Smith-Magenis syndrome: clinical evaluation in seven Brazilian patients

Smith-Magenis syndrome (SMS) is a complex congenital anomaly characterized by craniofacial anomalies, neurological and behavioral disorders. SMS is caused by a deletion in region 17p11.2, which includes the RAI1 gene (90% of cases), or by point mutation in the RAI1 gene (10% of cases). Laboratory diagnosis is through cytogenetic analysis by GTG banding and molecular cytogenetic analysis by FISH. We carried out an active search for patients in Associations of Parents and Friends of Exceptional Children (APAE) of S?o Paulo and genetic centers in Brazil. Forty-eight patients were screened for mental retardation, craniofacial abnormalities and stereotyped behavior with a diagnosis of SMS. In seven of them, chromosome banding at high resolution demonstrated chromosome 17p11.2 deletions, confirmed by FISH. We also made a meta-analysis of 165 cases reported between 1982 and 2010 to compare with the clinical data of our sample. We demonstrated differences between the frequencies of clinical signs among the cases reported and seven Brazilian cases of this study, such as dental anomalies, strabismus, ear infections, deep hoarse voice, hearing loss, and cardiac defects. Although the gold standard for diagnosis of SMS is FISH, we found that the GTG banding technique developed to evaluate chromosome 17 can be used for the SMS diagnosis in areas where the FISH technique is not available.     

Subtelomeric rearrangements in Polish subjects with intellectual disability and dysmorphic features

Fluorescent in situ hybridization (FISH) was performed in 76 patients referred to our department because of intellectual disability and dysmorphic features that can be related to subtelomeric microaberrations. In all the patients, conventional cytogenetic methods revealed normal karyotype. Four (5.3%) subtelomeric rearrangements were detected by FISH: 2 subtelomeric 1p36 deletions, an unbalanced translocation involving chromosomes 1 and 12 with 1p36 deletion, and a de novo balanced translocation involving chromosomes 19 and 22. Thus, 3 cases of 1p36 subtelomeric deletion were found (3.95%). To confirm subtelomeric rearrangements in 2 patients, comparative genomic hybridization (CGH) was applied. Moreover, 3 cases of polymorphism without phenotypic effects were found: in 2 patients, the polymorphism involved the long arm of chromosome 2 (maternal derivative in both patients), while in the third patient, a polymorphism of the long arm of chromosome 7 was diagnosed. The latter polymorphism was also found in the patient’s mother and grandfather.     

DNA rearrangements on both homologues of chromosome 17 in a mildly delayed individual with a family history of autosomal dominant carpal tunnel syndrome.

Disorders known to be caused by molecular and cytogenetic abnormalities of the proximal short arm of chromosome 17 include Charcot-Marie-Tooth disease type 1A (CMT1A), hereditary neuropathy with liability to pressure palsies (HNPP), Smith-Magenis syndrome (SMS), and mental retardation and congenital anomalies associated with partial duplication of 17p. We identified a patient with multifocal mononeuropathies and mild distal neuropathy, growth hormone deficiency, and mild mental retardation who was found to have a duplication of the SMS region of 17p11.2 and a deletion of the peripheral myelin protein 22 (PMP22) gene within 17p12 on the homologous chromosome. Further molecular analyses reveal that the dup(17)(p11.2p11.2) is a de novo event but that the PMP22 deletion is familial. The family members with deletions of PMP22 have abnormalities indicative of carpal tunnel syndrome, documented by electrophysiological studies prior to molecular analysis. The chromosomal duplication was shown by interphase FISH analysis to be a tandem duplication. These data indicate that familial entrapment neuropathies, such as carpal tunnel syndrome and focal ulnar neuropathy syndrome, can occur because of deletions of the PMP22 gene. The co-occurrence of the 17p11.2 duplication and the PMP22 deletion in this patient likely reflects the relatively high frequency at which these abnormalities arise and the underlying molecular characteristics of the genome in this region.     

