Cytogenetic, biochemical, and molecular analyses of a 22q13 deletion

Pharmacological modulation of developmental and synaptic phenotypes in human SHANK3 deficient stem cell-derived neuronal models

Phelan-McDermid syndrome (PMDS) arises from mutations in the terminal region of chromosome 22q13, impacting the SHANK3 gene. The resulting deficiency of the postsynaptic density scaffolding protein SHANK3 is associated with autism spectrum disorder (ASD). We examined 12 different PMDS patient and CRISPR-engineered stem cell-derived neuronal models and controls and found that reduced expression of SHANK3 leads to neuronal hyperdifferentiation, increased synapse formation, and decreased neuronal activity. We performed automated imaging-based screening of 7,120 target-annotated small molecules and identified three compounds that rescued SHANK3-dependent neuronal hyperdifferentiation. One compound, Benproperine, rescued the decreased colocalization of Actin Related Protein 2/3 Complex Subunit 2 (ARPC2) with ß-actin and rescued increased synapse formation in SHANK3 deficient neurons when administered early during differentiation. Neuronal activity was only mildly affected, highlighting Benproperine's effects as a neurodevelopmental modulator. This study demonstrates that small molecular compounds that reverse developmental phenotypes can be identified in human neuronal PMDS models.

Definition and clinical variability of SHANK3-related Phelan-McDermid syndrome

Phelan-McDermid syndrome (PMS) is an infrequently described syndrome that presents with a disturbed development, neurological and psychiatric characteristics, and sometimes other comorbidities. As part of the development of European medical guidelines we studied the definition, phenotype, genotype-phenotype characteristics, and natural history of the syndrome. The number of confirmed diagnoses of PMS in different European countries was also assessed and it could be concluded that PMS is underdiagnosed. The incidence of PMS in European countries is estimated to be at least 1 in 30,000. Next generation sequencing, including analysis of copy number variations, as first tier in diagnostics of individuals with intellectual disability will likely yield a larger number of individuals with PMS than presently known. A definition of PMS by its phenotype is at the present not possible, and therefore PMS-SHANK3 related is defined by the presence of SHANK3 haploinsufficiency, either by a deletion involving region 22q13.2-33 or a pathogenic/likely pathogenic variant in SHANK3. In summarizing the phenotype, we subdivided it into that of individuals with a 22q13 deletion and that of those with a pathogenic/likely pathogenic SHANK3 variant. The phenotype of individuals with PMS is variable, depending in part on the deletion size or, whether only a variant of SHANK3 is present. The core phenotype in the domains development, neurology, and senses are similar in those with deletions and SHANK3 variants, but individuals with a SHANK3 variant more often are reported to have behavioural disorders and less often urogenital malformations and lymphedema. The behavioural disorders may, however, be a less outstanding feature in individuals with deletions accompanied by more severe intellectual disability. Data available on the natural history are limited. Results of clinical trials using insulin-like growth factor I (IGF-1), intranasal insulin, and oxytocin are available, other trials are in progress. The present guidelines for PMS aim at offering tools to caregivers and families to provide optimal care to individuals with PMS.

Phelan-McDermid syndrome: a classification system after 30 years of experience

Phelan-McDermid syndrome (PMS) was initially called the 22q13 deletion syndrome based on its etiology as a deletion of the distal long arm of chromosome 22. These included terminal and interstitial deletions, as well as other structural rearrangements. Later, pathogenetic variants and deletions of the SHANK3 gene were found to result in a phenotype consistent with PMS. The association between SHANK3 and PMS led investigators to consider disruption/deletion of SHANK3 to be a prerequisite for diagnosing PMS. This narrow definition of PMS based on the involvement of SHANK3 has the adverse effect of causing patients with interstitial deletions of chromosome 22 to “lose” their diagnosis. It also results in underreporting of individuals with interstitial deletions of 22q13 that preserve SHANK3. To reduce the confusion for families, clinicians, researchers, and pharma, a simple classification for PMS has been devised. PMS and will be further classified as PMS-SHANK3 related or PMS-SHANK3 unrelated. PMS can still be used as a general term, but this classification system is inclusive. It allows researchers, regulatory agencies, and other stakeholders to define SHANK3 alterations or interstitial deletions not affecting the SHANK3 coding region.

