Identification of 14 novel mutations in DHCR7 causing the Smith-Lemli-Opitz syndrome and delineation of the DHCR7 mutational spectra in Spain and Italy

The Smith-Lemli-Opitz syndrome (SLOS) is a phenotypically variable metabolic malformation and mental retardation syndrome for which more than 80 mutations in the DHCR7 disease-causing gene have been described. The DHCR7 mutational spectra differ significantly in different areas of Europe, and several common putative founder mutations account for a substantial fraction of all mutations in some ethnic groups. Here we have analysed 47 SLOS patients and describe 14 newly identified mutations in 18 SLOS patients of Ashkenazi Jewish, Austrian, British, German, Italian, Irish, Polish, Portuguese, and Spanish origins. Half of the new mutations are in the transmembrane domains of the protein. In addition, there were two null mutations, one mutation in the 4th cytoplasmic loop, two mutations in the first and last codons, and three mutations in other regions such as the second cytoplasmic loop and the first endoplasmic loop. The analysis included 20 Spanish and 12 Italian SLOS patients and revealed very different mutation spectra in these patients compared to previously described patients from Czechoslovakia, Germany, Poland, and the UK and implicated p.Thr93Met on the J haplotype as the most frequent Mediterranean founder mutation.

Detection of a deletion of exons 8-16 of the UBE3A gene in familial Angelman syndrome using a semi-quantitative dosage PCR based assay

Angelman syndrome (AS) is a neurodevelopmental disorder caused by failure of expression of the maternal copy of the imprinted UBE3A gene through a variety of mechanisms detected by methylation studies, mutation analysis of UBE3A and FISH. In 10-15% of suspected cases of AS these investigations do not reveal a genetic abnormality. We report here the development of a semi-quantitative dosage PCR technique used to identify sub-microscopic deletions involving UBE3A. Using this method we analysed a panel of 26 patients from 24 families, all fulfilling the clinical criteria for AS. We identified a deletion of UBE3A exons 8-16 in a sibling pair. Analysis of parental samples revealed the same deletion in their phenotypically normal mother. This is an inexpensive and valuable method for detecting UBE3A deletions in a small but important proportion of AS cases of unidentifiable cause.

Twelve novel JAG1 gene mutations in Polish Alagille syndrome patients

Alagille syndrome (AGS) is an autosomal dominant disorder with developmental abnormalities of the liver, heart, eyes, vertebrae, and face. Mutations in the JAG1 (Jagged 1) gene, coding a ligand in the evolutionarily conserved Notch signaling pathway, are responsible for AGS. Here we present sixteen different JAG1 gene mutations, among them twelve novel, not described previously. Seven frameshift: c. 172_178del7 (p.Ala58fs), c.509delT (p.Leu170fs), c.1197delG (p.Val399fs), c.1485_1486delCT (p.Pro495fs), c.1809_1810insTGGG (p.Lys604fs), c.2122_2125delCAGT (p.Gln708fs), c.2753delT (p.Ile918fs); five nonsense: c.383G>A (p.Trp128X), c.496C>T (p.Glu166X), c.841C>T (p.Gln281X), c.1207C>T (p.Gln403X), c.1603C>T (p.Gln535X); two splice site: c.388-1G>C, c.3048+1_3048+2insG and two missense mutations: c.359T>A (p.Ile120Asn), c.560G>A (p.Cys187Tyr) were found. Forty percent of the changes were identified in exons 2 and 4, the remaining mutations are distributed along the entire coding sequence of the gene. Seventy-five percent of the mutations lead to creation of premature termination codons. Family studies revealed that the specific mutations were inherited in 3 out of 11 investigated cases. No correlation between genotype and phenotype was observed.

