Clinical experience of laboratory follow-up with noninvasive prenatal testing using cell-free DNA and positive microdeletion results in 349 cases.

OBJECTIVE: Screening via noninvasive prenatal testing (NIPT) involving the analysis of cell-free DNA (cfDNA) from plasma has become readily available to screen for chromosomal and DNA aberrations through maternal blood. This report reviews a laboratory's experience with follow-up of positive NIPT screens for microdeletions. METHODS: Patients that were screened positive by NIPT for a microdeletion involving 1p, 4p, 5p, 15q, or 22q who underwent diagnostic studies by either chorionic villus sampling or amniocentesis were evaluated. RESULTS: The overall positive predictive value for 349 patients was 9.2%. When a microdeletion was confirmed, 39.3% of the cases had additional abnormal microarray findings. Unrelated abnormal microarray findings were detected in 11.8% of the patients in whom the screen positive microdeletion was not confirmed. Stretches of homozygosity in the microdeletion were frequently associated with a false positive cfDNA microdeletion result. CONCLUSIONS: Overall, this report reveals that while cfDNA analysis will screen for microdeletions, the positive predictive value is low; in our series it is 9.2%. Therefore, the patient should be counseled accordingly. Confirmatory diagnostic microarray studies are imperative because of the high percentage of false positives and the frequent additional abnormalities not delineated by cfDNA analysis.

Interstitial deletion of proximal 8q including part of the centromere from unbalanced segregation of a paternal deletion/marker karyotype with neocentromere formation at 8p22.

BACKGROUND/AIMS: The 'McClintock mechanism' of chromosome breakage and centromere misdivision, in which a deleted chromosome with its concomitant excised marker or ring chromosome is formed, has been described in approximately one dozen reports. We report a case of a girl with short stature, developmental delay, and dysmorphic features. METHODS: Analysis was performed on the proband and father using cytogenetic chromosome analysis and the Affymetrix 6.0 SNP microarray. Fluorescence in situ hybridization (FISH) using a chromosome 8 alpha-satellite probe and immunofluorescence with antibodies to CENP-C were used to examine the centromere positions in these chromosomes. RESULTS: An abnormal chromosome 8 with a cytogenetically visible deletion was further defined by SNP array as a 10.6-Mb deletion from 8q11.1→q12.1. FISH with a chromosome 8 alpha-satellite probe demonstrated that the deletion removed a significant portion of the pericentromeric alpha-satellite repeat sequences and proximal q arm. The deleted chromosome 8 appeared to have a constriction at 8p22, suggesting the formation of a neocentromere, even though alpha-satellite sequences still appeared at the normal location. Chromosome analysis of the phenotypically normal father revealed the same deleted chromosome 8, as well as an apparently balancing mosaic marker chromosome 8. FISH studies revealed that the majority of the chromosome 8 alpha-satellite DNA resided in the marker chromosome. Immunofluorescence studies with antibodies to CENP-C, a kinetochore protein, proved the presence of a neocentromere at 8p22. The excision of the marker from the deleted chromosome 8 likely necessitated the formation of a new kinetochore at the 8p22 neocentromere to stabilize the chromosome during mitosis. CONCLUSION: This case clearly illustrates the utilization of classic cytogenetics, FISH, and array technologies to better characterize chromosomal abnormalities and provide information on recurrence risks. It also represents a rare case where a neocentromere can form even in the presence of existing alpha-satellite DNA.

Subtelomere FISH analysis of 11 688 cases: an evaluation of the frequency and pattern of subtelomere rearrangements in individuals with developmental disabilities.

BACKGROUND: Subtelomere fluorescence in situ hybridisation (FISH) analysis has increasingly been used as an adjunct to routine cytogenetic testing in order to detect small rearrangements. Previous reports have estimated an overall abnormality rate of 6%, with a range of 2-29% because of different inclusion criteria. METHODS: This study presents data compiled from 11 688 cases referred for subtelomere FISH testing in three clinical cytogenetic laboratories. RESULTS: In this study population, the detection rate for clinically significant subtelomere abnormalities was approximately 2.5%, with an additional 0.5% detection of presumed familial variants. Approximately half of the clinically significant abnormalities identified were terminal deletions, the majority of which were de novo. Most of the remaining cases were unbalanced translocations between two chromosomes or two arms of the same chromosome. Approximately 60% of the unbalanced translocations were inherited from a parent carrying a balanced form of the rearrangement. Other abnormalities identified included tandem duplications, apparently balanced translocations, partial deletions, and insertions. Interestingly, 9 cases (0.08%) were found to have interstitial deletions of non-telomeric control loci, either BCR on 22q or PML on 15q. The most common clinically significant imbalances found were deletions of 1p, 22q, 4p, 9q, 8p, 2q and 20p. The most common familial variants were a deletion or duplication of 10q, deletion of 4q, deletion of Yq, and duplication of X/Yp onto Xq. CONCLUSIONS: This study of subtelomere rearrangements is a 20 fold increase in number over the previously reported largest study and represents an unbiased analysis of subtelomere rearrangements in a large, unselected patient population.

