A familial cryptic subtelomeric deletion 12p with variable phenotypic effect

Erotomania and phenotypic continuum in a family frameshift variant of AUTS2: a case report and review

BACKGROUND

Pathogenic variants of the AUTS2 (Autism Susceptibility candidate 2) gene predispose to intellectual disability, autism spectrum disorder, attention deficit hyperactivity disorder, facial dysmorphism and short stature. This phenotype is therefore associated with neurocognitive disturbances and social cognition, indicating potential functional maladjustment in the affected subjects, and a potentially significant impact on quality of life. Although many isolated cases have been reported in the literature, to date no families have been described. This case reports on a family (three generations) with a frameshift variant in the AUTS2 gene.

CASE PRESENTATION

The proband is 13 years old with short stature, dysmorphic features, moderate intellectual disability and autism spectrum disorder. His mother is 49 years old and also has short stature and similar dysmorphic features. She does not have autism disorder but presents an erotomaniac delusion. Her cognitive performance is heterogeneous. The two aunts are also of short stature. The 50-year-old aunt has isolated social cognition disorders. The 45-year-old aunt has severe cognitive impairment and autism spectrum disorder. The molecular analysis of the three sisters and the proband shows the same AUTS2 heterozygous duplication leading to a frame shift expected to produce a premature stop codon, p.(Met593Tyrfs*85). Previously reported isolated cases revealed phenotypic and cognitive impairment variability. In this case report, these variabilities are present within the same family, presenting the same variant.

CONCLUSIONS

The possibility of a phenotypic spectrum within the same family highlights the need for joint psychiatry and genetics research.

Variable Phenotypes of Epilepsy, Intellectual Disability, and Schizophrenia Caused by 12p13.33-p13.32 Terminal Microdeletion in a Korean Family: A Case Report and Literature Review

A simultaneous analysis of nucleotide changes and copy number variations (CNVs) based on exome sequencing data was demonstrated as a potential new first-tier diagnosis strategy for rare neuropsychiatric disorders. In this report, using depth-of-coverage analysis from exome sequencing data, we described variable phenotypes of epilepsy, intellectual disability (ID), and schizophrenia caused by 12p13.33–p13.32 terminal microdeletion in a Korean family. We hypothesized that CACNA1C and KDM5A genes of the six candidate genes located in this region were the best candidates for explaining epilepsy, ID, and schizophrenia and may be responsible for clinical features reported in cases with monosomy of the 12p13.33 subtelomeric region. On the background of microdeletion syndrome, which was described in clinical cases with mild, moderate, and severe neurodevelopmental manifestations as well as impairments, the clinician may determine whether the patient will end up with a more severe or milder end-phenotype, which in turn determines disease prognosis. In our case, the 12p13.33–p13.32 terminal microdeletion may explain the variable expressivity in the same family. However, further comprehensive studies with larger cohorts focusing on careful phenotyping across the lifespan are required to clearly elucidate the possible contribution of genetic modifiers and the environmental influence on the expressivity of 12p13.33 microdeletion and associated characteristics.

Ectopic Posterior Pituitary, Polydactyly, Midfacial Hypoplasia and Multiple Pituitary Hormone Deficiency due to a Novel Heterozygous IVS11-2A>C(c.1957-2A>C) Mutation in the GLI2 Gene

A novel heterozygous IVS11-2A>C(c.1957-2A>C) mutation in the GLI2 gene is reported. There was an extremely distinct phenotypical expression in two siblings and their father. The index case was a boy who developed cholestasis and hypoglycaemia in the neonatal period. He had bilateral postaxial polydactyly, mid-facial hypoplasia, high palatal arch, micropenis, and bilateral cryptorchidism. Laboratory examination revealed a diagnosis of multiple pituitary hormone deficiency. There was severe anterior pituitary hypoplasia, absent pituitary stalk and ectopic posterior pituitary on magnetic resonance imaging which suggested pituitary stalk interruption syndrome with no other midline structural abnormality. Molecular genetic analysis revealed a novel heterozygous splicing IVS11-2A>C(c.1957-2A>C) mutation detected in the GLI2 gene. His father and a six-year-old brother with the identical mutation also had unilateral postaxial polydactyly and mid-facial hypoplasia although there was no pituitary hormone deficiency. This novel heterozygous GLI2 mutation detected appears to present with an extremely variable clinical phenotype, even in related individuals with an identical mutation, suggesting incomplete penetrance of this GLI2 mutation.