Characterisation of the clinical phenotype in Phelan-McDermid syndrome

BACKGROUND

Phelan-McDermid syndrome (PMS) is a rare genetic disorder compromising the 22q13 terminal region and affecting SHANK3, a gene crucial to the neurobehavioural phenotype and strongly linked to autism (ASD) and intellectual disability (ID). The condition is characterised by global developmental delay, ID, speech impairments, hypotonia and autistic behaviours, although its presentation and symptom severity vary widely. In this study, we provide a thorough description of the behavioural profile in PMS and explore differences related to deletion size and language ability.

METHODS

We used standard clinical assessment instruments to measure altered behaviour, adaptive skills and autistic symptomatology in sixty participants with PMS (30 females, median age 8.5 years, SD=7.1). We recorded background information and other clinical manifestations and explored associations with deletion size. We performed descriptive and inferential analyses for group comparison.

RESULTS

We found delayed gross and fine motor development, delayed and impaired language (~70% of participants non or minimally verbal), ID of different degrees and adaptive functioning ranging from severe to borderline impairment. Approximately 40% of participants experienced developmental regression, and half of those regained skills. Autistic symptoms were frequent and variable in severity, with a median ADOS-2 CSS score of 6 for every domain. Sensory processing anomalies, hyperactivity, attentional problems and medical comorbidities were commonplace. The degree of language and motor development appeared to be associated with deletion size.

CONCLUSIONS

This study adds to previous research on the clinical descriptions of PMS and supports results suggesting wide variability of symptom severity and its association with deletion size. It makes the case for suitable psychotherapeutic and pharmacological approaches, for longitudinal studies to strengthen our understanding of possible clinical courses and for more precise genomic analysis.

Looping Genomes: Diagnostic Change and the Genetic Makeup of the Autism Population

This article builds on Hacking's framework of "dynamic nominalism" to show how knowledge about biological etiology can interact with the "kinds of people" delineated by diagnostic categories in ways that "loop" or modify both over time. The authors use historical materials to show how "geneticization" played a crucial role in binding together autism as a biosocial community and how evidence from genetics research later made an important contribution to the diagnostic expansion of autism. In the second part of the article, the authors draw on quantitative and qualitative analyses of autism rates over time in several rare conditions that are delineated strictly according to genomic mutations in order to demonstrate that these changes in diagnostic practice helped to both increase autism's prevalence and create its enormous genetic heterogeneity. Thus, a looping process that began with geneticization and involved the social effects of genetics research itself transformed the autism population and its genetic makeup.

Molecular cytogenetic analysis of telomere rearrangements

Genomic imbalances involving the telomeric regions of human chromosomes, which contain the highest gene concentration in the genome, are proposed to have severe phenotypic consequences. For this reason, it is important to identify telomere rearrangements and assess their contribution to human pathology. This unit describes the structure and function of human telomeres and outlines several methodologies that can be employed to study these unique regions of human chromosomes. It is a revision of the original version of the unit published in 2000, now including an introductory section describing advances in the discipline that have taken place since the original publication.

A patient presenting a 22q13 deletion associated with an apparently balanced translocation t(16;22): An illustrative case in the investigation of patients with low ARSA activity

A 10-year-old speechless, mentally deficient male, with low arylsulfatase A (ARSA) activity, and presumably, methachromatic leukodystrophy, underwent genetic evaluation. As the clinical picture was not compatible with this diagnosisan ARSA gene and chromosome analysis were performed, showing the presence of a pseudodeficiency ARSA allele and a de novo apparently balanced t(16;22)(p11.2;q13) translocation. A deletion on the long arm of chromosome 22 encompassing the ARSA gene, as shown by FISH and array-CGH, indicated a 22q13 deletion syndrome. This case illustrates the importance of detailed cytogenetic investigation in patients presenting low arylsulfatase A activity and atypical/unspecific clinical features.