Maternal urinary steroid profiles in prenatal diagnosis of Smith-Lemli-Opitz syndrome: first patient series comparing biochemical and molecular studies

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive disorder caused by reduced activity of 7-dehydrocholesterol (7DHC) reductase, resulting in a decreased level of cholesterol and increased concentrations of 7DHC and 8DHC in body fluids and tissues. Ten pregnancies at 25% risk of SLOS underwent prenatal testing. Diagnostic studies included DHCR7 mutation analysis in chorionic villus samples, amniotic fluid sterol analysis and serial measurements of oestriol (E3), pregnanetriol (PT), 7-dehydropregnanetriol (7DHPT) and 8-dehydroesteriol (8DHE3) concentrations in maternal urine samples obtained between 9 and 20 weeks of gestation. All tests were diagnostic and revealed nine unaffected foetuses (two normal homozygotes and seven DHCR7 heterozygotes) and one affected foetus. In the affected pregnancy, 7DHC and 8DHC in amniotic fluid were 9.87 and 3.7 microg/ml, respectively [reference range (RR) 0.0026 +/- 0.0015 microg/ml and not detectable, respectively] and maternal urinary steroid analyses showed increased ratios of 7DHPT/PT and 8DHE3/E3 of 0.74 and 1.7, respectively (RR 0-0.0147 and 0-0.019). In the heterozygous foetuses, 7DHPT/PT and 8DHE3/E3 ratios did not exceed those found in 48 normal controls. This is the first series of prenatal diagnostic testing for SLOS where non-invasive biochemical testing was performed in tandem with invasive diagnostic testing. We conclude that steroid measurements in maternal urine are a reliable means of prenatal diagnosis for SLOS.

Carrier frequency of mutation 657del5 in the NBS1 gene in a population of polish pediatric patients with sporadic lymphoid malignancies

Nijmegen breakage syndrome (NBS) is a human autosomal recessive disease characterized by genomic instability and enhanced cancer predisposition, in particular to lymphoma and leukemia. Recently, significantly higher frequencies of heterozygous carriers of the Slavic founder NBS1 mutation, 657del5, were found in Russian children with sporadic lymphoid malignancies, and in Polish adults with non-Hodgkin lymphoma (NHL). In addition, the substitution 643C>T (R215W) has also been found in excess among children with acute lymphoblastic leukemia (ALL). In an attempt to asses the contribution of both mutations to the development of sporadic lymphoid malignancies, we analyzed DNA samples from a large group of Polish pediatric patients. The NBS1 mutation 657del5 on one allele was found in 3 of 270 patients with ALL and 2 of 212 children and adolescents with NHL; no carrier was found among 63 patients with Hodgkin lymphoma (HL). No carriers of the variant R215W were detected in any studied group. The relative frequency of the 657del5 mutation was calculated from a total of 6,984 controls matched by place of patient residence, of whom 42 were found to be carriers (frequency = 0.006). In the analyzed population with malignancies, an increased odds ratio for the occurrence of mutation 657del5 was found in comparison with the control Polish population (OR range 1.48-1.85, 95% confidence interval 1.18-2.65). This finding indicates that the frequency of the mutation carriers was indeed increased in patients with ALL and NHL (p < 0.05). Nonetheless, NBS1 gene heterozygosity is not a major risk factor for lymphoid malignancies in childhood and adolescence.

Maternal apo E genotype is a modifier of the Smith-Lemli-Opitz syndrome

BACKGROUND

Smith-Lemli-Opitz syndrome (MIM 270400) is an autosomal recessive malformation and mental retardation syndrome that ranges in clinical severity from minimal dysmorphism and mild mental retardation to severe congenital anomalies and intrauterine death. Smith-Lemli-Opitz syndrome is caused by mutations in the Delta7 sterol-reductase gene (DHCR7; EC 1.3.1.21), which impair endogenous cholesterol biosynthesis and make the growing embryo dependent on exogenous (maternal) sources of cholesterol. We have investigated whether apolipoprotein E, a major component of the cholesterol transport system in human beings, is a modifier of the clinical severity of Smith-Lemli-Opitz syndrome.

METHOD

Common apo E, DHCR7, and LDLR genotypes were determined in 137 biochemically characterised patients with Smith-Lemli-Opitz syndrome and 59 of their parents.

RESULTS

There was a significant correlation between patients' clinical severity scores and maternal apo E genotypes (p = 0.028) but not between severity scores and patients' or paternal apo E genotypes. In line with their effects on serum cholesterol levels, the maternal apo epsilon2 genotypes were associated with a severe Smith-Lemli-Opitz syndrome phenotype, whereas apo E genotypes without the epsilon2 allele were associated with a milder phenotype. The correlation of maternal apo E genotype with disease severity persisted after stratification for DHCR7 genotype. There was no association of Smith-Lemli-Opitz syndrome severity with LDLR gene variation.

