Hereditary cancer testing in a diverse sample across three breast imaging centers.

PURPOSE: Up to 10% of all breast cancers (BC) are attributed to inherited pathogenic variants (PV) in BC susceptibility genes; however, most carriers of PVs remain unidentified. Here, we sought to determine the yield of hereditary cancer gene PVs among diverse women attending breast imaging centers, who could benefit from enhanced surveillance and/or risk reduction interventions. METHODS: This cross-sectional retrospective cohort study included consecutive women, unselected for personal or family cancer history, who were offered genetic testing for hereditary cancer genes at the time of breast imaging at three centers (November 2020-March 2022). RESULTS: Among 1943 patients (median age: 66 years), self-reported race/ethnicity was White (34.5%), Hispanic (27.7%), African American (17.9%), Asian (4.5%), Ashkenazi Jewish (0.6%), Other (3.5%), and missing (13.0%). Thirty-nine patients (2%) were identified as carriers of a PV in an autosomal dominant clinically actionable hereditary breast and ovarian cancer (HBOC)-related or Lynch syndrome gene, most frequently, BRCA2 (6/39; 15.4%), PALB2 (8/39; 20.5%), CHEK2 (10/39; 25.6%), and PMS2 (5/39; 12.8%). Of the 34 PVs with known race/ethnicity, 47% were detected among non-White patients. Overall, 354/1,943 (18.2%) of patients met NCCN guidelines for HBOC gene testing and only 15/39 (38.5%) patients with an autosomal dominant clinically actionable PV met guidelines. CONCLUSION: This population health approach extended the reach of genetic cancer risk assessment in a diverse population and highlighted the limits of a guideline-based approach. This may help address inequity in access to risk-appropriate screening and cancer prevention.

Maternal Sex Chromosome Aneuploidy Identified through Noninvasive Prenatal Screening: Clinical Profile and Patient Experience.

OBJECTIVE: Noninvasive prenatal screening (NIPS) may incidentally identify maternal aneuploidies that have health implications. We evaluated patients' experience with counseling and follow-up diagnostic testing after NIPS flags a potential maternal sex chromosome aneuploidy (SCA). STUDY DESIGN: Patients who underwent NIPS at two reference laboratories between 2012 and 2021 and had test results that were consistent with possible or probable maternal SCA were contacted with a link to an anonymous survey. Survey topics included demographics, health history, pregnancy history, counseling, and follow-up testing. RESULTS: A total of 269 patients responded to the anonymous survey, and 83 of these individuals also completed one follow-up survey. Most received pretest counseling. A total of 80% were offered fetal genetic testing during the pregnancy, and 35% of patients completed diagnostic maternal testing. Monosomy X-related phenotypes such as short stature or hearing loss prompted follow-up testing that led to a diagnosis of monosomy X in 14 (6%) cases. CONCLUSION: Follow-up counseling and testing after a high-risk NIPS result suggestive of maternal SCA is heterogenous in this cohort and may be frequently incomplete. Health outcomes may be affected by these results and additional research could improve the provision, delivery, and quality of posttest counseling. KEY POINTS: · NIPS results showing potential SCA could have maternal health implications.. · Variations in counseling and testing after NIPS were observed for women with suspected SCA.. · Comprehensive counseling and diagnostic testing strategies are critical for these patients..

Role of genetic analysis of products of conception and PGT in managing early pregnancy loss.

This article considers the addition of comprehensive 24-chromosomal microarray (CMA) analysis of products of conception (POC) to a standard evaluation for recurrent pregnancy loss (RPL) to help direct treatment towards expectant management versus IVF with preimplantation genetic testing for aneuploidies (PGT-A). The review included retrospective data from 65,333 miscarriages, a prospective evaluation of 378 couples with RPL who had CMA testing of POC and the standard workup, and data from an additional 1020 couples who were evaluated for RPL but did not undergo CMA testing of POC. Aneuploidy in POC explained the pregnancy loss in 57.7% (218/378) of cases. In contrast, the full RPL evaluation recommended by the American Society for Reproductive Medicine identified a potential cause in only 42.9% (600/1398). Combining the data from the RPL evaluation and the results of genetic testing of POC provides a probable explanation for the loss in over 90% (347/378) of women. Couples with an unexplained loss after the standard evaluation with POC aneuploidy accounted for 41% of cases; PGT-A may be considered after expectant management. Conversely, PGT-A would have a limited role in those with a euploid loss and a possible explanation after the standard workup. Categorizing a pregnancy loss as an explained versus unexplained loss after the standard evaluation combined with the results of CMA testing of POC may help identify patients who would benefit from expectant management versus PGT-A.