12p deletion spectrum syndrome: a new case report reinforces the evidence regarding the potential relationship to autism spectrum disorder and related developmental impairments

BACKGROUND

Autism Spectrum Disorders (ASD) now encompass a broad heterogeneous group of people who present in the early developmental years with a wide range of social and communication deficits, which are typically also associated with complex repetitive behaviors and circumscribed interests. The target goal is to heighten readers' perception into the trend to personalize the distinct autistic and related developmental conditions encompassing the 12p region.

CASE PRESENTATION

This is a case-report of a 4-year-old male who presented the core signs of ASD, which were thought to be related to a rare 12p13.2 deletion.  We further reviewed the literature in order to outline the related developmental conditions in the 12p region. Aside from this patient reported here, we found an additional number of 43 cases described in the medical literature since 1974, that have been related to deletions in the 12p region. However, to the best of our knowledge, none of the previous had been specifically linked to the 12p13.2 band.

CONCLUSIONS

The 12p deletion spectrum is rarely described as part of the selective genotypes thought to be related to ASD. Even inside of a small piece of the puzzle, there might be ample variation in the behavioral and clinical phenotypes of children and adults presenting with this particular genetic profile. In that regard, the particular 12p13.2 distal deletion presentation is one of the possible genotypes encompassed by the "12p deletion spectrum syndrome", that might be potentially connected to the diagnosis of ASD and related developmental disorders.

Concurrent Loss of Heterozygosity and Mosaic Deletion of 12p13.32pter

Deletions in the short arm of chromosome 12 are the rarest subtelomeric imbalances. Less than 20 patients have been reported to date, and their microdeletions were identified either by FISH or array-CGH without SNP data. Here, we report a patient with a 12p13.32pter mosaic deletion detected by chromosome microarray analysis with loss of heterozygosity (LOH) of the deleted segment in addition to the adjacent distal segment. LOH is indicative of a complex rearrangement, suggestive of mitotic microhomology-mediated break-induced replication.

Subtle functional defects in the Arf-specific guanine nucleotide exchange factor IQSEC2 cause non-syndromic X-linked intellectual disability

Mutations in IQSEC2, a guanine nucleotide exchange factor for the ADP-ribosylation factor (Arf) family of small GTPases have recently been shown to cause non-syndromic X-linked intellectual disability (ID), characterised by substantial limitations in intellectual functioning and adaptive behaviour. This discovery was revealed by a combination of large-scale resequencing of the X chromosome, and key functional assays that revealed a reduction, but not elimination, of IQSEC2 GEF activity for mutations affecting conserved amino acids in the IQ-like and Sec7 domains. Compromised GTP binding activity of IQSEC2 leading to reduced activation of selected Arf substrates (Arf1, Arf6) is expected to impact on cytoskeletal organization, dendritic spine morphology and synaptic organisation. This study highlights the need for further investigation of the IQSEC gene family and Arf GTPases in neuronal morphology and synaptic function, and suggests that the genes encoding the ArfGEFs IQSEC1 and IQSEC3 should be considered as candidates for screening in autosomal ID.