CONCLUSIONS

These results suggest that the efficiency of cholesterol transport from the mother to the embryo is affected by the maternal apo E genotype and extend the role of apo E and its disease associations to modulation of embryonic development and malformations.

DHCR7 mutations and genotype-phenotype correlation in 37 Polish patients with Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive disorder of cholesterol biosynthesis caused by mutations in the DHCR7 gene. Thirty-seven ethnic Polish patients with SLOS underwent mutation analysis. The mutation frequencies in Polish patients were significantly different from those observed in Western European populations. Two mutations, W151X (22/68 alleles, 32%) and V326L (19/68 alleles, 28%), accounted for 60% of all observed in our cohort. Two missense mutations L68P and L360P have not been reported previously. In total, we report 15 DHCR7 mutations identified in Polish patients. By comparing clinical severity scores and the biochemical and molecular data, a genotype-phenotype correlation was attempted. In compound heterozygotes with one null mutation, the phenotype severity depends on the localization and type of the second mutation: mild phenotypes are correlated with mutations affecting the putative transmembrane domains TM1-TM6 or CT regions and severe phenotypes with mutations localized in TM7 and 4L region. The phenotypic differences of patients with the same genotype suggest that severity of the disease may be affected by other factors.

Schimke immuno-osseous dysplasia: two cases

We report two patients with Schimke immuno-osseous dysplasia (SIOD). SIOD is characterised by growth retardation, renal failure, spondylo-epiphyseal dysplasia, specific phenotype and defective cellular immunity. These two children demonstrated a bone dysplasia with characteristic radiographic appearances. We postulate that SIOD should be considered in all cases of growth failure with an unclassifiable bone dysplasia. Repeated urine tests for proteinuria could be helpful in reaching the correct diagnosis.

Alagille syndrome associated with a paracentric inversion 20p12.2p13 disrupting the JAG1 gene

Mutations in the human gene Jagged1 (JAG1) localized in 20p12 have been recently identified as causal for the anomalies found in patients with Alagille syndrome (AGS). This gene encodes a ligand for the Notch1 transmembrane receptor, which plays a key role in cell-to-cell signaling during differentiation and is conserved from C. elegans to human. We report a paracentric inversion (PAI) of chromosome 20p12.2p13 in an individual with AGS who also had alpha-1-antitrypsin deficiency. To our knowledge, this is the first published case of PAI involving the short arm of chromosome 20. Using FISH, fiberFISH, and molecular studies with a approximately 40 kb cosmid clone encompassing the entire 36 kb JAG1 gene, we demonstrate that the gene was disrupted by the inversion breakpoint between exons 5 and 6. An unusual association between two most common causes of chronic liver disease in childhood, AGS and alpha-1-antitrypsin deficiency, as well as their influence on the proband's abnormal phenotype are discussed.

Parental mosaicism of JAG1 mutations in families with Alagille syndrome

The Alagille syndrome (AGS), a congenital disorder affecting liver, heart, skeleton and eye in association with a typical face, is an autosomal dominant disease with nearly complete penetrance and variable expression. AGS is caused by mutations in the developmentally important JAG1 gene. In our mutation screening, where 61 mutations in JAG1 were detected, we identified five cases where mosaicism is present. Our results point to a significant frequency of mosaicism for JAG1 mutations in AGS of more than 8.2%. Because mosaicism may be associated with a very mild phenotype, the appropriate diagnosis of AGS and consequently the determination of the recurrence risk can be complicated.

X-linked hypophosphatemia in Polish patients. 2. Analysis of clinical features and genotype-phenotype correlation

Clinical and molecular data of 59 affected persons from 36 unrelated families with XLH (36 probands and 23 members of their families) were analysed. Characteristic phenotypic features (degree of leg deformities, growth failure, tooth abnormalities, tubular reabsorption of phosphate, serum phosphate and 1,25-dihydroxyvitamin D3 concentrations, head length and hearing defect in some cases) were assessed in relation to the type and localisation of 29 different PHEX gene mutations. The severity of clinical symptoms did not strictly depend upon the type and localisation of the PHEX gene mutation. A hearing defect was correlated with mutations in the beginning fragment, while tooth abnormalities and increased head length with the mutations in the beginning and the terminal fragment of the gene. Phosphate and vitamin D3 supplementation usually slowed progressive growth retardation and leg bowing. Our results point to the probability that alternative splicing occurs in the PHEX gene, producing several active forms of the PHEX protein. Some of them might be involved in bone turnover and dentin formation, others in renal phosphate uptake and vitamin D3 metabolism.