Epigenetic detection of human chromosome 14 uniparental disomy.

The recent demonstration of genomic imprinting of DLK1 and MEG3 on human chromosome 14q32 indicates that these genes might contribute to the discordant phenotypes associated with uniparental disomy (UPD) of chromosome 14. Regulation of imprinted expression of DLK1 and MEG3 involves a differentially methylated region (DMR) that encompasses the MEG3 promoter. We exploited the normal differential methylation of the DLK1/MEG3 region to develop a rapid diagnostic PCR assay based upon an individual's epigenetic profile. We used methylation-specific multiplex PCR in a retrospective analysis to amplify divergent lengths of the methylated and unmethylated MEG3 DMR in a single reaction and accurately identified normal, maternal UPD14, and paternal UPD14 in bisulfite converted DNA samples. This approach, which is based solely on differential epigenetic profiles, may be generally applicable for rapidly and economically screening for other imprinting defects associated with uniparental disomy, determining loss of heterozygosity of imprinted tumor suppressor genes, and identifying gene-specific hypermethylation events associated with neoplastic progression.

Twin-twin transfusion syndrome in a dizygotic monochorionic-diamniotic twin pregnancy.

We present a case of twin-twin transfusion syndrome with discordant gender. Monochorionicity was confirmed by surgical pathology. Cytogenetic analysis showed normal 46,XX and 46,XY karyotypes. Microsatellite analysis using reliable pericentromeric markers was consistent with dispermic fertilization of two separate ova. This suggests that monochorionicity, rather than zygosity, may be responsible for the development of placental vascular anastomoses.

Phenotypes Associated with SHOX Deficiency.

Leri-Weill dyschondrosteosis (LWD) (MIM 127300) is a dominantly inherited skeletal dysplasia characterized phenotypically by Madelung wrist deformity, mesomelia, and short stature. LWD can now be defined genetically by haploinsufficiency of the SHOX (short stature homeobox-containing) gene. We have studied 21 LWD families (43 affected LWD subjects, including 32 females and 11 males, ages 3-56 yr) with confirmed SHOX abnormalities. We investigated the relationship between SHOX mutations, height deficit, and Madelung deformity to determine the contribution of SHOX haploinsufficiency to the LWD and Turner syndrome (TS) phenotypes. Also, we examined the effects of age, gender, and female puberty (estrogen) on the LWD phenotype. SHOX deletions were present in affected individuals from 17 families (81%), and point mutations were detected in 4 families (19%). In the LWD subjects, height deficits ranged from -4.6 to +0.6 SD (mean +/- SD = -2.2 +/- 1.0). There were no statistically significant effects of age, gender, pubertal status, or parental origin of SHOX mutations on height z-score. The height deficit in LWD is approximately two thirds that of TS. Madelung deformity was present in 74% of LWD children and adults and was more frequent and severe in females than males. The prevalence of the Madelung deformity was higher in the LWD vs. a TS population. The prevalence of increased carrying angle, high arched palate, and scoliosis was similar in the two populations. In conclusion, SHOX deletions or mutations accounted for all of our LWD cases. SHOX haploinsufficiency accounts for most, but not all, of the TS height deficit. The LWD phenotype shows some gender- and age-related differences.

Prenatal molecular cytogenetic diagnosis of partial tetrasomy 10p due to neocentromere formation in an inversion duplication analphoid marker chromosome.