Relationship between paternal factors and embryonic aneuploidy of paternal origin.

OBJECTIVE: To determine if there is a relationship between paternal factors and embryonic aneuploidy of paternal origin using preimplantation genetic testing for aneuploidy (PGT-A). DESIGN: Retrospective cohort. SETTING: Academic. PARTICIPANTS: Couples undergoing in vitro fertilization with PGT-A. INTERVENTIONS: None. MAIN OUTCOME MEASURE: To determine if there is an association between paternal age, body mass index (BMI), or semen analysis parameters and paternal aneuploidy. RESULTS: From January 2015-2020, 453 in vitro fertilization cycles (1,720 embryos) underwent PGT-A using single nucleotide polymorphism microarrays with parental support bioinformatics. The mean (±SD) was 36.5 (±3.5) years for maternal age, 39.5 (±5.5) years for paternal age, 24.7 (±5.0) kg/m2 for maternal BMI, and 27.6 (±4.3) kg/m2 for paternal BMI. Embryonic aneuploidy of paternal origin was found in 8.4% (144/1,720) embryos. There were 1,533 embryos with a recorded paternal BMI. Rates of embryonic aneuploidy of paternal origin were similar between men across BMI groups: BMI 18-24.9 kg/m2 was 7.2% (referent); BMI 25-29.9 kg/m2 was 8.4% (odds ratio [OR], 1.12; 95% confidence interval [CI], 0.79-1.82); and BMI ≥30 kg/m2 was 9.1% (OR, 1.31; 95% CI, 0.83-2.08). There were 854 embryos from men with a normal and 866 from men with an abnormal semen analysis. No differences were found in the rate of embryonic aneuploidy of paternal origin between men with normal and abnormal sperm concentration, total count, motility, progressive motility, or morphology. No significant difference was seen in rates of aneuploidy between men aged <50 years and those aged ≥50 years (OR, 1.69; 95% CI, 0.96-2.98). CONCLUSION: No association was found between paternal age, BMI, or semen analysis parameters and paternal aneuploidy.

Novel ploidy analysis in ectopic pregnancy.

OBJECTIVE: To study whether a single-nucleotide polymorphism (SNP) array could be used to test tissue from ectopic pregnancy to distinguish whether ectopic pregnancies were aneuploid. DESIGN: Case series report. SETTING: Academic medical center. PATIENTS: One hundred seventy-eight women who underwent surgery for ectopic pregnancy at Northwestern Memorial Hospital between 2015 and 2018 were eligible for participation; written consent was obtained from 33 patients. Eight subjects had sufficient DNA samples and were included in the analysis. Maternal and paternal DNA samples were self-collected by buccal swab. Archived paraffin tissue containing chorionic villi from each surgically removed ectopic specimen was analyzed using SNP microarray technology to determine chromosome number and evaluate for maternal cell contamination. INTERVENTIONS: None. MAIN OUTCOME MEASURES: Prevalence of aneuploidy in ectopic pregnancy specimens as well as success of SNP array technology in formalin-fixed and paraffin-embedded specimens. RESULTS: Subjects had a mean (±SD) age of 33.4 ± 5.4 years, body mass index of 23.4 ± 5.7 kg/m2, 3.3 ± 1.8 prior pregnancies, and 1.5 ± 1.4 live births. Genetic testing revealed that all eight tested samples were euploid, 6 female and 2 male (two arr(1-22)x2, (X,Y)x1 and 6 arr(1-22, X)x2); maternal cell contamination was ruled out in all cases. CONCLUSIONS: This study showed proof of concept for the use of routinely stored formalin-fixed, paraffin-embedded tissue blocks with DNA extraction for SNP array to detect ploidy status of ectopic pregnancy. Although all tested samples were euploid, further research is needed to gain a definitive answer to this question and better understand the mechanism that leads to ectopic implantation.

Single-nucleotide polymorphism microarray detects molar pregnancies in 3% of miscarriages.

