Structural Abnormalities: Deletions (Cri-du-Chat Syndrome) and Duplications (Pallister-Killian
What are structural chromosome abnormalities?
Structural chromosome abnormalities occur when there is a change in the structure
or parts of a chromosome. The total number of chromosomes is typically 46 total per
cell. Structural chromosome abnormalities occur when part of a chromosome is missing,
a part of a chromosome is extra, or a part has switched places with another part.
Ultimately, this leads to having too much or too little genetic material. This is
a cause of some birth defects.
Each chromosome has many segments. These are usually divided into a "short arm" and
a "long arm" of the chromosome. The short arm, which is the upper half of the chromosome,
is known as the "p arm." The long arm, which is the lower half of the chromosome,
is the "q arm." The centromere is the center part of a chromosome that appears "pinched"
between the p and q arms.
What are deletions?
The term "deletion" simply means that a part of a chromosome is missing or "deleted."
A very small piece of a chromosome can contain many different genes. When genes are
missing, there may be errors in the development of a baby since some of the "instructions"
are missing. One example of a genetic syndrome caused by a deletion is called "Cri-du-chat"
or "5p minus syndrome," where part of the #5 chromosome is missing or deleted.
What is Cri-du-chat?
Cri-du-chat or "cat's cry syndrome" is found in approximately 1 in 20,000 to 50,000
live births in the U.S. Cri-du- chat is caused by a deletion of chromosome 5p, which
is written "5p-." Babies with Cri-du-chat have a high-pitched cry, poor muscle tone,
a small head size, and low birth weight. They also have problems with language and
may express themselves by using a small number of words or sign language. Other health
problems can be present. These include delays in walking, problems with feeding, hyperactivity,
scoliosis, and severe intellectual disability. Most people with Cri-du-chat may have
a normal life span, unless they are born with other serious organ defects. Educational
intervention at an early age, in addition to physical and language therapy, is important
for children with Cri-du-chat to reach their full potential.
What are duplications?
The term "duplication" simply means that a part of a chromosome is duplicated, or
present in 2 copies. This results in having extra genetic material, even though the
total number of chromosomes is usually normal. Since a very small piece of a chromosome
can contain many different genes, the extra genes present in a duplication may cause
those genes to not function properly. These "extra instructions" can lead to errors
in the development of a baby. One way to think of a duplication is to think of all
46 chromosomes as a cookbook, and each individual chromosome as a recipe. If a deletion
is a missing ingredient in the recipe, a duplication is an extra ingredient. One example
of a rare genetic disorder of duplication is called Pallister-Killian syndrome, where
part of the #12 chromosome is duplicated.
What is Pallister-Killian syndrome?
Pallister-Killian syndrome is a result of extra #12 chromosome material. There is
usually a mixture of cells (mosaicism), some with extra #12 material, and some that
are normal (46 chromosomes without the extra #12 material). Babies with this syndrome
have many problems. These include severe intellectual disability, poor muscle tone,
"coarse" facial features, and a prominent forehead. They tend to have a very thin
upper lip, with a thicker lower lip and a short nose. Other health problems include
seizures, poor feeding, stiff joints, cataracts in adulthood, hearing loss, and heart
defects. People with Pallister-Killian syndrome have a shortened life span but may
live into their 40s.