Two cases of terminal deletion of chromosome 13: clinical features, conventional and molecular cytogenetic analysis

We report the cases of two unrelated patients with psychomotor retardation and craniofacial abnormalities, in whom cytogenetic studies have revealed a terminal deletion of chromosome 13 confirmed by fluorescence in situ hybridization (FISH). This del(13)(q33.2) is the smallest terminal deletion of the 13q reported so far. Interestingly enough, the serum level of coagulation factors VII and X, whose genes are located in 13q34, were reduced in both patients. These cases illustrate the difficulties in identifying precisely chromosome deletions and demonstrate that FISH techniques allow to obtain a more precise correlation between clinical phenotype and cytogenetic abnormalities.     

Evidence for a distinct region causing a cat-like cry in patients with 5p deletions.

The cri-du-chat syndrome is a contiguous gene syndrome that results from a deletion of the short arm of chromosome 5 (5p). Patients present with a cat-like cry at birth, which is usually considered diagnostic of this syndrome. Additional features of the syndrome include failure to thrive, microcephaly, hypertelorism, epicanthal folds, hypotonia, and severe mental retardation. We report on four families in which patients with 5p deletions have only the characteristic cat-like cry, with normal to mildly delayed development. The precise locations of the deletions in each family were determined by FISH using lambda phage and cosmid clones. All of the deletion breakpoints map distal to a chromosomal region that is implicated with the facial features and severe mental and developmental delay in the cri-du-chat syndrome. DNA clones mapping in the chromosomal region associated with the cat-like cry feature will be useful diagnostic tools. They will allow for the distinction between 5p deletions that will result in the severe delay observed in most cri-du-chat syndrome patients and those deletions that result in the isolated cat-like cry feature, which is associated with a better prognosis.     

Aniridia as part of a WAGR syndrome in a girl whose brother presented hypospadias

Aniridia can arise as part of the WAGR syndrome (Wilms tumour. aniridia, genitourinary anomalies, and mental retardation), due to a deletion or chromosomal region 11p13. We report a girl with a complete WAGR syndrome, whose brother presented hypospadias. Cytogenetic, FISH and molecular studies showed a deletion in one chromosome 11 of the patient. No cytogenetic rearrangement or deletion affecting the genes included in this region (PAX6 and WT1) were observed in her brother and parents. This excludes a higher risk than that of the general population for developing Wilms tumour in the brother and supports that the presence of WAGR syndrome in the patient and hypospadias in her brother is a chance association. We conclude that the identification and definition of the deletions in the WAGR region, which include the WT1 locus are important in order to identify a high tumour risk in infant patients with aniridia including those without other WAGR anomalies.     

Delineation of two distinct 6p deletion syndromes

Deletions of the short arm of chromosome 6 are relatively rare, the main features being developmental delay, craniofacial malformations, hypotonia, and defects of the heart and kidney, with hydrocephalus and eye abnormalities occurring in some instances. We present the molecular cytogenetic investigation of six cases with 6p deletions and two cases with unbalanced translocations resulting in monosomy of the distal part of 6p. The breakpoints of the deletions have been determined accurately by using 55 well-mapped probes and fluorescence in situ hybridization (FISH). The cases can be grouped into two distinct categories: interstitial deletions within the 6p22–p24 segment and terminal deletions within the 6p24–pter segment. Characteristics correlating with specific regions are: short neck, clinodactyly or syndactyly, brain, heart and kidney defects with deletions within 6p23–p24; and corneal opacities/iris coloboma/Rieger anomaly, hypertelorism and deafness with deletions of 6p25. The two cases with unbalanced translocations presented with a Larsen-like syndrome including some characteristics of the 6p deletion syndrome, which can be explained by the deletion of 6p25. Such investigation of cytogenetic abnormalities of 6p using FISH techniques and a defined set of probes will allow a direct comparison of reported cases and enable more accurate diagnosis as well as prognosis in patients with 6p deletions. Received: 29 July 1998 / Accepted: 28 October 1998     

Strong correlation of elastin deletions,detected by FISH,with Williams syndrome: evaluation of 235 patients.

