Chromosome 18, Monosomy 18p

Article by Dr. Han Thi Thu Huong - Medical Genetics Block, Vinmec High-Tech Center.

Chromosome 18, Monosomy 18p, also known as 18p deletion syndrome, is a rare chromosomal disorder caused by partial or complete loss of the short arm of chromosome 18. This syndrome is usually characterized by low chromosomal abnormalities. short stature, mental retardation, speech delay, craniofacial malformation, and other physical abnormalities.

Monosomy 18p occurs in about 1:50,000 live births with a female to male ratio of about 3:2. Since this syndrome was originally described in 1963, more than 150 cases have been reported in the medical literature.

In many cases, infants with Monosomy 18p have low birth weight; mild to moderate growth deficiency; decreased muscle tone; moderate mental retardation; slow acquisition of speech and language skills, failing to say simple words or sentences before about seven to nine years of age. Some children may have behavioral or emotional abnormalities such as difficulty concentrating, restlessness, and rapid mood swings (emotional instability).

The degree of craniofacial abnormalities varies from case to case including: abnormally small head (microcephaly); distinctive round face; wide mouth, "carp-shaped"; flat or wide nose; large, deformed, lower-than-normal ears; ocular hypertelorism, drooping of the upper eyelid (ptosis), crossed eyes; Abnormally small, inverted mandible (microretrognathia).
About 10% of 18p deletion carriers have “holoprosencephaly,” a complex congenital abnormality of the brain caused by the inability of the forebrain to divide normally into hemispheres during embryonic development. Holoprosencephaly can lead to varying degrees of mental retardation, seizures, and other neurological symptoms. Holoprosencephaly can also lead to facial abnormalities such as mild to severe midline defects, with only a single maxillary incisor; eyes that are far away or too close together (ocular hypertelorism or hypotelorism); in extreme cases, the orbits merge into a single socket containing one eye (cyclopia); cleft palate; unusual flat nose; having one nostril or no nose (arhinia).
Some infants may have an abnormal accumulation of fluid in the soft tissues of the hands and feet, accompanied by swelling (lymphedema); short, webbed neck; unusually wide chest, relatively small hands and feet; short fingers; turn "little finger" (clinodactyly). Infected men may have an abnormally small penis (micropenis) and undescended testicles (cryptorchidism). Some cases of antibody deficiency (immunoglobulin A); congenital heart defects.

Most people are born with 23 pairs (for a total of 46) chromosomes in every cell of the body. Each chromosome consists of a long arm (q) and a short arm (p) separated from each other by a centromere. Monosomy 18p is a chromosomal disease caused by the loss of all or part of the short arm on chromosome 18. This disorder is not usually genetic, but occurs as a de novo of very unknown cause. early in embryonic development. In such cases, the parents of a child with the mutation usually have normal chromosomes and the risk of having another child with a chromosomal abnormality is relatively low.
In some cases the 18p deletion appears to be the result of a “balanced translocation” mutation in the parental pairs. If the chromosomal rearrangement is balanced, i.e. the set of chromosomes is altered but there is no excess or lack of genetic material, it may be harmless to the mutation carrier but may increase the risk of developing the disorder. chromosomal abnormalities in offspring.
In addition, in rare cases, the parents of an affected child also carry an 18p deletion in all or some cells (mosaic).

Monosomy 18p syndrome can be diagnosed prenatally by ultrasound, amniocentesis, and chorionic villus sampling (CVS) or postpartum on a peripheral blood sample to perform cytogenetic tests such as Karyotyping (Technology & Clinical Practices). chromosomal modality), FISH (Fluorescent In-situ Hybrid), Array-CGH (Comparative Genomics Hybrid).

Because Monosomy 18p affects each individual so differently, treatment and interventions must be tailored to the individual needs of the patient. Treatment may require the coordination of a team of medical professionals, such as a pediatrician; operated doctor; a doctor who specializes in disorders of the skeleton, muscles, joints and related tissues (orthopedist); neurologist; speech-language pathologists and other health care professionals.
Early intervention is important in helping children with 18p deletion syndrome develop normally. The primary approach is special education, speech therapy, physical therapy, and other medical, social and vocational services. Genetic counseling is recommended for families with children with Monosomy 18p Syndrome.

References: rarediseases.org