The 22q13.3 Deletion Syndrome (Phelan-McDermid Syndrome)

The 22q13.3 deletion syndrome, also known as Phelan-McDermid syndrome, is a contiguous gene disorder resulting from deletion of the distal long arm of chromosome 22. In addition to normal growth and a constellation of minor dysmorphic features, this syndrome is characterized by neurological deficits which include global developmental delay, moderate to severe intellectual impairment, absent or severely delayed speech, and neonatal hypotonia. In addition, more than 50% of patients show autism or autistic-like behavior, and therefore it can be classified as a syndromic form of autism spectrum disorders (ASD). The differential diagnosis includes Angelman syndrome, velocardiofacial syndrome, fragile X syndrome, and FG syndrome. Over 600 cases of 22q13.3 deletion syndrome have been documented. Most are terminal deletions of ∼100 kb to >9 Mb, resulting from simple deletions, ring chromosomes, and unbalanced translocations. Almost all of these deletions include the gene SHANK3 which encodes a scaffold protein in the postsynaptic densities of excitatory synapses, connecting membrane-bound receptors to the actin cytoskeleton. Two mouse knockout models and cell culture experiments show that SHANK3 is involved in the structure and function of synapses and support the hypothesis that the majority of 22q13.3 deletion syndrome neurological defects are due to haploinsufficiency of SHANK3, although other genes in the region may also play a role in the syndrome. The molecular connection to ASD suggests that potential future treatments may involve modulation of metabotropic glutamate receptors.

Protein interactome reveals converging molecular pathways among autism disorders

To uncover shared pathogenic mechanisms among the highly heterogeneous autism spectrum disorders (ASDs), we developed a protein interaction network that identified hundreds of new interactions among proteins encoded by ASD-associated genes. We discovered unexpectedly high connectivity between SHANK and TSC1, previously implicated in syndromic autism, suggesting that common molecular pathways underlie autistic phenotypes in distinct syndromes. ASD patients were more likely to harbor copy number variations that encompass network genes than were control subjects. We also identified, in patients with idiopathic ASD, three de novo lesions (deletions in 16q23.3 and 15q22 and one duplication in Xq28) that involve three network genes (NECAB2, PKM2, and FLNA). The protein interaction network thus provides a framework for identifying causes of idiopathic autism and for understanding molecular pathways that underpin both syndromic and idiopathic ASDs.

Genomic designation: how genetics can delineate new, phenotypically diffuse medical categories

This paper reports and discusses 'genomic designation' as a way of classifying people. In genomic designation the object of biomedical analysis--and the concomitant medical category that is subject to scientific, clinical, and social action--is delineated on a genomic basis, while the phenotype is decentralized and tabulated post factum. Unlike prominent sociological concepts such as biosociality or geneticization, where genetic proclivities for or explanations of phenotypic categories affect social processes, genomic designation treats characteristics of the genome as the essential referent of new categories of illness. I outline the relevant sociological literature and the shift to what Nikolas Rose has called the 'molecular gaze' before explicating the concept ofgenomic designation and its half-century history. I use 22q13 Deletion/Phelan-McDermid syndrome as an example of genomic designation: investigations into the deletion of genetic material at site q13 on the 22nd chromosome preceded and made practicable the delineation of a syndrome more than a decade later, even though the associated phenotype is not distinct enough for diagnosis. Finally, I discuss the implications of this turn to 'rigidly designate' kinds of people according to observations made at the level of the genome and outline directions for future research.

Multi-system disorders of glycosphingolipid and ganglioside metabolism

Glycosphingolipids (GSLs) and gangliosides are a group of bioactive glycolipids that include cerebrosides, globosides, and gangliosides. These lipids play major roles in signal transduction, cell adhesion, modulating growth factor/hormone receptor, antigen recognition, and protein trafficking. Specific genetic defects in lysosomal hydrolases disrupt normal GSL and ganglioside metabolism leading to their excess accumulation in cellular compartments, particularly in the lysosome, i.e., lysosomal storage diseases (LSDs). The storage diseases of GSLs and gangliosides affect all organ systems, but the central nervous system (CNS) is primarily involved in many. Current treatments can attenuate the visceral disease, but the management of CNS involvement remains an unmet medical need. Early interventions that alter the CNS disease have shown promise in delaying neurologic involvement in several CNS LSDs. Consequently, effective treatment for such devastating inherited diseases requires an understanding of the early developmental and pathological mechanisms of GSL and ganglioside flux (synthesis and degradation) that underlie the CNS diseases. These are the focus of this review.

