Chromosomal Deletion Syndromes
(See also Overview of Chromosome Disorders.)
Chromosomal deletion syndromes occur when part of a chromosome is missing.
Chromosomes are structures within cells that contain DNA and many genes. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or more types of cells in the body (see Genes and Chromosomes for a discussion about genetics). Genes contain instructions that determine how the body is supposed to function. When parts of chromosomes are missing, a number of syndromes can occur. These syndromes are called chromosomal deletion syndromes. They tend to cause severe birth defects and markedly limited intellectual development and physical development. A few chromosomal deletion syndromes are discussed here, but there are many others.
In this rare syndrome, part of chromosome 5 is missing. The size of the missing part varies, and people who have larger deletions are often more severely affected.
Children with this syndrome often have a characteristic high-pitched, mewing cry that sounds like a kitten crying. This cry may be heard immediately after birth, lasts several weeks, and then disappears. An infant with this syndrome may have a low birth weight and a small head with many abnormal features, including a round face, small jaw, wide nose, widely separated eyes, crossed eyes (strabismus), and abnormally shaped ears set low in the head. Often the infant seems limp. Webbed fingers and toes (syndactyly) and heart defects are common. There are significant limitations in mental and physical development. Many children with cri du chat syndrome survive to adulthood but have substantial disabilities.
About 70% of people who have this syndrome are missing part of chromosome 15. About 30% of people who have this syndrome have problems with the structure and function of chromosome 15.
Many symptoms of Prader-Willi syndrome vary according to the child's age. Newborns with the defect feel limp, feed poorly, and gain weight slowly. Eventually these symptoms resolve. Then, between the ages of 1 and 6, appetite increases, often becoming insatiable. Children rapidly gain weight. The hands and feet remain small, and children remain short. Abnormalities of the face include almond-shaped eyes and a mouth with thin upper lips and down-turned corners. Children have bone disorders (such as scoliosis and kyphosis). Obsessive-compulsive behaviors are common. The function of the reproductive organs is abnormally decreased, which limits growth and sexual development. Boys have undescended testes (cryptorchidism) and an underdeveloped penis and scrotum. Intellectual disability is common. Weight gain continues into adulthood and is excessive, which can lead to other health problems, such as obesity. Obesity can be severe enough to justify gastric bypass surgery.
Ongoing research is being done to find treatments that help improve the prognosis for children and adults who have Prader-Willi syndrome.
In this syndrome, part of chromosome 4 is missing. Children with this syndrome often have profound intellectual disability. Children may also have epilepsy, a broad or beaked nose, scalp defects, drooping upper eyelids (ptosis) and gaps or fissures (colobomas) of the iris, cleft palate, and delayed bone development. Boys may have undescended testes (cryptorchidism) and a misplaced opening of the urethra (hypospadias—see Hypospadias). Some children have an immune deficiency, which means their body is less able to fight off infections. Many affected children die during infancy. Children who survive into their 20s typically have severe disabilities.