Genotype-phenotype relationship in a child with 2.3 Mb de novo interstitial 12p13.33-p13.32 deletion

Microdeletion 12p13.33, though very rare, is an emerging condition associated with variable phenotype including a specific speech delay sound disorder, labelled childhood apraxia of speech (CAS), intellectual disability (ID) and neurobehavioral problems. Here we report a de novo 2.3 Mb interstitial 12p13.33-p13.32 deletion in a 5 year-old child with mild ID, speech delay, microcephaly, muscular hypotonia, and joint laxity. In contrast to previously reported patients with 12p13.33 monosomy, our patient's interstitial deletion spans the 12p13.33-12p13.32 region with the distal breakpoint within intron 12 of CACNA1C. Phenotype-genotype comparison between our case, previously reported patients, and subjects with 12p13.33 deletions led us to propose that haploinsufficiency of CACNA1C may influence the variability of the patients' phenotype, since the gene resulted disrupted or entirely deleted in the majority of reported patients. In addition, phenotypic features such as microcephaly, muscular hypotonia, and joint laxity are mainly present in patients with monosomy of 12p13.33 extending to the 12p13.32 portion. A common region of ~300 kb, harbouring EFCAB4B and PARP11, is deleted in patients with microcephaly while a second region of ~700 kb, including TSPAN9 and PMTR8, could be associated with muscle hypotonia and joint laxity. These data reinforce the hypothesis that multiple haploinsufficient genes and age-dependent observation may concur to generate the variable phenotype associated with 12p13.33 deletion.

Chromosomal-array analysis reveals partial 11q duplication and partial 12p deletion in a mildly affected case

Partial trisomy 11q is a rare syndrome and may be observed due to an intra-chromosomal duplication or an inter-chromosomal insertion. The deletions of the short arm of chromosome 12 are also uncommon structural aberrations. Only a small fraction of structural chromosome anomalies are related to the unbalanced progeny of balanced translocation carrier parents. We here report on a 10-month-old baby boy who shows a very mild phenotype related to unique chromosomal abnormality, partial trisomy of 11q, and partial monosomy of 12p, due to the maternal balanced reciprocal translocation (11;12). The proband showed a 49.64 Mb duplication of 11q14.1-q25 and 0.44 Mb deletion of 12p13.33 in chromosomal array analysis. Since it is known that the duplications may cause a milder phenotype than deletions. Dysmorphic facial features, minor cardiac anomalies, respiratory distress, central nervous system anomalies, and psychomotor delay observed in the patient was similar to the reported pure 11q duplication cases, while behavioral problems observed in pure monosomy 12p cases could not be evaluated due to the young age of the patient. Phenotype-genotype correlation will be discussed in view of all the reported pure partial 11q trisomies and pure partial 12p deletion cases.

A 1.5Mb terminal deletion of 12p associated with autism spectrum disorder

We report a patient with a terminal 12p deletion associated with autism spectrum disorder (ASD). This 12p13.33 deletion is 1.5Mb in size and encompasses 13 genes (B4GALNT3, CCDC77, ERC1, FBXL14, IQSEC3, KDM5A, LINC00942, LOC574538, NINJ2, RAD52, SLC6A12, SLC6A13 and WNK1). All previous cases reported with partial monosomy of 12p13.33 are associated with neurodevelopmental delay, and we suggest that ERC1, which encodes a regulator of neurotransmitter release, is the best gene candidate contributing to this phenotype as well as to the ASD of our patient.

12p13.33 microdeletion including ELKS/ERC1, a new locus associated with childhood apraxia of speech

Speech sound disorders are heterogeneous conditions, and sporadic and familial cases have been described. However, monogenic inheritance explains only a small proportion of such disorders, in particular in cases with childhood apraxia of speech (CAS). Deletions of <5 Mb involving the 12p13.33 locus is one of the least commonly deleted subtelomeric regions. Only four patients have been reported with such a deletion diagnosed with fluorescence in situ hybridisation telomere analysis or array CGH. To further delineate this rare microdeletional syndrome, a French collaboration together with a search in the Decipher database allowed us to gather nine new patients with a 12p13.33 subtelomeric or interstitial rearrangement identified by array CGH. Speech delay was found in all patients, which could be defined as CAS when patients had been evaluated by a speech therapist (5/9 patients). Intellectual deficiency was found in 5/9 patients only, and often associated with psychiatric manifestations of various severity. Two such deletions were inherited from an apparently healthy parent, but reevaluation revealed abnormal speech production at least in childhood, suggesting variable expressivity. The ELKS/ERC1 gene, which encodes for a synaptic factor, is found in the smallest region of overlap. These results reinforce the hypothesis that deletions of the 12p13.33 locus may be responsible for variable phenotypes including CAS associated with neurobehavioural troubles and that the presence of CAS justifies a genetic work-up.