OBJECTIVE: To determine the frequency of molar pregnancy in miscarriage cases based on single-nucleotide polymorphism (SNP) microarray testing on products of conception (POC) tissue and to estimate the sensitivity of ultrasound and histopathologic evaluation for cases identified to be at risk for gestational trophoblastic disease. DESIGN: Retrospective cohort analysis. SETTING: Reference laboratory. PATIENT(S): POC specimens from 26,101 consecutive miscarriages. INTERVENTION(S): POC samples were sent to a single laboratory for analysis. Maternal age, gestational age, egg donor use, indication for testing, and additional clinical information were obtained from the requisition form and a survey sent to ordering providers. MAIN OUTCOME MEASURE(S): Rates of full paternal uniparental disomy (UPD), indicative of complete molar pregnancy, and triploidy of paternal origin, indicative of partial molar pregnancy, in POC samples. RESULT(S): Paternal triploidy was detected in 638 cases (2.8%) and full paternal UPD in 72 cases (0.3%). Of the cases with complete clinical information (224/710; 31.5%), histopathology and/or ultrasound did not detect 71% of partial molar pregnancies and 30% of complete molar pregnancies. CONCLUSION(S): Molar pregnancy, detected in 3.1% of all miscarriages in this study with the use of 24-chromosome SNP microarray testing, occurred significantly more frequently than previously estimated with the use of ultrasound and/or histopathology.

Neutropenia in Barth syndrome: characteristics, risks, and management.

PURPOSE OF REVIEW: Barth syndrome (BTHS) is an X-linked disease characterized by defective remodeling of phospholipid side chains in mitochondrial membranes. Major features include neutropenia, dilated cardiomyopathy, motor delay and proximal myopathy, feeding problems, and constitutional growth delay. We conducted this review of neutropenia in BTHS to aid in the diagnosis of this disease, and to improve understanding of both the consequences of neutropenia and the benefits of treatment with granulocyte colony-stimulating factor (G-CSF). RECENT FINDINGS: In 88 patients with BTHS, neutropenia, that is, at least one count below 1.5 × 10/l, was detected in 74 (84%) and 44% had severe chronic neutropenia, with multiple counts below 0.5 × 10/l. The pattern of neutropenia varied between intermittent and unpredictable, chronic and severe, or cyclical with mathematically regular oscillations. Monocytosis, that is, monocytes more than 1.0 × 10/l, was observed at least once in 64 of 85 (75%) patients. G-CSF was administered to 39 of 88 patients (44%). Weekly average G-CSF doses ranged from 0.12 to 10.92 µg/kg/day (mean 1.16 µg/kg/day, median 1.16 µg/kg/day). Antibiotic prophylaxis was additionally employed in 21 of 26 neutropenic patients. Pretreatment bone marrow evaluations predominantly showed reduced myeloid maturation which normalized on G-CSF therapy in seven of 13 examined. Consistent clinical improvement, with reduced signs and symptoms of infections, was observed in response to prophylactic G-CSF ±â€Šprophylactic antibiotics. However, despite G-CSF and antibiotics, one adult patient died with multiple infections related to indwelling medical devices and gastrostomy site infection after 15.5 years on G-CSF and a pediatric patient required gastrostomy removal for recurrent abdominal wall cellulitis. SUMMARY: BTHS should be considered in any men with neutropenia accompanied by any of the characteristic features of this syndrome. Prophylaxis with G-CSF ±â€Šantibiotics prevents serious bacterial infections in the more severe neutropenic patients although infections remain a threat even in patients who are very compliant with therapy, especially in those with indwelling devices.

Incidence of the 22q11.2 deletion in a large cohort of miscarriage samples.

BACKGROUND: The 22q11.2 deletion syndrome is the most common microdeletion syndrome in livebirths, but data regarding its incidence in other populations is limited and also include ascertainment bias. This study was designed to determine the incidence of the 22q11.2 deletion in miscarriage samples sent for clinical molecular cytogenetic testing. RESULTS: Twenty-six thousand one hundred one fresh product of conception (POC) samples were sent to a CLIA- certified, CAP-accredited laboratory from April 2010--May 2016 for molecular cytogenetic miscarriage testing using a single-nucleotide polymorphism (SNP)-based microarray platform. A retrospective review determined the incidence of the 22q11.2 deletion in this sample set. Fetal results were obtained in 22,451 (86%) cases, of which, 15 (0.07%) had a microdeletion in the 22q11.2 region (incidence, 1/1497). Of those, 12 (80%) cases were found in samples that were normal at the resolution of traditional karyotyping (i.e., had no chromosome abnormalities above 10 Mb in size) and three (20%) cases had additional findings (Trisomy 15, Trisomy 16, XXY). Ten (67%) cases with a 22q11.2 deletion had the common ~3 Mb deletion; the remaining 5 cases had deletions ranging in size from 0.65 to 1.5 Mb. A majority (12/15) of cases had a deletion on the maternally inherited chromosome. No significant relationship between maternal age and presence of a fetal 22q11.2 deletion was observed. CONCLUSIONS: The observed incidence of 1/1497 for the 22q11.2 deletion in miscarriage samples is higher than the reported general population prevalence (1/4000-1/6000). Further research is needed to determine whether the 22q11.2 deletion is a causal factor for miscarriage.