Williams syndrome (WS) is generally characterized by mental deficiency, gregarious personality, dysmorphic facies, supravalvular aortic stenosis, and idiopathic infantile hypercalcemia. Patients with WS show allelic loss of elastin (ELN), exhibiting a submicroscopic deletion, at 7q11.23, detectable by FISH. Hemizygosity is likely the cause of vascular abnormalities in WS patients. A series of 235 patients was studied, and molecular cytogenetic deletions were seen in 96% of patients with classic WS. Patients included 195 solicited through the Williams Syndrome Association (WSA), plus 40 clinical cytogenetics cases referred by primary-care physicians. Photographs and medical records of most WSA subjects were reviewed, and patients were identified as "classic" (n = 114) or "uncertain" (n = 39). An additional 42 WSA patients were evaluated without clinical information. FISH was performed with biotinylated ELN cosmids on metaphase cells from immortalized lymphoblastoid lines from WSA patients and after high-resolution banding analysis on clinical referral patients. An alpha-satellite probe for chromosome 7 was included in hybridizations, as an internal control. Ninety-six percent of the patients with classic WS showed a deletion in one ELN allele; four of these did not show a deletion. Of the uncertain WS patients, only 3 of 39 showed a deletion. Of the 42 who were not classified phenotypically, because of lack of clinical information, 25 patients (60%) showed a deletion. Thirty-eight percent (15/40) of clinical cytogenetics cases showed an ELN deletion and no cytogenetic deletion by banded analysis. These results support the usefulness of FISH for the detection of elastin deletions as an initial diagnostic assay for WS.     

Genomic findings in patients with clinical suspicion of 22q11.2 deletion syndrome

Chromosome 22q11.2 deletion syndrome, one of the most common human genomic syndromes, has highly heterogeneous clinical presentation. Patients usually harbor a 1.5 to 3 Mb hemizygous deletion at chromosome 22q11.2, resulting in pathognomic TBX1, CRKL and/or MAPK1 haploinsufficiency. However, there are some individuals with clinical features resembling the syndrome who are eventually diagnosed with genomic disorders affecting other chromosomal regions. The objective of this study was to evaluate the additive value of high-resolution array-CGH testing in the cohort of 41 patients with clinical features of 22q11.2 deletion syndrome and negative results of standard cytogenetic diagnostic testing (karyotype and FISH for 22q11.2 locus). Array-CGH analysis revealed no aberrations at chromosomes 22 or 10 allegedly related to the syndrome. Five (12.2 %) patients were found to have other genomic imbalances, namely 17q21.31 microdeletion syndrome (MIM#610443), 1p36 deletion syndrome (MIM#607872), NF1 microduplication syndrome (MIM#613675), chromosome 6pter-p24 deletion syndrome (MIM#612582) and a novel interstitial deletion at 3q26.31 of 0.65 Mb encompassing a dosage-dependent gene NAALADL2. Our study demonstrates that the implementation of array-CGH into the panel of classic diagnostic procedures adds significantly to their efficacy. It allows for detection of constitutional genomic imbalances in 12 % of subjects with negative result of karyotype and FISH targeted for 22q11.2 region. Moreover, if used as first-tier genetic test, the method would provide immediate diagnosis in ～40 % phenotypic 22q11.2 deletion subjects.     