A 4.5 Mb terminal deletion of chromosome 12p helps further define a psychosis-associated locus

A 12 year-old girl presented with cognitive disability and dysmorphic features. Chromosome microarray analysis revealed a de novo, approximately 4.5 Mb terminal deletion of the short arm of chromosome 12 at 12p13.33 region: chr12:100712-4607067. At 13 years this patient developed psychotic manifestations and was admitted to a psychiatric department for treatment. She started hearing voices, talking to herself and laughing without reason. We have previously reported a male individual with psychotic manifestations and a larger (6.2 Mb) terminal deletion in the same chromosomal region. The present case along with previous reports, define a 2 Mb region on chromosome 12p, where a psychosis-associated gene may be located. Included in this psychosis-associated area are 18 OMIM listed genes.

12p13 rearrangements: 6 Mb deletion responsible for ID/MCA and reciprocal duplication without clinical responsibility

Congenital balanced reciprocal translocations are one of the most frequent structural chromosomal aberrations in the population. We report a familial translocation t(12;22)(p13.3;pter) responsible for intellectual disabilities and congenital anomalies characterized by FISH and array CGH. Two patients carried a der(12)t(12;22)(p13.3;pter), resulting in a 6 Mb 12pter deletion. Patients presented with intellectual disabilities, pre- and post-natal growth retardation, ponderal development delay, global hypotonia, feeding problems and dysmorphic features. Two relatives presented with the reciprocal 12pter duplication, which had no clinical manifestations associated. For this translocation, we propose a mechanism based on a non-allelic recombination model, in which recombination of direct oriented segmental duplications between non-homologous chromatids leads to the reciprocal translocation. The characterization of this translocation has been critical for the family. Translocation carriers have a risk of 40% of having offspring carrying unbalanced products. 12p13.3 deletion carriers present with a recognizable syndrome and on the contrary, 12p13.3 duplication carriers present without clinical manifestations. Other published cases of 12p13.3 duplication show that this syndrome has a variable phenotype. It is advisable to delineate the duplication size and to discard other genetic aberrations, in order to give an accurate genetic counseling in patients carrying 12pter duplications.

1.39 Mb inherited interstitial deletion in 12p13.33 associated with developmental delay

We identified a novel 1.39 Mb interstitial deletion of chromosome 12p13.33 in an 8 year-old Caucasian female propositus and her affected father and brother using microarray-based comparative genomic hybridization (aCGH). They share a history of developmental delay and staring episodes. The deleted region contains eight annotated genes (ERC1, FBXL14, WNT5B, ADIPOR2, CACNA2D4, LRTM2, DCP1B, and CACNA1C). Hemizygous deletions of ERC1, FBXL14, or WNT5B genes may be involved in the development of neurological disorders in these individuals. Furthermore, the centromeric breakpoint of the 1.39 Mb deleted region is the same as the centromeric breakpoint of a 2.3 Mb terminal deletion of 12p13.33 reported recently, indicating the presence of an unstable structure near the breakpoint facilitating recurrent genomic rearrangements.

Subtelomeric deletion of 12p: Description of a third case and review

Only 12 cases with a cytogenetically visible deletion of the short arm of chromosome 12 (12p) have been reported so far. The difference in clinical features observed in these patients indicates that there is no distinct phenotype associated with this short arm deletion, although the existence of a del(12p) syndrome was previously suggested. Besides those 12 reports, only two patients have been described with a subtelomeric 12p deletion; both present in the same family in which the son showed a mild phenotype of moderate mental retardation and behavioral problems and his carrier mother had no apparent phenotype. In this article, we describe the third known patient with a subtelomeric 12p deletion in a young boy with mental retardation and microcephaly, and review the literature.