Chromosome 22q13.3 deletion syndrome with a de novo interstitial 22q13.3 cryptic deletion disrupting SHANK3

BACKGROUND

The 22q13.3 deletion syndrome (or Phelan-McDermid syndrome, MIM 606232) is characterized by developmental delay, absent or severely delayed speech, neonatal hypotonia, autistic behavior, normal to accelerated growth, and minor dysmorphic facial features. Among the three genes in the minimal critical region (from the centromere to the telomere: SHANK3, ACR and RABL2B), the defect in the SHANK3 gene is considered to be the cause of the neurobehavioral symptoms.

OBJECTIVE

We describe the molecular characterization of a de novo interstitial del(22)(q13.3q13.3) disrupting the SHANK3 gene in a child with a phenotype compatible with the 22q13.3 deletion syndrome.

METHODS

Clinical work-up included clinical histories, physical, neurological, and ophthalmological examinations, and imaging of the brain. Commercially available MLPA for subtelomeric analysis, FISH specific probes and quantitative real-time PCR were used to characterize the rearrangement.

RESULTS

Subtelomere analysis by MLPA showed a discrepancy between P036B and P070 kits (MCR Holland): the P070 MLPA 22q probe (targeting the ARSA gene) showed a deletion but the P036B one (targeting the RABL2B gene) showed a normal result. FISH analysis using LSI TUPLE1/LSI ARSA (Vysis) probes confirmed deletion of ARSA, whereas FISH with N25/N85A3 (Cytocell) probes, targeting the SHANK3 locus was normal. Supplemented FISH analysis using BAC clones allowed us to specify the centromeric breakpoint region of the interstitial deletion between clones RP11-354I12 and RP11-232E17, at less than 2 Mb from the telomere. Quantitative real-time PCR of exon 5, 22 and 24 and intron 9 of SHANK3 showed that the telomeric breakpoint occurred between intron 9 and exon 22.

CONCLUSIONS

These data highlight the difficulty of performing an appropriate test aimed at looking for cryptic 22q13.3 deletion. Furthermore, the molecular characterization of this interstitial 22q13.3 deletion contributes to the clinical and genetic delineation of the 22q13.3 deletion syndrome.

Terminal 22q deletion syndrome: a newly recognized cause of speech and language disability in the autism spectrum

OBJECTIVE

Cryptic subtelomeric chromosome rearrangements account for 6% to 10% of idiopathic mental retardation. As cytogenetic and molecular techniques have become more sophisticated, the number of genetic syndromes attributed to these microdeletions has increased. To date, 64 patients have been described in the literature with a more recently recognized microdeletion syndrome, del 22q13.3. The purpose of this study is to present 11 new cases of this recently described syndrome to delineate further the phenotype and to alert the clinician to another genetic condition that should be considered in the differential diagnosis of early hypotonia, delayed speech acquisition, and autistic behavior.

METHODS

Eleven patients were evaluated in 3 academic institutions. Clinical features and results of cytogenetic testing were recorded and tabulated. Reasons for referral for genetic evaluation included developmental delay, severe expressive speech and language delay, and dysmorphic features.

RESULTS

Age of presentation ranged from 5 months to 46 years. There were 10 female patients and 1 male patient. All of the patients exhibited delayed motor development, some degree of hypotonia, and severe expressive speech and language delay. Dysmorphic facial features included epicanthal folds, large cupped ears, underdeveloped philtrum, loss of cupid's bow, and full supraorbital ridges. Six patients exhibited autistic-like behaviors. Microscopically visible chromosome deletions were observed in 6 patients. In the remainder, the deletion was detected with the use of fluorescence in situ hybridization.

CONCLUSIONS

Hypotonia and developmental delay are nonspecific findings observed in many malformation and genetic syndromes. However, in association with severe speech and language delay and autistic-like behavior, this phenotype may be a significant indication to consider the 22q13 deletion syndrome as a potential cause.