Genetic features of B-cell chronic lymphocytic leukemia

The genetic features of B-cell chronic lymphocytic leukemia (CLL) are currently being reassessed by molecular cytogenetic techniques such as fluorescence in situ hybridization (FISH). Conventional cytogenetic studies by chromosome banding are difficult in CLL mainly because of the low in vitro mitotic activity of the tumor cells, which leads to poor quantity and quality of metaphase spreads. Molecular genetic analyses are limited because candidate genes are known for only a few chromosomal aberrations that are observed in CLL. FISH was found to be a powerful tool for the genetic analysis of CLL as it overcomes both the low mitotic activity of the CLL cells and the lack of suitable candidate genes for analysis. Using FISH, the detection of chromosomal aberrations can be performed at the single cell level in both dividing and non-dividing cells, thus circumventing the need of metaphase preparations from tumor cells. Probes for the detection of trisomies, deletions and translocation breakpoints can be applied to the regions of interest with the growing number of clones available from genome-wide libraries. Using the interphase cytogenetic FISH approach with a disease specific set of probes, chromosome aberrations can be found in more than 80% of CLL cases. The most frequently observed abnormalities are losses of chromosomal material, with deletions in band 13q14 being the most common, followed by deletions in 11q22-q23, deletions in 17p13 and deletions in 6q21. The most common gains of chromosomal material are trisomies 12q, 8q and 3q. Translocation breakpoints, in particular involving the immunoglobulin heavy chain locus at 14q32, which are frequently observed in other types of non-Hodgkin's lymphoma, are rare events in CLL. Genes affected by common chromosome aberrations in CLL appear to be p53 in cases with 17p deletion and ataxia telangiectasia mutated (ATM), which is mutated in a subset of cases with 11q22-q23 aberrations. However, for the other frequently affected genomic regions, the search for candidate genes is ongoing. In parallel, the accurate evaluation of the incidence of chromosome aberrations in CLL by FISH allows the correlation of genetic abnormalities with clinical disease manifestations and outcome. In particular, 17p abnormalities and deletions in 11q22-q23 have already been shown to be among the most important independent prognostic factors identifying subgroups of patients with rapid disease progression and short survival. In addition, deletion 17p has been associated with resistance to treatment with purine analogs. Therefore, genetic abnormalities may allow a risk assessment for individual patients at the time of diagnosis, thus giving the opportunity for a risk-adapted management.     

Frequent chromosome aberrations revealed by molecular cytogenetic studies in patients with aniridia

Seventy-seven patients with aniridia, referred for cytogenetic analysis predominantly to assess Wilms tumor risk, were studied by fluorescence in situ hybridization (FISH), through use of a panel of cosmids encompassing the aniridia-associated PAX6 gene, the Wilms tumor predisposition gene WT1, and flanking markers, in distal chromosome 11p13. Thirty patients were found to be chromosomally abnormal. Cytogenetically visible interstitial deletions involving 11p13 were found in 13 patients, 11 of which included WT1. A further 13 patients had cryptic deletions detectable only by FISH, 3 of which included WT1. Six of these, with deletions <500 kb, share a similar proximal breakpoint within a cosmid containing the last 10 exons of PAX6 and part of the neighboring gene, ELP4. Two of these six patients were mosaic for the deletion. The remaining four had chromosomal rearrangements: an unbalanced translocation, t(11;13), with a deletion including the WAGR (Wilms' tumor, aniridia, genitourinary abnormalities, and mental retardation) region, and three balanced rearrangements with what appear to be position effect breakpoints 3' of PAX6: (a) a t(7;11) with the 11p13 breakpoint approximately 30 kb downstream of PAX6, (b) a dir ins(12;11) with a breakpoint >50 kb from PAX6, and (c) an inv(11)(p13q13) with a breakpoint >75 kb downstream of PAX6. The proportion and spectrum of chromosome anomalies in familial (4/14, or 28.5%) and sporadic (26/63, or 41%) cases are not significantly different. An unexpectedly high frequency of chromosomal rearrangements is associated with both sporadic and familial aniridia in this cohort.     

Physical mapping of the holoprosencephaly critical region in 18p11.3.