2.3 Mb terminal deletion in 12p13.33 associated with oculoauriculovertebral spectrum and evaluation of WNT5B as a candidate gene

We describe a patient presenting with developmental delay, patent foramen ovale, moderate short QT interval, and facial dysmorphism including left microtia, preauricular tag and pit, wide left corner of the mouth, and left hemifacial microsomia, fitting with the oculoauriculovertebral spectrum. We identified a de novo 2.3 Mb deletion in the 12p13.33 region that contains eighteen genes. Amongst those, the WNT5B gene stands out as a possible candidate. However, we did not find any mutation of this gene neither in our patient nor in a series of 53 OAVS patients. The CACNA1C gene is interrupted by the centromeric breakpoint of the deletion and its inactivation probably accounts for the short QT interval of the patient. We speculate that the phenotype of our patient may be explained by the combined effect of the loss of several of the genes contained in the deleted chromosomal segment and of the inactivation of CACNA1C.

Psychotic manifestations in a patient with mental retardation and a 6.2 megabase deletion at the distal short arm of chromosome 12

Genetic factors are known to contribute to the development of schizophrenia and related psychoses. Cytogenetic abnormalities have been occasionally found in patients with psychotic disorders and, thus, have helped identify candidate gene contributors for these conditions. The individual described here first presented with mental retardation and anxiety disorder in his mid-childhood. In his early 20s, the patient started exhibiting various psychotic manifestations, including delusions and hallucinations. His psychotic symptoms were difficult to control with psychotropic medications. The family history was negative for psychiatric disorders. This patient was found to have a 6.2 megabase deletion of the terminal portion of the short arm of chromosome 12 that was characterized using fluorescence in situ hybridization and microarray comparative genomic hybridization analysis. The maternal chromosomes were normal, but the paternal chromosomes could not be tested. To-date such a chromosomal abnormality has not been described in association with schizophrenia/psychosis. This case suggests that psychosis-associated gene(s) may be located in the terminal region of the short arm of chromosome 12.

A fourteen years follow-up of a case of partial trisomy 12q and monosomy 12p recombinants of a familial pericentric inversion of chromosome 12: Clinical, cytogenetic and molecular observations

Partial trisomy 12q and monosomy 12p lead to multiple malformation syndromes. Only four cases were previously reported with the association of these two aneusomies resulting from a familial pericentric inversion of chromosome 12. We report on the clinical, cytogenetic and molecular findings in a boy with an unbalanced karyotype which resulted from a familial pericentric inversion of chromosome 12. The patient was evaluated at birth and followed up until 14 years of age. He showed severe mental retardation, seizures, and dysmorphic features related both to a trisomy 12q and a monosomy 12p. Chromosome breakpoint BAC-FISH mapping revealed that the rec(12) chromosome had a terminal deletion of a 6.7Mb region extending from 12pter to 12p13.31 and a duplicated region of 19.8Mb extending from 12qter to 12q24.13. The findings from the case reported here emphasize the occurrence of some consistent clinical features and illustrate the deficiencies associated with the recombinants from the inversion inv(12)(p13.31q24.13)mat.

Idiopathic learning disability and genome imbalance

Learning disability (LD) is a very common, lifelong and disabling condition, affecting about 3% of the population. Despite this, it is only over the past 10-15 years that major progress has been made towards understanding the origins of LD. In particular, genetics driven advances in technology have led to the unequivocal demonstration of the importance of genome imbalance in the aetiology of idiopathic LD (ILD). In this review we provide an overview of these advances, discussing technologies such as multi-telomere FISH and array CGH that have already emerged as well as new approaches that show diagnostic potential for the future. The advances to date have highlighted new considerations such as copy number polymorphisms (CNPs) that can complicate the interpretation of genome imbalance and its relevance to ILD. More importantly though, they have provided a remarkable approximately 15-20% improvement in diagnostic capability as well as facilitating genotype/phenotype correlations and providing new avenues for the identification and understanding of genes involved in neurocognitive function.