An investigation into sub-telomeric deletions of chromosome 22 and pervasive developmental disorders

Deletions of the sub-telomeric region of chromosome 22 have been associated with mental retardation, developmental delay, and autistic behaviors. This study investigated sub-telomeric anomalies of chromosome 22 using fluorescent in situ hybridization (FISH) probes in 82 subjects diagnosed with autism and atypical autism. No microdeletions were detected in this group. Similar FISH analyses were undertaken on two children with developmental delay, who were ascertained to be ring 22 during routine cytogenetic investigations. One subject was shown to have a microdeletion in the sub-telomeric region tested. Both children met the social and communication cut off for autism on the ADI and but did not meet the cut off for restrictive and repetitive behaviors. Only one of the two children met the criteria for PDD on the ADOS.

22q13 deletion syndrome: an update and review for the primary pediatrician

Recent advances in genetic testing can help to provide a specific diagnosis to children born with syndromes that result in congenital anomalies and developmental delay. One such emerging condition is the 22q13 deletion syndrome. With the introduction of subtelomeric fluorescence-in-situ hybridization (FISH) analysis, the 22q13 deletion has become recognized as a relatively widespread and underdiagnosed cause of mental retardation. Primary-care physicians play an important role in the care of children with 22q13 deletion syndrome, from suspecting the diagnosis in a developmentally delayed child through the medical, developmental, and behavioral aspects of their care. Furthermore, they serve as a valuable source of support and advocacy for the family and a resource for other care providers. The remainder of this article addresses the current state of knowledge regarding 22q13 deletion syndrome and offers the primary-care physician a framework in which to provide care and information.

Subtle trisomy 12q24.3 and subtle monosomy 22q13.3: three new cases and review

The high resolution G-bands (850 bands) karyotype have made it possible to identify small chromosome anomalies (5 megabases) which are now microscopically visible. New techniques have been improved, such as the Fluorescent in situ hybridization (FISH) with subtelomeric probes, which can be employed to detect cryptic chromosome alterations not visible microscopically. We present three cases which had been remitted for a high resolution karyotype. The high resolution G-band karyotype and the FISH techniques led us to conclude that the three cases were carriers of a similar subtle chromosomal alteration. Case I is a new born female with developmental and psychomotor delay, hypotonia, and long limbs with arachnodactily. A high resolution G-band karyotype showed an abnormal chromosome 22. FISH techniques confirmed a der(22)t(12;22)(q24.31;q13.3). Case II is a 12-year-old girl, with growth retardation, long shaped face with thick eyebrows, smooth philtrum, and thin upper lip with severe mental retardation (still no language), with a phenotype very similar to that of his sister: long shaped face, thick eyebrows, smooth philtrum, and thin upper lip. A high resolution G-band karyotype also showed in Case II and III an abnormal chromosome 22, studied by FISH techniques which confirmed a der(22)t(12;22)(q24.3;q13.3) in both cases.

Molecular characterisation of the 22q13 deletion syndrome supports the role of haploinsufficiency of SHANK3/PROSAP2 in the major neurological symptoms

METHODS

The 22q13 deletion syndrome (MIM 606232) is characterised by moderate to profound mental retardation, delay/absence of expressive speech, hypotonia, normal to accelerated growth, and mild dysmorphic features. We have determined the deletion size and parent of origin in 56 patients with this syndrome.

RESULTS

Similar to other terminal deletion syndromes, there was an overabundance of paternal deletions. The deletions vary widely in size, from 130 kb to over 9 Mb; however all 45 cases that could be specifically tested for the terminal region at the site of SHANK3 were deleted for this gene. The molecular structure of SHANK3 was further characterised. Comparison of clinical features to deletion size showed few correlations. Some measures of developmental assessment did correlate to deletion size; however, all patients showed some degree of mental retardation and severe delay or absence of expressive speech, regardless of deletion size.

CONCLUSION

Our analysis therefore supports haploinsufficiency of the gene SHANK3, which codes for a structural protein of the postsynaptic density, as a major causative factor in the neurological symptoms of 22q13 deletion syndrome.