Neocentromeres are fully functional centromeres found on rearranged or marker chromosomes that have separated from endogenous centromeres. Neocentromeres often result in partial tri- or tetrasomy because their formation confers mitotic stability to acentric chromosome fragments that would normally be lost. We describe the prenatal identification and characterization of a de novo supernumerary marker chromosome (SMC) containing a neocentromere in a 20-wk fetus by the combined use of comparative genomic hybridization (CGH) and fluorescence in situ hybridization (FISH). GTG-banding of fetal metaphases revealed a 47,XY,+mar karyotype in 100% of cultured amniocytes; parental karyotypes were both normal. Although sequential tricolor FISH using chromosome-specific painting probes identified a chromosome 10 origin of the marker, a complete panel of chromosome-specific centromeric satellite DNA probes failed to hybridize to any portion of the marker. The presence of a neocentromere on the marker chromosome was confirmed by the absence of hybridization of an all-human-centromere alpha-satellite DNA probe, which hybridizes to all normal centromeres, and the presence of centromere protein (CENP)-C, which is associated specifically with active kinetochores. Based on CGH analysis and FISH with a chromosome 10p subtelomeric probe, the marker was found to be an inversion duplication of the distal portion of chromosome 10p. Thus, the proband's karyotype was 47,XY,+inv dup(10)(pter-->p14 approximately 15::p14 approximately 15-->neo-->pter), which is the first report of partial tetrasomy 10p resulting from an analphoid marker chromosome with a neocentromere. This study illustrates the use of several molecular strategies in distinguishing centric alphoid markers from neocentric analphoid markers.

XX sex reversal: molecular analysis of the SRY/ZFY regions.

PURPOSE: The mammalian sex determining gene, sex region Y chromosome (SRY), is now firmly established as the testis determining locus. The SRY locus is close to the short arm Y terminus and just distal to zinc finger Y region (ZFY), a locus previously thought to be involved in testicular differentiation and the male phenotype. We report on XX sex reversal, a rare sex chromosomal disorder in humans. MATERIALS AND METHODS: Routine amniocentesis revealed an XX fetal karyotype, although at birth the neonate was phenotypically male. Radiographic evaluation showed a normal male urethra and the absence of any female internal genitalia. Subsequent molecular analysis with polymerase chain reaction amplified sequences of the SRY and ZFY loci were positive. RESULTS: This case is the fourth in our series of XX sex reversed male individuals and to our knowledge the first to be diagnosed perinatally. In all cases the SRY and ZFY loci are present, presumably on the paternal X chromosome, as well as a Klinefelter phenotype. These sex reversing translocations are thought to be due to an unequal meiotic recombination of the distal X and Y short arms during male gametogenesis. The tendency for XY translocations to break between the SRY and ZFY loci was not seen in these apparent microtranslocation cases. CONCLUSIONS: These 4 cases demonstrate the usefulness of molecular followup of clinically perplexing sexual discordance. We conclude that SRY and ZFY polymerase chain reaction amplification studies should be performed when sexual discrepancies are noted on prenatal ultrasound and karyotype analysis.

Cytogenetic, morphologic and oncogene analysis of a cell line derived from a heterologous mixed mullerian tumor of the ovary.

A cell line was established from a mixed mullerian tumor of the ovary and designated LN1. Histopathologic analysis of the fresh tumor specimen demonstrated a highly aneuploid heterologous tumor comprised of undifferentiated mesodermal components with carcinomatous cells present as a smaller population. Long-term in vitro culture resulted in the establishment of a cell line that exhibits an epithelial-like morphology and expresses epithelial antigens cytokeratin, epithelial membrane antigen, and carcinoma antigen TAG-72. These cells also express mesenchymal intermediate filaments, vimentin, and desmin. Karyotypic analysis revealed a basic triploid pattern with multiple chromosomal abnormalities, most notably an isochromosome of the short arm of five present in three copies. Analysis of oncogene expression revealed that LN1 cells constitutively express mRNA for c-ras, c-erbB2, and p53. The expression of mRNA for cellular oncogenes correlated with the presence of corresponding oncoproteins, p21H-ras, p21K-ras, and p185erB2 and mutant p53 protein. In summary, coexpression of epithelial and mesenchymal antigens by LN1 cells lends support to the hypothesis that epithelial and mesenchymal elements comprising mixed mullerian tumors of the ovary are derived from a common stem cell precursor. Furthermore, this cell line represents a functional in vitro model to evaluate the biologic activities of these unusual and highly aggressive ovarian malignancies.

Refined molecular characterization of the breakpoints in small inv dup(15) chromosomes.