Familial deletion 18p syndrome: case report

BACKGROUND

Deletion 18p is a frequent deletion syndrome characterized by dysmorphic features, growth deficiencies, and mental retardation with a poorer verbal performance. Until now, five families have been described with limited clinical description. We report transmission of deletion 18p from a mother to her two daughters and review the previous cases.

CASE PRESENTATION

The proband is 12 years old and has short stature, dysmorphic features and moderate mental retardation. Her sister is 9 years old and also has short stature and similar dysmorphic features. Her cognitive performance is within the borderline to mild mental retardation range. The mother also presents short stature. Psychological evaluation showed moderate mental retardation. Chromosome analysis from the sisters and their mother revealed the same chromosomal deletion: 46, XX, del(18)(p11.2). Previous familial cases were consistent regarding the transmission of mental retardation. Our family differs in this regard with variable cognitive impairment and does not display poorer verbal than non-verbal abilities. An exclusive maternal transmission is observed throughout those families. Women with del(18p) are fertile and seem to have a normal miscarriage rate.

CONCLUSION

Genetic counseling for these patients should take into account a greater range of cognitive outcome than previously reported.

Microarray-based comparative genomic hybridization and its applications in human genetics

Through the years, several techniques capable of detecting DNA copy number changes have been developed. A number of those, such as karyotyping and fluorescence in situ hybridization (FISH), have proven to be valuable tools in both research and diagnostics. Recently, a new technique, called microarray-based comparative genomic hybridization (array CGH), has been introduced. Array CGH has proven to be a specific, sensitive, and fast technique, with considerable advantages compared to other methods used for the analysis of DNA copy number changes. Array CGH enables analysis of the whole genome in a single experiment. Until now, its applications have been mainly directed at detecting genomic abnormalities in cancer. However, array CGH is also suitable for the analysis of DNA copy number aberrations that cause human genetic disorders. This review gives an overview of array CGH and its applications in human genetics. Advantages, limitations, and future perspectives of array CGH are discussed.

Screening for subtelomeric rearrangements in 210 patients with unexplained mental retardation using multiplex ligation dependent probe amplification (MLPA)

BACKGROUND

Subtelomeric rearrangements contribute to idiopathic mental retardation and human malformations, sometimes as distinct mental retardation syndromes. However, for most subtelomeric defects a characteristic clinical phenotype remains to be elucidated.

OBJECTIVE

To screen for submicroscopic subtelomeric aberrations using multiplex ligation dependent probe amplification (MLPA).

METHODS

210 individuals with unexplained mental retardation were studied. A new set of subtelomeric probes, the SALSA P036 human telomere test kit, was used.

RESULTS

A subtelomeric aberration was identified in 14 patients (6.7%) (10 deletions and four duplications). Five deletions were de novo; four were inherited from phenotypically normal parents, suggesting that these were polymorphisms. For one deletion, DNA samples of the parents were not available. Two de novo submicroscopic duplications were detected (dup 5qter, dup 12pter), while the other duplications (dup 18qter and dup 22qter) were inherited from phenotypically similarly affected parents. All clinically relevant aberrations (de novo or inherited from similarly affected parents) occurred in patients with a clinical score of >or=3 using an established checklist for subtelomeric rearrangements. Testing of patients with a clinical score of >or=3 increased the diagnostic yield twofold to 12.4%. Abnormalities with clinical relevance occurred in 6.3%, 5.1%, and 1.7% of mildly, moderately, and severely retarded patients, respectively, indicating that testing for subtelomeric aberrations among mildly retarded individuals is necessary.

CONCLUSIONS

The value of MLPA is confirmed. Subtelomeric screening can be offered to all mentally retarded patients, although clinical preselection increases the percentage of chromosomal aberrations detected. Duplications may be a more common cause of mental retardation than has been appreciated.