No evidence for submicroscopic 22qter deletions in patients with features suggestive for Angelman syndrome

Patients with monosomy 22q13.3 --> qter have, in addition to (usually severe) developmental delay, hypotonia, severe expressive language delay leading to absence of speech, pervasive developmental abnormalities, and subtle facial anomalies. Thus far, it has been one of the more common submicroscopic telomere deletions seen in patients with mental retardation. Due to the phenotypic overlap between monosomy 22q13.3 and Angelman syndrome (AS), 44 patients with AS features but without one of the characteristic molecular 15q abnormalities were tested for 22qter deletions. In the study group, 31/44 (70%) were heterozygous for locus D22S163 with probe cMS607 (distance 0.125 Mb from telomere). The remaining 13/44 (30%) patients were heterozygous for one or more of four microsatellite markers centromeric from D22S163 in the 22qter region (distances 1.5-4.3 Mb from telomere). Based on the present study, there is no evidence that patients with an "Angelman-like" phenotype are more likely to have a 22qter deletion than other individuals with mental retardation.

Tandem translocation of chromosomes 22 and 15 with two preserved satellite stalk regions and deletion 22q13.3-qter

We describe here a case of a tandem 22/15 translocation with deletion of the 22q13.3-qter region and retention of the NOR of chromosome 15. A 2(1/2)-year-old Korean girl was referred for chromosome analysis after a clinical evaluation for developmental delay. Physical examination revealed hypotonia, developmental delay, delay of gross motor milestones and speech delay. No dysmorphic features of face, hands or feet were evident in the patient. G-banded peripheral blood lymphocyte chromosomes showed a tandem translocation between chromosomes 22 and 15, with the satellite stalks of chromosome 15 apparently being retained. All-telomere FISH analysis using a TTAGGG repeat probe showed absent signals at the junction of the translocation. Sequential G-banding and FISH analysis using a beta satellite probe showed positive signals close to the junction of the translocation, an indication that the short arms of the chromosome 15 involved in the translocation are retained. FISH with a probe for arylsulfatase, mapped to 22q13.3 region, was negative on the translocation chromosome. Therefore, the 22q13.3 region is deleted.

22q13 deletion syndrome

We have recently collected clinical information on 37 individuals with deletion of 22q13 and compared the features of these individuals with 24 previously reported cases. The features most frequently associated with this deletion are global developmental delay, generalized hypotonia, absent or severely delayed speech, and normal to advanced growth. Minor anomalies include dolicocephaly, abnormal ears, ptosis, dysplastic toenails, and relatively large hands. As with many terminal deletions involving pale G-band regions, the deletion can be extremely subtle and can go undetected on routine cytogenetic analysis. In fact, 32% of the individuals in our study had previous chromosome analyses that failed to detect the deletion. Eight of 37 individuals had deletion of 22q13 secondary to an unbalanced chromosome translocation. In the newborn, this deletion should be considered in cases of hypotonia for which other common causes have been excluded. In the older child, this syndrome should be suspected in individuals with normal growth, profound developmental delay, absent or delayed speech, and minor dysmorphic features. We recommend high-resolution chromosome analysis and fluorescence in situ hybridization studies, or molecular analysis to exclude this diagnosis.

A boy with a submicroscopic 22qter deletion, general overgrowth and features suggestive of FG syndrome

Over recent years, submicroscopic subtelomeric rearrangements have been shown to be a significant cause of mental retardation and, therefore, such abnormalities should be considered in every child with moderate to severe retardation with additional features suggestive of a chromosomal abnormality. The FG syndrome is an X-linked recessive mental retardation syndrome with congenital hypotonia, relative macrocephaly, a characteristic facies and constipation. We describe a severely mentally retarded boy with a history of severe constipation, truncal hypotonia, facial dysmorphism, fetal pads, and joint laxity, leading to an initial diagnosis of FG syndrome at the age of 3 years. Clinical re-evaluation at the age of 6 years, when he showed signs of general overgrowth, initiated a telomere screen, and a submicroscopic 22q13.3 telomere deletion was detected. The features suggestive of FG syndrome in this boy with a 22q13.3--> qter deletion may indicate testing for submicroscopic 22qter deletions in patients with atypical features of FG syndrome without a definite X-linked family history.