Inv dup(15) is the most common supernumerary marker chromosome in humans. To investigate the mechanism responsible for this frequent chromosome rearrangement, we characterized the breakpoints in 18 individuals with small inv dup(15) chromosomes [i.e., negative for the Prader-Willi (PWS)/Angelman syndrome (AS) critical region]. Since two proximal breakpoint regions ("hotspots") for PWS/AS deletions have been previously identified with the most proximal 15q markers D15S541/S542 and S543, we hypothesized that formation of the small inv dup(15) chromosomes may involve one or both of these breakpoint hotspots. By analysis with S542, both breakpoint regions were found to be involved in approximately equal frequencies. In ten cases, the inv dup(15) was negative for S542 (Class I), indicating the breakpoint is between the centromere and the most proximal marker on chromosome 15. For the other eight cases, S542 was positive by fluorescence in situ hybridization (5/5) and/or microsatellite analysis (7/7), but S543 was negative (Class II). These two breakpoint regions appear to be the same as the two proximal breakpoints reported in the common PWS/AS deletions. To initiate cloning and sequencing of the Class II breakpoint, the gap in the yeast artificial chromosome (YAC) contig between S541/S542 and S543 was filled by screening the CEPH YAC and mega-YAC libraries. YACs 705C2 and 368H3 were found to bridge this gap, and therefore contain the more distal breakpoint region. The finding of consistent breakpoints in small inv dup(15), like that found in PWS/AS deletions, provides strong evidence for hotspots for chromosome breakage in this region. In addition, our results show that two extra copies (tetrasomy) of the region from 15cen to the euchromatic region containing S542 are present in individuals with Class II breakpoints. Since most individuals carrying a small inv dup(15) are phenotypically normal, the euchromatin region included in the small inv dup(15) chromosomes does not appear to contain genes with clinically significant dosage effects.

Spectral karyotyping refines cytogenetic diagnostics of constitutional chromosomal abnormalities.

Karyotype analysis by chromosome banding is the standard method for identifying numerical and structural chromosomal aberrations in pre- and postnatal cytogenetics laboratories. However, the chromosomal origins of markers, subtle translocations, or complex chromosomal rearrangements are often difficult to identify with certainty. We have developed a novel karyotyping technique, termed spectral karyotyping (SKY), which is based on the simultaneous hybridization of 24 chromosome-specific painting probes labeled with different fluorochromes or fluorochrome combinations. The measurement of defined emission spectra by means of interferometer-based spectral imaging allows for the definitive discernment of all human chromosomes in different colors. Here, we report the comprehensive karyotype analysis of 16 samples from different cytogenetic laboratories by merging conventional cytogenetic methodology and spectral karyotyping. This approach could become a powerful tool for the cytogeneticists, because it results in a considerable improvement of karyotype analysis by identifying chromosomal aberrations not previously detected by G-banding alone. Advantages, limitations, and future directions of spectral karyotyping are discussed.

Xp pseudoautosomal gene haploinsufficiency and linear growth deficiency in three girls with chromosome Xp22;Yq11 translocation.

Colony stimulating factor-2 receptor alpha (CSF2RA) and interleukin-3 receptor alpha (IL3RA), two genes from the chromosome Xp and Yp pseudoautosomal region (PAR), have been suggested as candidate genes for short stature in Turner syndrome. We report three girls with X;Y translocation (46,X,der(X)t(X;Y)(p22;q11) initially detected by amniocentesis. The terminal portion of the X chromosome distal to the translocation breakpoint at Xp22 was deleted on the derivative X chromosome in all three patients. Each had normal stature at birth, with greater than expected deceleration of growth velocity by the second year. Using fluorescence in situ hybridisation (FISH), we have shown deletion of the CSF2RA and IL3RA loci on the derivative X chromosomes of all three patients. The role of CSF2RA and IL3RA haploinsufficiency in linear growth and final adult stature is discussed. Additional studies, particularly of molecular deletions within the PAR, are needed to improve our understanding of the role of these and other PAR loci in the genetic control of adult stature.

Distinct 15q genotypes in Russell-Silver and ring 15 syndromes.

