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.

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

22q13 deletion syndrome

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.

https://jmg.bmj.com/content/jmedgenet/37/6/401.full.pdf

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

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 cupid9s 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.

Terminal 22q Deletion Syndrome: A Newly Recognized Cause of Speech and Language Disability in the Autism Spectrum

Summary

The present study explores the origin of human Robertsonian translocations (RT) and the causes of the nonrandom participation of the different acrocentrics in them. Satellite associations have been analysed in 966 cells from 8 persons, and 1266 RT with ascertainment have been collected from the literature. The observation that the chromosomes preferentially taking part in satellite associations vary between individuals is confirmed. However, since a preferred chromosome appears to associate at random with the others, this phenomenon should not add to the nonrandomness of the RT.



Most RT presumably arise through adjacent chromatid exchanges corresponding to mitotic chiasmata, in the pericentric regions of the acrocentrics. Our working hypothesis is that there is a basic exchange rate between any two acrocentrics. The surplus of t(14q21q) is presumed to depend on these two chromosomes having a homologous pericentric region. The 10–20 times higher incidence of t(13q14q) as compared with other RT is best explained by crossing-over between homologous, but relatively inverted, segments in these chromosomes. Of the 246 RT ascertained through repeated abortions or infertility, 56 were found through the latter. Of these, chromosome 14 was involved in 51. The infertility may be caused by a small deletion of 14q, as is often the case in 15q in Prader-Willi syndrome. In all RT ascertained through 21 or 13 trisomy, respectively, the relevant chromosome is one of the participants. Our data thus do not give any support to the idea of interchromosomal effects exerted by RT.

The nonrandom participation of human acrocentric chromosomes in Robertsonian translocations

An 18 month old girl with partial monosomy for the long arm of chromosome 22 is described. The karyotype was 46,XX,del(22)(pter----q13.1::q13.33----qter). To our knowledge this is the first report of monosomy for this specific segment of chromosome 22. Clinical features include developmental delay in all areas, hypotonia, macrosomia, full cheeks, eyebrows, and eyelids, mild epicanthus, wide nasal bridge, long philtrum, and thick lower lip. Parental chromosome studies were normal.

https://jmg.bmj.com/content/jmedgenet/27/9/588.full.pdf

Partial monosomy for chromosome 22 in a patient with del(22)(pter----q13.1::q13.33----qter).

Three cases of partial trisomy 7q are described. One case had duplication of region 7q22.1----q31.2 owing to a de novo direct intra-arm intrachromosomal duplication. The other two cases, first cousins, were trisomic for 7q34----qter, resulting from recombination within the inserted segment of a dir ins(7;17)(q34;q23.1q25.3)mat. All three cases had a number of the already recorded manifestations of partial trisomy 7q, namely strabismus, low set ears, depressed nasal bridge, small nose, hypotonia, and mental retardation.

https://jmg.bmj.com/content/jmedgenet/27/2/109.full.pdf

Three cases of partial trisomy 7q owing to rare structural rearrangements of chromosome 7.

A 22 year old woman with partial trisomy for the long arm of chromosome 2 is described. The karyotype is 46,XX, dir dup(2)(q33.1q35) de novo confirmed by FISH using a chromosome 2 specific paint. Parental chromosome studies were normal. To our knowledge this is the first report of trisomy for this specific segment of 2q and only the sixth case of de novo direct duplication of 2q, one of which was mosaic. Clinical features include epicanthus, clinodactyly, scoliosis, broad, flat nasal bridge, thin upper lip, long philtrum, and short neck.

https://jmg.bmj.com/content/jmedgenet/31/8/652.full.pdf

Partial trisomy for 2q in a patient with dir dup(2) (q33.1q35).

A 32-year-old mentally retarded woman was found to have a complex rearrangement of one chromosome 4. Her karyotype is interpreted as 46,XX,inv(4) (pter----p14::q12----p14::q12----qter) del (4) (pter----15.33::p15.2----qter). Clinically she does not show the features of the Wolf-Hirschhorn syndrome. Her phenotype and cytogenetic findings are compared with 2 other reported cases of 4p-without Wolf-Hirschhorn syndrome.

A complex structural rearrangement of chromosome 4 in a woman without phenotypic features of Wolf-Hirschhorn syndrome

Trisomy for the distal part of the long arm of chromosome 8(q24.13----qter) is described in three sibs. The anomaly arose as an adjacent 1 meiotic segregation from a balanced reciprocal translocation t(1;8)(q44; q24.13)mat.

https://jmg.bmj.com/content/jmedgenet/26/2/133.full.pdf

R G Parfitt

Papers

Partial monosomy for chromosome 22 in a patient with del(22)(pter----q13.1::q13.33----qter).

Three cases of partial trisomy 7q owing to rare structural rearrangements of chromosome 7.

Partial trisomy for 2q in a patient with dir dup(2) (q33.1q35).

A complex structural rearrangement of chromosome 4 in a woman without phenotypic features of Wolf-Hirschhorn syndrome

Familial distal trisomy 8(q24.13----qter).