy 2 mosaicism was found on placental biopsy. This case presented prenatally with severe intrauterine growth restriction (IUGR) and oligohydramnios. The diploid newborn had hypospadias and features consistent with oligohydramnios sequence. He died shortly after birth of severe pulmonary hypoplasia. The term placenta had high levels of trisomy 2 in both the trophoblast and the stroma. A comparison of this case with others reported in the literature suggests that the IUGR and oligohydramnios are likely related to placental insufficiency due to the high levels of trisomy 2 present in the trophoblast of the term placenta and the presence of UPD 2 in the diploid placental line.

Comprehensive 4-year follow-up on a case of maternal heterodisomy for chromosome 16

Uniparental disomy for chromosome 16 has been previously identified in fetal deaths and newborn infants with limited follow-up. Thus there is a lack of information about the long-term effects of maternal uniparental disomy 16 on growth and development. We present a case of maternal heterodisomy for chromosome 16 and a comprehensive 4-year physical and cognitive evaluation. Cytogenetic analysis of chorionic villus obtained at 10 weeks gestation for advanced maternal age showed trisomy 16. At 15 weeks, amniocentesis demonstrated low level mosaicism 47,XY,+16[1]/46,XY[25]. Decreased fetal growth was noted in the last 2 months of pregnancy and the infant was small for gestational age at birth. Molecular studies revealed only maternal alleles for chromosome 16 in a peripheral blood sample from the child, consistent with maternal uniparental heterodisomy 16. Although short stature remains a concern, there appears to be no major cognitive effects of maternal disomy 16. Clinical evaluation and follow-up on additional cases should further clarify the role of placental mosaicism and maternal disomy 16 in intrauterine growth retardation and its effects on long-term growth in childhood.