Cri du Chat syndrome, also known as 5p- syndrome, is a rare genetic condition. It is characterized by a distinctive high-pitched cry in infants that sounds similar to a cat’s mewing, from which the syndrome gets its French name. Individuals with Cri du Chat syndrome often exhibit low birth weight, a small head size (microcephaly), and distinctive facial features. They also experience varying degrees of intellectual disability and developmental delays.
The Chromosomal Change
Cri du Chat syndrome arises from a missing piece of genetic material on chromosome 5. A deletion refers to a type of genetic change where a segment of a chromosome or a sequence of DNA is lost. In Cri du Chat syndrome, this specific genetic event is a deletion on the short arm of chromosome 5, often referred to as a 5p deletion. The “p” designates the short arm, while “q” refers to the long arm of a chromosome. The size and exact location of this deletion can vary among affected individuals, influencing the range and severity of their symptoms.
How Cri du Chat Occurs
The deletion on chromosome 5 that causes Cri du Chat syndrome primarily occurs in two ways. The most frequent cause, accounting for approximately 80-90% of cases, is a de novo mutation. This means the deletion happens spontaneously and randomly during the formation of reproductive cells (sperm or egg) or in the very early stages of fetal development. In these de novo instances, the parents typically have normal chromosomes.
In a smaller proportion of cases, about 10-15%, Cri du Chat syndrome is inherited from a parent. This occurs when a parent carries a balanced translocation involving chromosome 5. A balanced translocation is a chromosomal rearrangement where segments of two different chromosomes have swapped places, but no genetic material is gained or lost overall. While a parent with a balanced translocation is usually healthy because they have a complete set of genetic material, they can pass on an unbalanced form to their child. If the child inherits an unbalanced translocation, they will have missing genetic material from chromosome 5, leading to Cri du Chat syndrome.
Understanding Family Risk
For families where Cri du Chat syndrome has occurred due to a de novo deletion, the likelihood of it happening again in a future pregnancy is very low. The recurrence risk for these spontaneous cases is very low, less than 1%. This is because the initial event was a random error in cell division.
Conversely, if Cri du Chat syndrome is a result of an inherited balanced translocation from a parent, the recurrence risk for future pregnancies is significantly higher. The exact risk depends on the specific type of translocation and how chromosomes segregate during the formation of reproductive cells, but it can range from approximately 5% to 20% or even higher. Genetic counseling is important for affected families to understand their specific risks and explore options. Genetic counselors can provide detailed information about inheritance patterns and discuss prenatal diagnostic options, such as amniocentesis, which can detect the deletion in an unborn baby.