Genomic imbalance in products of conception: single-nucleotide polymorphism chromosomal microarray analysis.

OBJECTIVE: To report the full cohort of identifiable anomalies, regardless of known clinical significance, in a large-scale cohort of postmiscarriage products-of-conception samples analyzed using a high-resolution single-nucleotide polymorphism (SNP)-based microarray platform. High-resolution chromosomal microarray analysis allows for the identification of visible and submicroscopic cytogenomic imbalances; the specific use of SNPs permits detection of maternal cell contamination, triploidy, and uniparental disomy. METHODS: Miscarriage specimens were sent to a single laboratory for cytogenomic analysis. Chromosomal microarray analysis was performed using a SNP-based genotyping microarray platform. Results were evaluated at the cytogenetic and microscopic (greater than 10 Mb) and submicroscopic (less than 10 Mb) levels. Maternal cell contamination was assessed using information derived from fetal and maternal SNPs. RESULTS: Results were obtained on 2,389 of 2,392 specimens (99.9%) that were less than 20 weeks of gestation. Maternal cell contamination was identified in 528 (22.0%) specimens. The remaining 1,861 specimens were considered to be of true fetal origin. Of these, 1,106 (59.4%) showed classical cytogenetic abnormalities: aneuploidy accounted for 945 (85.4%), triploidy for 114 (10.3%), and structural anomalies or tetraploidy for the remaining 47 (4.2%). Of the 755 (40.6%) cases considered normal at the cytogenetic level, SNP chromosomal microarray analysis revealed a clinically significant copy number change or whole-genome uniparental disomy in 12 (1.6%) and three (0.4%) cases, respectively. CONCLUSION: Chromosomal microarray analysis of products-of-conception specimens yields a high diagnostic return. Using SNPs extends the scope of detectable genomic abnormalities and facilitates reporting "true" fetal results. This supports the use of SNP chromosomal microarray analysis for cytogenomic evaluation of miscarriage specimens when clinically indicated. LEVEL OF EVIDENCE: III.

Reliability of 46,XX results on miscarriage specimens: a review of 1,222 first-trimester miscarriage specimens.

OBJECTIVE: To examine the rate of maternal contamination in miscarriage specimens. DESIGN: Retrospective review of 1,222 miscarriage specimens submitted for chromosome testing with detection of maternal cell contamination (MCC). SETTING: Referral centers requesting genetic testing of miscarriage specimens at a single reference laboratory. PATIENT(S): Women with pregnancy loss who desire complete chromosome analysis of the pregnancy tissue. INTERVENTION(S): Analysis of miscarriage specimens using single-nucleotide polymorphism (SNP) microarray technology with bioinformatics program to detect maternal cell contamination. MAIN OUTCOME MEASURE(S): Chromosome content of miscarriages and incidence of 46,XX results due to MCC. RESULT(S): Of the 1,222 samples analyzed, 592 had numeric chromosomal abnormalities, and 630 were normal 46,XX or 46,XY (456 and 187, respectively). In 269 of the 46,XX specimens, MCC with no embryonic component was found. With the exclusion of maternal 46,XX results, the chromosomal abnormality rate increased from 48% to 62%, and the ratio for XX to XY results dropped from 2.6 to 1.0. CONCLUSION(S): Over half of the normal 46,XX results in miscarriage specimens were due to MCC. The use of SNPs in MCC testing allows for precise identification of chromosomal abnormalities in miscarriage as well as MCC, improving the accuracy of products of conception testing.

Mutations in SLC20A2 are a major cause of familial idiopathic basal ganglia calcification.