Individuals with a ring 15 chromosome [r(15)] and those with Russell-Silver syndrome have short stature, developmental delay, triangular face, and clinodactyly. To assess whether the apparent phenotypic overlap of these conditions reflects a common genetic cause, the extent of deletions in chromosome 15q was determined in 5 patients with r(15), 1 patient with del 15q26.1-qter, and 5 patients with Russell-Silver syndrome. All patients with Russell-Silver syndrome were diploid for genetic markers in distal 15q, indicating that Russell-Silver syndrome in these individuals was unlikely to be related to the expression of single alleles at these or linked genetic loci. At least 3 distinct sites of chromosome breakage close to the telomere were found in the r(15) and del 15q25.1-qter patients, with 1 r(15) patient having both a terminal and an interstitial deletion. Although the patient with del 15q25.1-qter exhibited the largest deletion and the most profound growth retardation, the degree of growth impairment among the r(15) patients was not correlated with the size of the deleted interval. Rather, the parental origin of the ring chromosome in several patients was associated with phenotypes that are also seen in patients with either Prader-Willi (PWS) or Angelman (AS) syndromes, conditions that result from uniparental expression of genes on chromosome 15. In fact, unequal representation of chromosome 15 alleles in 1 patient with r(15) suggests the possibility that a mosaic karyotype composed of the constitutional cell line and cell line(s) possibly deficient in the ring chromosome might be present. The PWS-like or AS-like phenotypes could be explained by postzygotic loss of the ring chromosome, leading to uniparental inheritance of the intact chromosome in some tissues of r(15) patients.

Uniparental isodisomy of chromosome 14 in two cases: an abnormal child and a normal adult.

Uniparental disomy (UPD) of a number of different chromosomes has been found in associated with abnormal phenotypes. A growing body of evidence for an imprinting effect involving chromosome 14 has been accumulating. We report on a case of paternal UPD of chromosome 14 studied in late gestation due to polyhydramnios and a ventral wall hernia. A prenatal karyotype documented a balanced Robertsonian 14:14 translocation. The baby was born prematurely with hairy forehead, retrognathia, mild puckering of the lips and finger contractures. Hypotonia has persisted since birth and at age one year, a tracheostomy for laryngomalacia and gastrostomy for feeding remain necessary. Absence of maternal VNTR polymorphisms and homozygosity of paternal polymorphisms using chromosome 14 specific probes at D14S22 and D14S13 loci indicated paternal uniparental isodisomy (pUPID). Parental chromosomes were normal. We also report on a case of maternal UPD in a normal patient with a balanced Robertsonian 14:14 translocation and a history of multiple miscarriages. Five previous reports of chromosome 14 UPD suggest that an adverse developmental effect may be more severe whenever the UPD is paternal in origin. This is the second reported patient with paternal UPD and the fifth reported with maternal UPD, and only few phenotypic similarities are apparent. Examination of these chromosome 14 UPD cases of maternal and paternal origin suggests that there are syndromic imprinting effects.

Case report of spontaneous remission of cytogenetic relapse of chronic myelogenous leukemia suggestive of progression to blast crisis after allogeneic bone marrow transplantation.

Detection of the chronic myelogenous leukemia (CML)-related marker, the bcr/abl m-RNA transcript, in blood or bone marrow of patients with CML in hematologic remission after allogeneic bone marrow transplantation (allo-BMT) may be associated with the presence of minimal residual disease but does not uniformly predict hematologic relapse. In contrast, when there is cytogenetic reappearance of the Philadelphia (Ph1) translocation [t(9;22)(q34;q11)] along with additional cytogenetic abnormalities, especially more than 2 years after BMT, progression to hematologic relapse and acceleration of CML usually occur. An exception to this rule may be our patient, who was a 29-year old white woman diagnosed with Ph1-positive CML by cytogenetics. She was initially treated with hydroxyurea. An allo-BMT was performed 4 months after the diagnosis, while the patient was still in the first chronic phase of her disease, her HLA-identical brother serving as bone marrow (BM) donor. The conditioning regimen for BMT consisted of cytosine arabinoside, cyclophosphamide, total body irradiation, splenic irradiation, and intrathecal methotrexate. Graft-versus-host disease (GVHD) prophylaxis consisted of cyclosporin A and methotrexate. Her hospital course was unremarkable and without evidence of acute GVHD. Six months after transplantation, the patient had mild chronic GVHD and was treated with azathioprine and prednisone for 6 months. A year later, she recurred with mild chronic GVHD. She was treated with azathioprine alone for 5 months. Subsequently, she received cyclosporin A and prednisone for 8 months, with resolution of her symptoms. Serial BM cytogenetic studies showed normal male donor karyotypes 12 and 24 months after BMT. At 36, 42, and 50 months after BMT, reappearance of the Ph1 was noted along with some cells with additional cytogenetic abnormalities, including t(6;14)(p21;q32). The breakpoint involvement of 14q32, the heavy chain Ig locus, in the new clone may be indicative of B-lymphoid lineage-based evolution. The abnormal clones disappeared 56 months from BMT and remained absent through 69 months after BMT. The patient has remained in hematologic remission during her entire post-BMT course. Clinically, she continues to do well without immunosuppressants at presently 69 months after BMT. The reappearance of the Ph1 chromosome could be associated with the immunosuppressive therapy given for chronic GVHD. This case supports the concept that immunologic mechanisms may be important in the eradication of CML after allo-BMT, and even cytogenetic evidence of blast crisis CML may spontaneously remit after allo-BMT.