Holoprosencephaly (HPE) is a common developmental defect that results in a spectrum of craniofacial malformations. HPE is genetically heterogeneous, some cases being associated with deletions of the short arm of chromosome 18. In order to map the putative HPE gene located on 18p (HPE4) more precisely, six patients with various cytogenetic 18p deletions and clinical features of HPE have been characterized by using a combination of somatic cell hybrid analysis and FISH. By using a set of 27 chromosome 18p-specific markers, the deletion in each patient was characterized. The HPE minimal critical region on 18p was defined on a molecular level, localizing the HPE4 gene to 18p11.3.     

Molecular cytogenetic aberrations in Tunisian patients with multiple myeloma identified by cIg-FISH in fixed bone marrow cells

Cytogenetic studies in multiple myeloma (MM) are hampered by the hypo-proliferative nature of plasma cells. In order to circumvent this problem, we have used a combination of immunolabeling of cytoplasmic Ig light chains (λ or κ) and FISH (cIg-FISH), which allowed a comprehensive detection of the most common and/or recurrent molecular cytogenetic aberrations on fixed bone marrow cells of 70 Tunisian patients. Translocations involving the chromosome 14q32 region were observed in 32 cases (45.7%), including 18 cases with a t(11;14), 8 cases with a t(4;14), and 2 cases with a t(14;16). Deletions of the 13q14 region (D13S319/RB1) were detected in 18.6%, and deletions of the 17p13 region (TP53) in 5.7% of the cases, respectively. Of all patients with a D13S319/RB1 deletion, 61.5% also carried a 14q32 translocation, whereas TP53 deletions were associated with a t(11;14) in 2 cases (50%) and a D13S319 deletion in 1 case (25%). Our results suggest that there is a correlation between the presence of 14q32 translocations and chromosome 13q14 deletions in MM patients and that cIg-FISH is more sensitive as compared to conventional karyotyping in detecting molecular cytogenetic abnormalities in this disease.     

Molecular analyses of 17p11.2 deletions in 62 Smith-Magenis syndrome patients.

Smith-Magenis syndrome (SMS) is a clinically recognizable, multiple congenital anomalies/mental retardation syndrome caused by an interstitial deletion involving band p11.2 of chromosome 17. Toward the molecular definition of the interval defining this microdeletion syndrome, 62 unrelated SMS patients in conjunction with 70 available unaffected parents were molecularly analyzed with respect to the presence or absence of 14 loci in the proximal region of the short arm of chromosome 17. A multifaceted approach was used to determine deletion status at the various loci that combined (i) FISH analysis, (ii)PCR and Southern analysis of somatic cell hybrids retaining the deleted chromosome 17 from selected patients, and (iii) genotype determination of patients for whom a parent(s) was available at four microsatellite marker loci and at four loci with associated RFLPs. The relative order of two novel anonymous markers and a new microsatellite marker was determined in 17p11.2. The results confirmed that the proximal deletion breakpoint in the majority of SMS patients is located between markers D17S58 (EW301) and D17S446 (FG1) within the 17p11.1-17p11.2 region. The common distal breakpoint was mapped between markers cCI17-638, which lies distal to D17S71, and cCI17-498, which lies proximal to the Charcot Marie-Tooth disease type 1A locus. The locus D17S258 was found to be deleted in all 62 patients, and probes from this region can be used for diagnosis of the SMS deletion by FISH. Ten patients demonstrated molecularly distinct deletions; of these, two patients had smaller deletions and will enable the definition of the critical interval for SMS.     

