Trisomy 20p (ORPHA:261318)
Trisomy 20p is a chromosomal disorder resulting from duplication of all or part of the short arm of chromosome 20. It is mostly characterized by normal growth, mild to moderate intellectual disability, speech delay, poor coordination and evocative facial features.
Epidemiology
Trisomy 20p is rarely reported. To date, fewer than 40 patients have been described.
Clinical description
Clinical manifestations linked to trisomy 20p comprise a variable degree of developmental delay and mild to moderate intellectual disability, poor motor coordination, marked speech delay with difficulties in the articulation of some sounds. Typical facial dysmorphisms include a round face with full cheeks, thick coarse and usually straight hair, laterally arched eyebrows, upward slanting palpebral fissures, flared nostrils, high arched palate, abnormal teeth, occipital flattening and large misshapen ears. Other common features include variable skeletal anomalies including vertebral malformations (fusion of vertebrae, reduction of intervertebral spaces, spina bifida, scoliosis and kyphosis), hip deformity, and malposition of fingers and toes. Osteoporosis/osteopenia is reported in a few patients. Congenital heart defects and non-specific kidney abnormalities are present in some patients. Birth weight and growth pattern are usually normal. Less frequent physical abnormalities include umbilical and/or inguinal hernias and hypospadias. The extent and severity of clinical manifestations described above is contingent on the size and location of the duplication in 20p.
Etiology
Trisomy 20p is a chromosomal abnormality resulting from duplication of a fraction of the short arm of chromosome 20, variable in length, with no recurrent breakpoints. It may occur de novo, but most reported cases arise from a reciprocal translocation or, as described in a few cases, a parental inversion. Trisomy 20p is therefore frequently associated with another chromosomal imbalance that may modify the clinical picture. Pure trisomy 20p resulting from isochromosome formation and whole arm translocation has been reported. Precise genotype-phenotype correlations remain elusive due to the small number of patients and heterogeneity of breakpoints.
Diagnostic methods
Diagnosis is based on clinical manifestations leading to chromosomal analysis. Depending on their size, partial 20p duplications may be diagnosed by classical or molecular karyotyping. Molecular techniques are necessary for the genetic characterization of the duplication (FISH, MLPA, CGH array).
Antenatal diagnosis
Prenatal diagnosis of 20p duplication is possible by amniocentesis or chorionic villus sampling and cytogenetic analysis. Molecular techniques may be required depending on the size of the duplication.
Genetic counseling
Genetic counseling is recommended and requires parental karyotyping to evaluate their risk of having another affected child.
Management and treatment
Management is multi-disciplinary and requires evaluation and treatment by a pediatrician, and appropriate specialists. Patients will benefit from an early assessment and intervention with physiotherapy and occupational therapy.
Prognosis
The prognosis is variable, depending on the size and location of the duplication and on the quality and timing of treatment. Exact life expectancy is unknown, and depends on whether severe congenital anomalies are present. The majority of individuals with trisomy 20p will live into adulthood.

                            Last update: 
                            November 2012


                            - Expert reviewer(s): 
                            Dr Catherine TURLEAU