Frequency gradients of DHCR7 mutations in patients with Smith-Lemli-Opitz syndrome in Europe: evidence for different origins of common mutations

Smith-Lemli-Opitz syndrome/RSH (SLOS) is a multiple congenital anomaly syndrome caused by mutations in the gene for Delta7-sterol reductase (DHCR7) which catalyses the last step in the biosynthesis of cholesterol. SLOS is among the common recessive disorders in Europeans but almost absent in most other populations. More than 40 mutations in the DHCR7 gene some of which are frequent have been described in SLOS patients of various origins. Here we report mutation analysis of the DHCR7 gene in SLOS patients from Poland (n = 15), Germany/Austria (n = 22) and Great Britain (n = 22). Altogether 35 different mutations were identified and the two null mutations IVS8-1G > C and W151X were the most frequent in the total sample. In all three populations three mutations accounted for >0.5 of SLOS chromosomes. The mutational spectra were, however, significantly different across these populations with each of the common mutations showing an east-west gradient (W151X, V326L) or vice versa (IVS8-1G > C). W151X is the most frequent (0.33) mutation in Polish SLOS patients. It has an intermediate frequency in German/Austrian patients (0.18) and is rare among British patients (0.02). V326L shows the same distribution pattern (Poland 0.23, Germany/Austria 0.18, Britain 0.02). In contrast IVS8-1G > C is most frequent in Britain (0.34) intermediate in Germany/Austria (0.20) and rare in Poland (0.03). All analysed IVS8-1G > C and V326L alleles shared the same DHCR7 haplotype, whereas the W151X mutation occurred on different haplotypes. There is evidence for both recurrent mutations and founder effects. Together this suggests that the common SLOS mutations in Europe have different geographic and historic origins and spread across the continent in opposite directions.

SURF1 gene mutations in Polish patients with COX-deficient Leigh syndrome

One of the most frequent forms of Leigh syndrome (LS), a severe neurodegenerative, genetically heterogenous disease, is associated with cytochrome c oxidase (COX) deficiency. No mutations in any of the 13 polypeptide subunits of human COX have been detected in LS patients. Recently, SURF1, a positional candidate gene for LS has been identified on chromosome 9q34. We present the identification of SURF1 mutations in a randomly chosen group of Polish patients with a classical form of LS. Sequence analysis revealed the presence of a novel 704T-->C transition (Met235Thr), and two recurrent dinucleotide deletions (758delCA, 845delCT), as well as one novel polymorphic 573C-->G transversion (Thr191Thr). 845delCT was identified in 66% of all our patients in homozygous or heterozygous form. Our study confirms the recent observations that SURF1 is consistently involved in disorders of the mitochondrial respiratory chain in patients with typical Leigh syndrome.

Clinical ascertainment of Nijmegen breakage syndrome (NBS) and prevalence of the major mutation, 657del5, in three Slav populations

Nijmegen breakage syndrome (NBS) is a chromosomal instability disorder, clinically characterised by microcephaly, immunodeficiency, radiosensitivity and a very high predisposition to lymphoid malignancy. Recently, it was demonstrated that mutations in the NBS1 gene are responsible for NBS. Most of the NBS patients known so far are of Slav origin and carry a major founder mutation 657del5 in exon 6 of the NBS1 gene. In this study we estimated the prevalence of the 657del5 mutation in the Czech Republic, Poland and the Ukraine. We found an unexpectedly high carrier frequency of the 657del5 mutation (1/177) in the three Slav populations, a factor that may contribute to cancer frequency in those countries. In addition, we show that NBS patients are often diagnosed late and therefore receive inappropriate therapy.