Familial idiopathic basal ganglia calcification (IBGC) or Fahr's disease is a rare neurodegenerative disorder characterized by calcium deposits in the basal ganglia and other brain regions, which is associated with neuropsychiatric and motor symptoms. Familial IBGC is genetically heterogeneous and typically transmitted in an autosomal dominant fashion. We performed a mutational analysis of SLC20A2, the first gene found to cause IBGC, to assess its genetic contribution to familial IBGC. We recruited 218 subjects from 29 IBGC-affected families of varied ancestry and collected medical history, neurological exam, and head CT scans to characterize each patient's disease status. We screened our patient cohort for mutations in SLC20A2. Twelve novel (nonsense, deletions, missense, and splice site) potentially pathogenic variants, one synonymous variant, and one previously reported mutation were identified in 13 families. Variants predicted to be deleterious cosegregated with disease in five families. Three families showed nonsegregation with clinical disease of such variants, but retrospective review of clinical and neuroimaging data strongly suggested previous misclassification. Overall, mutations in SLC20A2 account for as many as 41% of our familial IBGC cases. Our screen in a large series expands the catalog of SLC20A2 mutations identified to date and demonstrates that mutations in SLC20A2 are a major cause of familial IBGC. Non-perfect segregation patterns of predicted deleterious variants highlight the challenges of phenotypic assessment in this condition with highly variable clinical presentation.

Genomic and genic deletions of the FOX gene cluster on 16q24.1 and inactivating mutations of FOXF1 cause alveolar capillary dysplasia and other malformations.

Alveolar capillary dysplasia with misalignment of pulmonary veins (ACD/MPV) is a rare, neonatally lethal developmental disorder of the lung with defining histologic abnormalities typically associated with multiple congenital anomalies (MCA). Using array CGH analysis, we have identified six overlapping microdeletions encompassing the FOX transcription factor gene cluster in chromosome 16q24.1q24.2 in patients with ACD/MPV and MCA. Subsequently, we have identified four different heterozygous mutations (frameshift, nonsense, and no-stop) in the candidate FOXF1 gene in unrelated patients with sporadic ACD/MPV and MCA. Custom-designed, high-resolution microarray analysis of additional ACD/MPV samples revealed one microdeletion harboring FOXF1 and two distinct microdeletions upstream of FOXF1, implicating a position effect. DNA sequence analysis revealed that in six of nine deletions, both breakpoints occurred in the portions of Alu elements showing eight to 43 base pairs of perfect microhomology, suggesting replication error Microhomology-Mediated Break-Induced Replication (MMBIR)/Fork Stalling and Template Switching (FoSTeS) as a mechanism of their formation. In contrast to the association of point mutations in FOXF1 with bowel malrotation, microdeletions of FOXF1 were associated with hypoplastic left heart syndrome and gastrointestinal atresias, probably due to haploinsufficiency for the neighboring FOXC2 and FOXL1 genes. These differences reveal the phenotypic consequences of gene alterations in cis.

Identification of copy number variants associated with BPES-like phenotypes.

Blepharophimosis-Ptosis-Epicanthus inversus syndrome (BPES) is a well-characterized rare syndrome that includes an eyelid malformation associated with (type I) or without premature ovarian failure (type II). Patients with typical BPES have four major characteristics: blepharophimosis, ptosis, epicanthus inversus and telecanthus. Mutations in the FOXL2 gene, encoding a forkhead transcription factor, are responsible for the majority of both types of BPES. However, many patients with BPES-like features, i.e., having at least two major characteristics of BPES, have an unidentified cause. Here, we report on a group of 27 patients with BPES-like features, but without an identified genetic defect in the FOXL2 gene or flanking region. These patients were analyzed with whole-genome high-density arrays in order to identify copy number variants (CNVs) that might explain the BPES-like phenotype. In nine out of 27 patients (33%) CNVs not previously described as polymorphisms were detected. Four of these patients displayed psychomotor retardation as an additional clinical characteristic. In conclusion, we demonstrate that BPES-like phenotypes are frequently caused by CNVs, and we emphasize the importance of whole-genome copy number screening to identify the underlying genetic causes of these phenotypes.

Craniosynostosis: another feature of the 22q11.2 deletion syndrome.

We report on the presence of craniosynostosis in four patients with the 22q11.2 deletion. In light of previous reports of the association, we propose that the occurrence is higher than the general population incidence. Therefore, we suggest that craniosynostosis should be considered a manifestation of the 22q11.2 deletion and conversely that the 22q11.2 deletion should be considered in the differential diagnosis of craniosynostosis.

Molecular analysis of congenital scoliosis: a candidate gene approach.