Physical map of 1p36, placement of breakpoints in monosomy 1p36, and clinical characterization of the syndrome

Monosomy 1p36 is the most common terminal deletion syndrome. This contiguous gene deletion syndrome is presumably caused by haploinsufficiency of a number of genes. We have constructed a contig of overlapping large-insert clones for the most distal 10.5 Mb of 1p36, evaluated the deletion sizes in 61 subjects with monosomy 1p36 from 60 families, and created a natural deletion panel. We found pure terminal deletions, interstitial deletions, derivative chromosomes, and more complex rearrangements. Breakpoints were "binned" into 0.5-Mb regions. Analyses revealed some clustering of breakpoints but no single common breakpoint. Determination of the parental origin showed that 60% of de novo 1p36 terminal deletions arose from the maternally inherited chromosome. Of the 61 subjects, 30 were examined systematically through a protocol at the Texas Children's Hospital General Clinical Research Center. Specifically, we report hearing evaluations, palatal and ophthalmological examinations, echocardiograms, neurological assessments, and thyroid function tests. To our knowledge, this systematic molecular and clinical characterization of monosomy 1p36 is the largest and most comprehensive study of this deletion syndrome to date. Many cytogenetically visible, apparent terminal deletions are more complex than anticipated by cytogenetics, as revealed at the molecular level by our study. Our clinical findings allow for the more accurate recognition of the syndrome and for proper medical evaluation.     

Familial mental retardation syndrome ATR-16 due to an inherited cryptic subtelomeric translocation, t(3;16)(q29;p13.3)

In the search for genetic causes of mental retardation, we have studied a five-generation family that includes 10 individuals in generations IV and V who are affected with mild-to-moderate mental retardation and mild, nonspecific dysmorphic features. The disease is inherited in a seemingly autosomal dominant fashion with reduced penetrance. The pedigree is unusual because of (1) its size and (2) the fact that individuals with the disease appear only in the last two generations, which is suggestive of anticipation. Standard clinical and laboratory screening protocols and extended cytogenetic analysis, including the use of high-resolution karyotyping and multiplex FISH (M-FISH), could not reveal the cause of the mental retardation. Therefore, a whole-genome scan was performed, by linkage analysis, with microsatellite markers. The phenotype was linked to chromosome 16p13.3, and, unexpectedly, a deletion of a part of 16pter was demonstrated in patients, similar to the deletion observed in patients with ATR-16 syndrome. Subsequent FISH analysis demonstrated that patients inherited a duplication of terminal 3q in addition to the deletion of 16p. FISH analysis of obligate carriers revealed that a balanced translocation between the terminal parts of 16p and 3q segregated in this family. This case reinforces the role of cryptic (cytogenetically invisible) subtelomeric translocations in mental retardation, which is estimated by others to be implicated in 5%-10% of cases.     

Pericentric inversion with partial 7(q35-->qter) duplication and 7pter deletion: diagnosis by cytogenetic and fish analysis in a 29-year-old male patient

We report on a 29-year-old male patient with an inverted 7(q35-qter) duplication diagnosed by combining cytogenetic and FISH studies. Traditional G-banding detected an abnormally long chromosome 7 which was further demonstrated to be entirely of chromosome 7 origin by using fluorescent whole chromosome 7 painting. The presence within the additional segment of a signal for 7q36 region (Williams control probe) and the absence of signals for 7q33 (Y938G5 probe) and 7q34 (Y815G5 probe) regions indicated that the breakpoint for this rearrangement was distal to 7q34 and proximal to 7q36. A distal 7p22 deletion was confirmed by the absence of signal for the 7p subtelomeric probe. Apart from kyphosis, developmental/mental retardation and abnormal ears, the clinical features of the present patient, who is the oldest individual ever reported with this duplication/deletion, were not typical for partial 7q trisomy syndrome. A review of the cases reported with 7(q35-qter) duplication is made and shows important clinical variability but constantly normal pre- and postnatal growth, a feature which can therefore be confirmed as distinctive of distal 7q trisomy syndrome.     

Molecular definition of the smallest region of deletion overlap in the Wolf-Hirschhorn syndrome.

