DELETION 18

Alternate Names: Distal 18q- and proximal 18q- collectively and originally known as De Grouchy syndrome

A is showing broad nose, square shaped tip of nose, small philtrum, hypertelorism, telecanthus, squint and low set ears. B is showing 22q11.2 FISH with deletion (most cells) and without deletion (arrow) on interphase cells obtained from peripheral blood.

Overview: As what the name suggests, it refers to the deletion on the long arm of chromosome 18. May refer to the interstitial deletion (proximal) or terminal deletion (distal). No common breakpoints were identified so the extent of deletion varies widely.

Characteristic/Phenotype: Phenotypes differ on the kind of deletion and are highly variable depending on the size of deletion. It is generally characterized by mental retardation, short stature, hypotonia (reduced muscle tone), auditory impairment, dysmorphic features, and foot deformities.

For distal 18 deletion, phenotypes include: rocker-bottom feet or clubfoot, cleft lip and palate, kidney abnormalities (horseshoe kidney, hydronephrosis, polycystic kidney, and absent kidney), cryptorchidism and hypospadias, strabismus and nystagmus, myopia and coloboma on some cases, ear canal atresia, hearing loss, hernias, scoliosis, microcephaly, hypothyroidism, low IgA levels, psychiatric conditions (depression, anxiety, manic and psychotic symptoms), and autism.

Heart abnormalities are present in 25–35% of people with distal 18q-. Hypotonia is a common finding. Approximately 10% of people with distal 18q- have seizures.

For proximal 18 deletion, phenotypes include: clubfoot, cryptochordism (uncommon), otitis media, strabismus, hypotonia and 50% have seizures, scoliosis and tapering fingers, developmental disabilities.

Cardiac anomalies are observed in 24% of cases, including atrial and ventricular septal defects, and pulmonary stenosis.

The overall level of mental retardation appears to be mild in patients with deletions distal to 18q and severe in patients with deletions proximal to 18q.

Frequency: About 1 in 50,000 babies is born with a deletion of 18p and about 1 in every 40,000 babies is born with distal 18q-. The female to male ratio is 3:2.

Diagnosis: The chromosomal aberration is typically suspected when the child has developmental delays. Diagnosis is confirmed by chromosome studies or with microarray analysis. Karyotyping can also be done with a blood sample.

Prenatal diagnosis can be done with amniocentesis or chrionic villus sampling but may be difficult when it involves smaller deletions.

Causes: Most cases are sporadic, but an autosomal dominant transmission was also reported. 80 percent of the cases are usually caused by a de novo chromosomal deletion during the formation of reproductive cells or during early fetal development.

Ten percent of cases are caused by parental translocations resulting in unbalanced chromosomes, the remaining 10 percent is due to mosaicism, a less severe phenotype.

Treatment/Recommendations/Therapies: At present, treatment for 18q- is symptomatic, meaning that the focus is on treating the signs and symptoms of the conditions as they arise. To ensure early diagnosis and treatment, it is suggested that people with 18q- undergo routine screenings for thyroid, hearing, and vision problems.

• Treatment varies considerable depending on the type and severity of symptom that develop
• Surgery may be needed to correct defects or abnormalities e.g. heart defects, skull and facial abnormalities
• Access to programs and services as required e.g. physical therapy, speech therapy, educational support, social, vocational and medical service
• Various other symptomatic and supportive measures
• Genetic counseling and joining a support group is recommended. If a parent has a deletion, there is a 50% chance that they will have a child with distal 18q-. 

Support Groups:

Chromosome 18 Research and Registry Society

http://www.chromosome18.org/

UT Health Science Center

http://www.pediatrics.uthscsa.edu/centers/chromosome18/

References:

Budsiteanu, M. Et al. (2010). 18q deletion syndrome – A case report. Retrieved March 12, 2013 from the URL: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3150013/

Maranda, B. (2006). Familial deletion 18p syndrome: case report. Retrieved March 12, 2013 from the URL: http://www.biomedcentral.com/1471-2350/7/60

Photo from:

http://www.molecularcytogenetics.org/content/figures/1755-8166-1-18-1.jpg