Perfect endings: a review of subtelomeric probes and their use in clinical diagnosis

Chromosomal rearrangements involving the ends of chromosomes (telomeres) are emerging as an important cause of human genetic diseases. This review describes the development of first and second generation sets of telomere specific clones, together with advances in fluorescence in situ hybridisation (FISH) technology, which have made the prospect of screening for telomeric rearrangements a realistic goal. Initial FISH studies using the telomere specific clones indicate that they will be a valuable diagnostic tool for the investigation of mental retardation, the characterisation of known abnormalities detected by conventional cytogenetic analysis, spontaneous recurrent miscarriages, infertility, haematological malignancies, and preimplantation diagnosis, as well as other fields of clinical interest. In addition, they may help investigate telomere structure and function and can be used in the identification of dosage sensitive genes involved in human genetic disease.

Rationale for a conditional knockout mouse model to study carnitine palmitoyltransferase I deficiencies

Several severe congenital cardiomyopathies are known to be associated with deficiencies in long-chain fatty acid transport and oxidation. Our studies are focused on a key enzyme in the regulation of intracellular long-chain fatty acid transport: carnitine palmitoyltransferase 1. Of this enzyme, two isoforms are expressed in the neonatal heart: L-CPT1 (the "liver-type" isoform) and M-CPT1 (the "muscle-type" isoform). It is known from studies in rats that chemical inhibition of both CPT1 isoforms results in hypertrophy of the cardiomyocytes, leading to an increase in heart-weight of up to 25%. With the aid of expressed sequence tag database analyses, cDNA- and genomic sequence information, we analysed the human gene for M-CPT1 in detail, and obtained partial clones of the murine genes for both CPT1 isoforms. We now started the development of a conditional knockout model to analyse and dissect deficiencies in these genes. While of the other mitochondrial components of the carnitine system deficiencies are known, some with severe cardiac consequences, M-CPT1 deficiencies have never been described. This suggests that M-CPT1 deficiency either (1) has not been recognised within the pool of congenital disorders, (2) is detrimental in an early stage of reproduction or embryogenesis, or (3) does not lead to physiological problems, probably due to the existence of a rescue system. If (1) is the case, the phenotypic effects of M-CPT1 deficiency have to be studied in order to generate criteria for clinical decision making and diagnosis. Option (2) demonstrates the necessity to use novel vector systems to create conditional gene disruptions. Hypothesis (3) implies a possible role for L-CPT1, and a knockout model allows a study of the interaction between the genes for L-CPT1 and M-CPT1. Applicable strategies to develop such a model system will be discussed.

Genetic evaluation of pervasive developmental disorders: the terminal 22q13 deletion syndrome may represent a recognizable phenotype

The evaluation of mental retardation is always a challenge to clinicians. The recognition of specific physical or behavioral characteristics can vastly improve diagnostic yield. Several genetic disorders have been identified to have certain behavioral characteristics, such as Williams syndrome, Smith-Magenis syndrome, and the velocardiofacial syndrome (VCFS). The deletion affecting the chromosome 22q in the most distal band (22q13) appears to define yet another neurobehavioral phenotype. In addition to our report, there are about 17 other cases published of this particular deletion syndrome. We describe three children who share features of developmental delay and pervasive behaviors in addition to normal to advanced growth patterns. Results of cytogenetic analysis suggest that the 3 patients share a deletion affecting the terminal 22q13 region. Two were found to have a cryptic deletion, in the third it was detected by conventional cytogenetics. The cryptic deletions were demonstrated using fluorescent in situ hybridization (FISH), where the control probe for the DiGeorge/VCFS region was deleted. While there remain gaps in our understanding of this particular deletion syndrome, we propose that patients with normal or advanced growth, significantly delayed speech, deviant development and pervasive behaviors, with minor facial dysmorphism, be screened for this deletion.

Prenatal diagnosis of mosaicism for a del(22)(q13)

A case of prenatally detected mosaicism for a del(22)(q13) is reported. CVS was performed because of abnormal fetal ultrasound findings: cystic 'tumour' in the fetal neck and the upper thoracic aperture. Karyotypes from chorionic villi were suspicious of an aberration concerning the long arm of one chromosome 22. FISH analysis demonstrated mosaicism for a distal 22q deletion in fetal fibroblasts. The deletion was postnatally confirmed by FISH with a chromosome-specific 22q probe. The 'tumour' on autopsy turned out to be cystic thymic tissue. Apart from this, no other obvious fetal anomalies were found.