Complex chromosome rearrangement with ankyloblepharon filiforme adnatum.

A Caucasian boy with a de novo complex chromosome rearrangement owing to six chromosome breaks was small for gestation with microcephaly, complex heart defect, hypotonia, left auricular pit, simian creases, and ankyloblepharon filiforme adnatum. The rearrangement included two translocation, t(15;21) (q22;q22) and t(3;11)(q21;q11), with the derivative 3 showing in addition pericentric inversion (p11q11) and interstitial deletion (q11q21). Based on parental satellite polymorphisms of chromosomes 15 and 21, the paternal gamete appeared to be the source of the chromosome rearrangement. There was no evidence of mitotic chromosome instability. A review of 36 reported patients with complex chromosome rearrangements secondary to more than four breaks indicates that complex chromosome rearrangements are compatible with gamete survival, zygote formation, and postnatal life. The latter is usually compromised by structural defects, growth retardation, and often mental retardation.

Cleft palate and complex chromosome rearrangements.

Two of three unrelated children with de novo congenital complex chromosome rearrangements (CCR) with more than four chromosome breaks had cleft lip and palate as one of several congenital anomalies. In patient 1, unilateral complete cleft of the primary and secondary palates accompanied severe ectrodactyly, bilateral posterior choanal atresia and several minor congenital anomalies. Karyotypes of peripheral lymphocytes and skin fibroblasts showed five derivative chromosomes with six break points. There were two translocations, t(2;5), t(3;11) and an interstitial deletion, del(13)(q12q14). Patient 2 had a bilateral complete cleft of the lip and palate, in addition to slow pre- and postnatal growth and minor congenital anomalies. Peripheral lymphocytes and palatal mucosa fibroblasts karyotypes showed five derivative chromosomes with six break points. A partial deletion of 10p, two translocations, t(2;3), t(7;18) and an inversion of the derivative chromosome 2 were present. In both patients, a "major catastrophe" of unknown etiology in one of the parental gametes appeared to be the event leading to the stable CCR without evidence of persistent chromosome instability. All four parents had normal karyotypes. The presence of palatal clefts in these patients indicates that dysmorphologists and pediatricians have to consider CCR whenever taking care of a patient with cleft palate, particularly if additional anomalies, no matter how subtle, are present. The detection and interpretation of the latter anomalies are essential for the diagnosis and management of these patients. Accurate cytogenetic diagnosis determines the short- and long-term prognosis and facilitates genetic counseling in regard to life-span, quality of life and reproductive plans of patients and parents.

Lowe oculocerebrorenal syndrome in a female with a balanced X;20 translocation: mapping of the X chromosome breakpoint.

A Hispanic girl with Lowe oculocerebrorenal syndrome (OCRL), an X-linked recessive condition characterized by cataracts, glaucoma, mental retardation, and proteinuria, is reported. A balanced X;20 chromosomal translocation with the X chromosome breakpoint at q26.1 was found with high-resolution trypsin-Giemsa banding. Somatic cell hybridization was used to separate the X chromosome derivative and the chromosome 20 derivative in order to position, with respect to the translocation breakpoint, several DNA loci that are linked to the Lowe syndrome locus (Xq24-q26). DXS10 and DXS53 were found to be distal to the breakpoint, whereas DXS37 and DXS42 were located proximal to it. These studies suggest that the OCRL locus lies in the region between these probes. The translocation chromosome originated from an unaffected male without a visible translocation, indicating that the most likely cause of OCRL in this patient is the de novo translocation that disrupted the OCRL locus.