De novo deletions of SNRPN exon 1 in early human and mouse embryos result in a paternal to maternal imprint switch

Prader-Willi syndrome (PWS) is a neurogenetic disease characterized by infantile hypotonia, gonadal hypoplasia, obsessive behaviour and neonatal feeding difficulties followed by hyperphagia, leading to profound obesity. PWS is due to a lack of paternal genetic information at 15q11-q13 (ref. 2). Five imprinted, paternally expressed genes map to the PWS region, MKRN3 (ref. 3), NDN (ref. 4), NDNL1 (ref. 5), SNRPN (refs 6-8 ) and IPW (ref. 9), as well as two poorly characterized framents designated PAR-1 and PAR-5 (ref. 10). Imprinting of this region involves a bipartite 'imprinting centre' (IC), which overlaps SNRPN (refs 10,11). Deletion of the SNRPN promoter/exon 1 region (the PWS IC element) appears to impair the establishment of the paternal imprint in the male germ line and leads to PWS. Here we report a PWS family in which the father is mosaic for an IC deletion on his paternal chromosome. The deletion chromosome has acquired a maternal methylation imprint in his somatic cells. We have made identical findings in chimaeric mice generated from two independent embryonic stem (ES) cell lines harbouring a similar deletion. Our studies demonstrate that the PWS IC element is not only required for the establishment of the paternal imprint, but also for its postzygotic maintenance.

Parental origin and mechanisms of formation of cytogenetically recognisable de novo direct and inverted duplications

Cytogenetic, FISH, and molecular results of 20 cases with de novo tandem duplications of 18 different autosomal chromosome segments are reported. There were 12 cases with direct duplications, three cases with inverted duplications, and five in whom determination of direction was not possible. In seven cases a rearrangement between non-sister chromatids (N-SCR) was found, whereas in the remaining 13 cases sister chromatids (SCR) were involved. Paternal and maternal origin (7:7) was found almost equally in cases with SCR (3:4) and N-SCR (4:3). In the cases with proven inversion, there was maternal and paternal origin in one case each. Twenty three out of 43 cytogenetically determined breakpoints correlated with common or rare fragile sites. In five cases, including all those with proven inverse orientation, all breakpoints corresponded to common or rare fragile sites. In at least two cases, one with an interstitial duplication (dup(19)(q11q13)) and one with a terminal duplication (dup(8) (p10p23)), concomitant deletions (del(8) (p23p23.3) and del(19)(q13q13)) were found.

Maternal uniparental disomy 7--review and further delineation of the phenotype

Uniparental disomy (UPD) is defined as the inheritance of both homologous chromosomes from only one parent. So far, maternal UPD 7 has been described in 28 cases. Here, we report 4 new cases, present clinical information of 5 cases previously reported by us, and review the clinical and molecular findings of all 32 cases. We found a phenotype characterized by pre- and postnatal growth retardation, occipitofrontal head circumference in the lower normal range, a triangular face, and retarded bone maturation. Findings of the facial gestalt included a high and broad forehead and a pointed chin. A broad mouth with down-turned corners, prominent ears, café-au-lait spots, hemihypotrophy, or clinodactyly were rarely present. Psychomotor development was delayed in 6 cases. The clinical findings strikingly resemble the phenotype of the heterogeneous Silver-Russell syndrome (SRS). Other anomalies were less frequently found than in SRS. Molecular investigations revealed 11 cases with isodisomy and 17 cases with heterodisomy. In 4 cases this information was not available. From the allelic distribution of the microsatellites investigated, 9 cases might be the consequence of an error at maternal meiosis I, and 6 cases might be due to non-disjunction at maternal meiosis II. Three of the 17 heterodisomic cases had trisomy 7 in chorionic villi, in the remaining cases no prenatal diagnosis through chorionic villus sampling was reported.

CONCLUSION

Maternal UPD 7 should investigated in children with pre- and postnatal growth retardation anda facial gestalt characterized by a high and broad forehead and a pointed chin, as well as in cofined placental mosaicism for trisomy 7.

Cranial MRI in the Nijmegen breakage syndrome

We present the results paragraph signof MRI examinations in ten patients with documented Nijmegen paragraph signbreakage syndrome (NBS), aged 1.75-19 years. T1-, Proton-Density- and T2-weighted spin-echo sequences were performed in three planes. All patients showed microcephaly with decreased size of the frontal lobes and narrow frontal horns. In four patients agenesis of the posterior part of the corpus callosum was found, with colpocephaly and temporal horns dilatation. In one patient callosal hypoplasia was accompanied by abnormal cerebrospinal fluid spaces and wide cerebral cortex, suspicious of pachygyria. Sinusitis was present in all ten patients, as a result of primary immunodeficiency. As in ataxia teleangiectasia and other breakage syndromes, patients with NBS show an inherited susceptibility to malignancy and hypersensitivity to X- and gamma-radiation. CT is therefore contraindicated in these patients and MRI should be the method of choice for diagnostic imaging.