The etiology of congenital scoliosis is largely unknown. The severe vertebral disorder, spondylocostal dysostosis type 1, is associated with a homozygous delta-like 3 (DLL3) mutation. Scoliosis has been observed in a heterozygous DLL3 carrier, raising the possibility of its involvement in congenital scoliosis. We present the first molecular study of congenital scoliosis by analysis of the candidate gene DLL3 and demonstrate one novel missense variant. However, no novel or previously described mutations are present in our cohort, indicating that DLL3 mutations may not be a major cause of congenital scoliosis. Additionally, we have evaluated patients with congenital scoliosis not diagnosed with a known syndrome and identified a significant number of associated renal and cardiac anomalies and familial incidence of idiopathic scoliosis in this group.

Congenital scoliosis and vertebral malformations: characterization of segmental defects for genetic analysis.

The developmental and genetic etiology of most congenital vertebral malformation disorders remains unknown. The objective of this study was to evaluate and classify congenital vertebral defect cases into groupings based on developmental etiology for clinical genetic studies. This classification is intended to be distinct from but complementary to traditional groupings based on spinal curvature or progression. In the first step of this analysis, the authors identified 84 cases of vertebral segmentation disorders by radiologic screening and prospectively recruited 39 of these patients into a clinical genetic study. Next, the authors quantified the extent of contiguous defects and organized cases by craniocaudal localization. Finally, the authors used available clinical association data to identify syndromic and nonsyndromic subcategories, and identified a high rate of orthopaedic and neurologic associations in nonsyndromic patients. This type of analysis has identified subgroups of patients with multiple, contiguous segmental defects and orthopaedic associations that are particularly suitable for further genetic analysis.

Increased prevalence of unprovoked seizures in patients with a 22q11.2 deletion.

Many neurologic abnormalities have been identified in patients with a deletion of chromosome region 22q11.2, including recurrent, apparently unprovoked seizures. We reviewed the database of patients with a 22q11.2 deletion at the Children's Hospital of Philadelphia to assess the prevalence of idiopathic epilepsy in this population. The records of 383 patients with a confirmed 22q11.2 deletion were reviewed for documentation of seizures; precipitating events such as hypocalcemia, fever, and recent surgery; MRI and EEG findings (to aid in seizure classification); and potential risk factors for epilepsy. Of 348 patients with adequately detailed histories, 27 patients had apparently unprovoked seizures (7% of the total population). A disproportionate number of these patients met criteria for generalized epilepsy. Cardiac disease and prematurity were not risk factors for the development of unprovoked seizures in this population. The prevalence of unprovoked seizures in individuals meeting criteria for epilepsy in patients with a 22q11.2 deletion evaluated at our institution is much greater than in the general population. The association with generalized epilepsy suggests that this increased risk is a primary manifestation of the genetic syndrome, not a secondary result of the other manifestations of 22q11.2 deletions. These results suggest that diagnostic screening for the 22q11.2 deletion syndrome should be considered in patients with epilepsy and other signs suggestive of this interstitial deletion syndrome, and have implications for the identification of potential genetic loci for idiopathic epilepsy.

Aberrant interchromosomal exchanges are the predominant cause of the 22q11.2 deletion.

Chromosome 22q11.2 deletions are found in almost 90% of patients with DiGeorge/velocardiofacial syndrome (DGS/VCFS). Large, chromosome-specific low copy repeats (LCRs), flanking and within the deletion interval, are presumed to lead to misalignment and aberrant recombination in meiosis resulting in this frequent microdeletion syndrome. We traced the grandparental origin of regions flanking de novo 3 Mb deletions in 20 informative three-generation families. Haplotype reconstruction showed an unexpectedly high number of proximal interchromosomal exchanges between homologs, occurring in 19/20 families. Instead, the normal chromosome 22 in these probands showed interchromosomal exchanges in 2/15 informative meioses, a rate consistent with the genetic distance. Meiotic exchanges, visualized as MLH1 foci, localize to the distal long arm of chromosome 22 in 75% of human spermatocytes tested, also reflecting the genetic map. Additionally, we found no effect of proband gender or parental age on the crossover frequency. Parental origin studies in 65 de novo 3 Mb deletions (including these 20 patients) demonstrated no bias. Unlike Williams syndrome, we found no chromosomal inversions flanked by LCRs in 22 sets of parents of 22q11 deleted patients, or in eight non-deleted patients with a DGS/VCFS phenotype using FISH. Our data are consistent with significant aberrant interchromosomal exchange events during meiosis I in the proximal region of the affected chromosome 22 as the likely etiology for the deletion. This type of exchange occurs more often than is described for deletions of chromosomes 7q11, 15q11, 17p11 and 17q11, implying a difference in the meiotic behavior of chromosome 22.