Wolf-Hirschhorn syndrome (WHS), associated with a deletion of chromosome 4p, is characterized by mental and growth retardation and typical facial dysmorphism. A girl with clinical features of WHS was found to carry a subtle deletion of chromosome 4p. Initially suggested by high-resolution chromosome analysis, her deletion was confirmed by fluorescence in situ hybridization (FISH) with cosmid probes, E13 and Y2, of D4S113. To delineate this 4p deletion, we performed a series of FISH and pulsed-field gel electrophoresis analyses by using probes from 4p16.3. A deletion of approximately 2.5 Mb with the breakpoint at approximately 80 kb distal to D4S43 was defined in this patient and appears to be the smallest WHS deletion so far identified. To further refine the WHS critical region, we have studied three unrelated patients with presumptive 4p deletions, two resulting from unbalanced segregations of parental chromosomal translocations and one resulting from an apparently de novo unbalanced translocation. Larger deletions were identified in two patients with WHS. One patient who did not clinically present with WHS had a smaller deletion that thus eliminates the distal 100-300 kb from the telomere as being part of the WHS region. This study has localized the WHS region to approximately 2 Mb between D4S43 and D4S142.     

Complex chromosome 17p rearrangements associated with low-copy repeats in two patients with congenital anomalies

Recent molecular cytogenetic data have shown that the constitution of complex chromosome rearrangements (CCRs) may be more complicated than previously thought. The complicated nature of these rearrangements challenges the accurate delineation of the chromosomal breakpoints and mechanisms involved. Here, we report a molecular cytogenetic analysis of two patients with congenital anomalies and unbalanced de novo CCRs involving chromosome 17p using high-resolution array-based comparative genomic hybridization (array CGH) and fluorescent in situ hybridization (FISH). In the first patient, a 4-month-old boy with developmental delay, hypotonia, growth retardation, coronal synostosis, mild hypertelorism, and bilateral club feet, we found a duplication of the Charcot-Marie–Tooth disease type 1A and Smith-Magenis syndrome (SMS) chromosome regions, inverted insertion of the Miller-Dieker lissencephaly syndrome region into the SMS region, and two microdeletions including a terminal deletion of 17p. The latter, together with a duplication of 21q22.3-qter detected by array CGH, are likely the unbalanced product of a translocation t(17;21)(p13.3;q22.3). In the second patient, an 8-year-old girl with mental retardation, short stature, microcephaly and mild dysmorphic features, we identified four submicroscopic interspersed 17p duplications. All 17 breakpoints were examined in detail by FISH analysis. We found that four of the breakpoints mapped within known low-copy repeats (LCRs), including LCR17pA, middle SMS-REP/LCR17pB block, and LCR17pC. Our findings suggest that the LCR burden in proximal 17p may have stimulated the formation of these CCRs and, thus, that genome architectural features such as LCRs may have been instrumental in the generation of these CCRs.     

Prader-Willi syndrome with an unusually large 15q deletion due to an unbalanced translocation t(4;15)

Prader-Willi syndrome (PWS) is a neurobehavioral disorder caused by deletions in the 15q11-q13 region, by maternal uniparental disomy of chromosome 15 or by imprinting defects. Structural rearrangements of chromosome 15 have been described in about 5% of the patients with typical or atypical PWS phenotype. An 8-year-old boy with a clinical diagnosis of PWS, severe neurodevelopmental delay, absence of speech and mental retardation was studied by cytogenetic and molecular techniques, and an unbalanced de novo karyotype 45,XY,der(4)t(4;15)(q35;q14),-15 was detected after GTG-banding. The patient was diagnosed by SNURF-SNRPN exon 1 methylation assay, and the extent of the deletions on chromosomes 4 and 15 was investigated by microsatellite analysis of markers located in 4qter and 15q13-q14 regions. The deletion of chromosome 4q was distal to D4S1652, and that of chromosome 15 was located between D15S1043 and D15S1010. Our patient's severely affected phenotype could be due to the extent of the deletion, larger than usually seen in PWS patients, although the unbalance of the derivative chromosome 4 cannot be ruled out as another possible cause. The breakpoint was located in the subtelomeric region, very close to the telomere, a region that has been described as having the lowest gene concentrations in the human genome.