An unusual familial chromosome 9 "variant" with variable phenotype: characterization by CGH analysis

Heterochromatin confined to pericentromeric and secondary constriction regions plays a major role in morphological variation of chromosome 9, because of its size and affinity for pericentric inversion. We report on a 6-year-old boy with growth and language delay, minor facial anomalies and unusual chromosome 9 variant with an extra-band in the centromeric region on the conventional karyotype. Subsequent analysis by FISH and CGH identified this variant as a dicentric chromosome 9 with a duplication of the 9p12-q21 region. An identical chromosome 9 variant was found in the mild language retarded brother and in the phenotypically normal father and grandfather. The presumed mechanism accounting for the phenotypic discordance observed in this family and the usefulness of CGH in characterization of such variants are discussed. To our knowledge, this is the first investigation of an unusual chromosome 9 variant by CGH.

Subtelomere deletions and translocations are frequently familial

In recent years, strategies have been developed to investigate the possible role of chromosomal subtelomere regions in genetic disorders. The present study was to determine the incidence of familial subtelomeric abnormalities among individuals with developmental delay, idiopathic mental retardation, or non-specific congenital abnormalities. A review was conducted for patients and their relatives on whom subtelomeric DNA fluorescence in situ hybridization (telo-FISH) studies were performed. Patients were identified through a search of the Mayo Genetics System (MGS) database. Of 2,170 consecutive telo-FISH index case studies completed in our laboratory between January 2002 and December 2003, 121 or 5.6% had abnormalities of the subtelomere region. The present report includes 18 other abnormal index cases seen prior to 2002 to yield a total of 139 abnormal index cases. This represents 71 index patients with deletions, 53 index patients with derivative chromosomes, and 15 index patients with balanced rearrangements. A familial abnormality was identified in 29 (51.8%) of 56 families in whom parents and/or sibs were available for testing. Among 28 patients with deletions, 9 (32%) had an inherited deletion, whereas 19 (68%) were de novo. Family members of 20 index patients with derivative chromosomes were tested. Of these, 13 (65%) patients inherited the abnormality from a parent (12 from a parent who had a balanced translocation and 1 from a parent with the same abnormality), while 7 (35%) apparently arose de novo. Seven (88%) of 8 with balanced translocations inherited the translocation from one parent. The most common familial abnormalities involved 8pter deletion or rearrangement. The incidence of familial subtelomeric abnormalities is significantly high making parental telo-FISH studies an essential part of the investigation of patients with subtelomeric chromosome abnormalities.

Two cousins with partial trisomy 12q and monosomy 12p recombinants of a familial pericentric inversion of the chromosome 12

Partial trisomy 12q and monosomy 12p lead to multiple malformation syndromes. Instead of trisomy 12q that has been reported as a clinically identifiable syndrome, monosomy 12p is characterized by a wide phenotypic spectrum. We report two cousins suffering from severe mental retardation, seizures, and dysmorphic features related to a trisomy 12q24.3-->qter and a monosomy 12p13-->pter resulting from a familial pericentric inversion of chromosome 12. In an attempt to improve the clinical delineation of these two syndromes, we compared our two patients with previous reports of these aneusomies. This review emphasizes the high frequency of familial translocations, including a breakpoint at 12q24 involved in trisomy 12q whereas monosomy 12p occurs most frequently de novo. Despite the poor specificity of the signs, this comparison allowed us to determine the clinical features present in more than 20% of patients with trisomy 12q or monosomy 12p. We particularly emphasize some consistent leading features of monosomy 12p, including microcephaly, dental, cardio-vascular, extremity, and sensorial abnormalities, initially not reported as recurrent in this syndrome.

Telomeres: a diagnosis at the end of the chromosomes

In recent years, subtelomeric rearrangements have been identified as a major cause of mental retardation and/or malformation syndromes. So far, over 2500 subjects with mental retardation have been tested and reported of whom approximately 5% appeared to have a subtelomeric rearrangement. In this review, the clinical aspects of each known (submicroscopic) subtelomeric deletion will be presented and the various methods available for detecting subtelomeric abnormalities will be discussed. Not only will the patients and their families benefit from a good collection and report of the various telomeric abnormalities and their clinical phenotype, but it will also give more insight into the aetiology of mental retardation and malformation syndromes.