X-linked hypophosphatemia in Polish patients. 1. Mutations in the PHEX gene

We present twenty-nine PHEX gene mutations extending our previous work, giving it to a total of 37 different mutations identified in Polish patients with familial or sporadic X-linked hypophosphatemia. Deletions, insertions and nucleotide substitutions leading to frameshift (27%), stop codon (29%), splice site (24%), and missense mutations (20%) were found. The mutations are distributed along the gene; exons 3, 4, 11, 12, 14, 15, 17, 20 and 22 are regions with the most frequent mutation events. Four mutations, P534L, G579R, R549X and IVS15+1nt, recurred in three, four, two and three unrelated patients, respectively. They have also been detected in affected persons from other countries. Twenty-eight mutations are specific for Polish population and almost all of them are unique. Most of the identified mutations are expected to result in major changes in protein structure and/or function.

Imprinting-mutation mechanisms in Prader-Willi syndrome

Microdeletions of a region termed the "imprinting center" (IC) in chromosome 15q11-q13 have been identified in several families with Prader-Willi syndrome (PWS) or Angelman syndrome who show epigenetic inheritance for this region that is consistent with a mutation in the imprinting process. The IC controls resetting of parental imprints in 15q11-q13 during gametogenesis. We have identified a larger series of cases of familial PWS, including one case with a deletion of only 7.5 kb, that narrows the PWS critical region to <4. 3 kb spanning the SNRPN gene CpG island and exon 1. Identification of a strong DNase I hypersensitive site, specific for the paternal allele, and six evolutionarily conserved (human-mouse) sequences that are potential transcription-factor binding sites is consistent with this region defining the SNRPN gene promoter. These findings suggest that promoter elements at SNRPN play a key role in the initiation of imprint switching during spermatogenesis. We also identified three patients with sporadic PWS who have an imprinting mutation (IM) and no detectable mutation in the IC. An inherited 15q11-q13 mutation or a trans-factor gene mutation are unlikely; thus, the disease in these patients may arise from a developmental or stochastic failure to switch the maternal-to-paternal imprint during parental spermatogenesis. These studies allow a better understanding of a novel mechanism of human disease, since the epigenetic effect of an IM in the parental germ line determines the phenotypic effect in the patient.

Further delineation of the classical Smith-Lemli-Opitz syndrome phenotype at different patient ages: clinical and biochemical studies

We present here extensive clinical and biochemical data on thirteen SLOS (type I) patients with proven defect in cholesterol biosynthesis for further delineation of the classical SLOS phenotype at different patient ages.

High level of unequal meiotic crossovers at the origin of the 22q11. 2 and 7q11.23 deletions

Interstitial chromosomal deletions at 22q11.2 and 7q11.23 are detected in the vast majority of patients affected by CATCH 22 syndromes and the Williams-Beuren syndrome, respectively. In a group of 15 Williams-Beuren patients, we have shown previously that a large number of 7q11.23 deletions occur in association with an interchromosomal rearrangement, indicative of an unequal crossing-over event between the two homologous chromosomes 7. In this study, we show that a similar mechanism also underlies the formation of the 22q11.2 deletions associated with CATCH 22. In eight out of 10 families with a proband affected by CATCH 22, we were able to show that a meiotic recombination had occurred at the critical deleted region based on segregation analysis of grandparental haplotypes. The incidences of crossovers observed between the closest informative markers, proximal and distal to the deletion, were compared with the expected recombination frequencies between the markers. A significant number of recombination events occur at the breakpoint of deletions in CATCH 22 patients (P = 2.99x10(-7)). The segregation analysis of haplotypes in three-generation families was also performed on an extended number of Williams-Beuren cases (22 cases in all). The statistically significant occurrence of meiotic crossovers (P = 4.45x10(-9)) further supports the previous findings. Thus, unequal meiotic crossover events appear to play a relevant role in the formation of the two interstitial deletions. The recurrence risk for healthy parents in cases where such meiotic recombinations can be demonstrated is probably negligible. Such a finding is in agreement with the predominantly sporadic occurrence of the 22q11.2 and 7q11. 23 deletions. No parent-of-origin bias was observed in the two groups of patients with regard to the origin of the deletion and to the occurrence of inter- versus intrachromosomal rearrangements.