Two 22q telomere deletions serendipitously detected by FISH

Cryptic telomere deletions have been proposed to be a significant cause of idiopathic mental retardation. We present two unrelated subjects, with normal G banding analysis, in whom 22q telomere deletions were serendipitously detected at two different institutions using fluorescence in situ hybridisation (FISH). Both probands presented with several of the previously described features associated with 22q deletions, including hypotonia, developmental delay, and absence of speech. Our two cases increase the total number of reported 22q telomere deletions to 19, the majority of which were identified by cytogenetic banding analysis. With the limited sensitivity of routine cytogenetic studies (approximately 2-5 Mb), these two new cases suggest that the actual prevalence of 22q telomere deletions may be higher than currently documented. Of additional interest is the phenotypic overlap with Angelman syndrome (AS) as it raises the possibility of a 22q deletion in patients in whom AS has been ruled out. The use of telomeric probes as diagnostic reagents would be useful in determining an accurate prevalence of chromosome 22q deletions and could result in a significantly higher detection rate of subtelomeric rearrangements.

The phenotypic effects of chromosome rearrangement involving bands 7q21.3 and 22q13.3

We report a family in which the proband has a direct insertion of band 7q21.3 into chromosome 22 at 22q13.3, karyotype 46,XX,dir ins(22;7)(q13.3;q21.2q22.1). Two of her children have unbalanced chromosome rearrangements involving 7q21.3, with one girl monosomic for the region and a boy trisomic for the region. The child monosomic for band 7q21.3 has a split hand/split foot (SHSF) anomaly and her clinical features are consistent with the 7q21-q22 contiguous gene deletion syndrome. In situ hybridisation studies have shown that the proband and her son have a submicroscopic deletion of chromosome band 22q13.3. Interstitial deletions of this chromosome band have rarely been reported.

Cryptic terminal rearrangement of chromosome 22q13.32 detected by FISH in two unrelated patients

Two unrelated patients with cryptic subtelomeric deletions of 22q13.3 were identified using FISH with the commercially available Oncor probe, D22S39. Proband 1 was found to have a derivative chromosome 22 resulting from the unbalanced segregation of a t(1;22)(q44;q13.32) in her mother. Additional FISH analysis of proband 1 and her mother placed the breakpoint on chromosome 22 in this family proximal to D22S55 and D22S39 and distal to D22S45. We have mapped D22S39 to within 170 kb of D22S21 using pulsed field gel electrophoresis. D22S21 is genetically mapped between D22S55 and D22S45. These data indicate that the deletion in proband 1 is smaller than in eight of nine reported del(22)(q13.3) patients. Probands 1 and 2 share features of hypotonia, developmental delay, and expressive language delay, also seen in previously reported del(22)(q13.3) patients, although proband 1 appears to be more mildly affected. Proband 1 is also trisomic for the region 1q44-->qter. This very small duplication has been previously reported only once and the patient had idiopathic mental retardation. This is the first report where 22q13.3 terminal deletion patients have been identified through the use of FISH, and the first report of a deletion of this region occurring because of missegregation of a parental balanced cryptic translocation. We feel that investigation of the frequency of del(22)(q13.3) in the idiopathic mentally retarded population is warranted and may be aided by the ability to use a commercially available probe (D22S39), which is already currently in use in a large number of cytogenetic laboratories.

Metachromatic leucodystrophy (MLD) in a patient with a constitutional ring chromosome 22

Metachromatic leucodystrophy (MLD) is an autosomal recessive lysosomal storage disease resulting from a severe deficiency of arylsulphatase A. The arylsulphatase A gene is located on chromosome 22 at q13.3. An MLD patient is described who carries a common splicing mutation (“I” allele) and a de novo ring 22 deleted for the arylsulphatase A gene. The fatehr was determined to be a heterozygous carrier of the “I” allele and the mother a heterozygous carrier of the arylsulphatase A pseudodeficiency allele. The ring 22 was shown by Southern blotting to be deleted in one copy of the arylsulphatase A gene. Minisatellite analysis showed the extent of the deletion and confirmed the biparental inheritance of chromosome 22 sequences. The carrier status of the parents and the patient's 46,XX,r(22) karyotype complicated the initial diagnosis in this family. However, the causal relationship of the ring 22 and MLD have implications for the recurrence risk in this family.