Chromosome 5

5q minus syndrome

The deletion of a region of DNA from the long (q) arm of chromosome 5 is involved in a condition called 5q minus (5q-) syndrome. 5q- syndrome is a type of myelodysplastic syndrome (MDS). In people with myelodysplastic syndrome, immature blood cells do not develop normally. Individuals with 5q- syndrome often have a shortage of red blood cells (anemia) and abnormalities in blood cells called megakaryocytes, which produce the cells that are involved in blood clotting (platelets). Affected individuals also have an increased risk of developing a fast-growing blood cancer called acute myeloid leukemia (AML). The deletion that causes 5q minus syndrome is somatic, which means that it occurs during a person's lifetime.

Most people with 5q- syndrome are missing a sequence of about 1.5 million DNA base pairs, also written as 1.5 megabases (Mb). This region of DNA contains 40 genes. Research suggests that the loss of one copy of multiple genes in this region contributes to the features of 5q- syndrome. In particular, the loss of the RPS14 gene leads to problems with red blood cell development, while the loss of the MIR145 or MIR146A gene contributes to the megakaryocyte abnormalities. Scientists are still determining how the loss of other genes in the deleted region might contribute to the features of 5q- syndrome and the development of AML.

5q31.3 microdeletion syndrome

5q31.3 microdeletion syndrome is caused by the deletion of a small piece of chromosome 5 in each cell. This rare condition is characterized by a severe delay in speaking and walking, weak muscle tone (hypotonia), breathing problems, seizures, and distinctive facial features. The deletion occurs on the long arm of the chromosome at a position designated q31.3. The size of the deletion can range from several thousand to several million base pairs. The deleted region typically contains at least three genes. The loss of one of these genes, PURA, is thought to lead to most of the characteristic features of the condition.

The protein produced from the PURA gene is especially important for normal brain development. It helps direct the growth and division of nerve cells (neurons). It may also be involved in the formation or maturation of myelin, the protective substance that insulates nerves and promotes the efficient transmission of nerve impulses. The loss of one copy of the PURA gene is thought to alter normal brain development and impair the function of neurons, which leads to the developmental delays, hypotonia, seizures, and the other neurological problems seen in people with 5q31.3 microdeletion syndrome.

Some studies suggest that the loss of another nearby gene on chromosome 5, called NRG2, increases the severity of the signs and symptoms. It is unclear how the loss of other genes in the deleted region contributes to the development of 5q31.3 microdeletion syndrome.

Cri-du-chat syndrome

Cri-du-chat (cat's cry) syndrome is caused by a deletion of the end of the short (p) arm of chromosome 5. This chromosomal change is written as 5p- (5p minus). Infants with this condition often have hypotonia and a high-pitched cry that sounds like that of a cat. Affected individuals also tend to have intellectual disabilities, developmental delays, and a small head size (microcephaly). The signs and symptoms of cri-du-chat syndrome are probably related to the loss of multiple genes in this region.

Researchers are working to determine how the loss of these genes leads to the features of the disorder. They have discovered that larger deletions tend to result in more severe intellectual disabilities and developmental delays than smaller deletions. Researchers have also defined regions of the short arm of chromosome 5 that are associated with particular features of cri-du chat syndrome. A specific region designated 5p15.3 is associated with a cat-like cry, and a nearby region called 5p15.2 is associated with intellectual disabilities, microcephaly, and distinctive facial features.

Liebenberg syndrome

Genetic changes in the long arm of chromosome 5 that affect the PITX1 gene cause Liebenberg syndrome. This condition is characterized by the abnormal development of the elbows, wrists, and hands. Affected individuals typically have joint deformities called contractures that limit the movement of these structures. The PITX1 gene provides instructions for producing a protein that plays a critical role in the development of the legs and feet.

People with Liebenberg syndrome have deletions, insertions, or rearrangements of the long arm of chromosome 5 that affect regions of DNA known as regulatory elements. These regulatory elements help turn genes on (enhancers) or off (repressors) during development. The chromosome changes that cause Liebenberg syndrome typically move an enhancer that normally activates the genes that are involved in the development of the arms and hands closer to the PITX1 gene.

As a result of these changes, the PITX1 gene becomes abnormally active during the development of the arms and hands. Because the PITX1 protein normally directs the development of the legs and feet, the bones and tissues in the arms and hands develop more like those in the legs and feet.

PDGFRB-associated chronic eosinophilic leukemia

Translocations that involve chromosome 5 can cause a type of blood cell cancer called PDGFRB-associated chronic eosinophilic leukemia. This condition is characterized by an increased number of eosinophils, which are a type of white blood cell. These cells help fight certain infections and are involved in the inflammation that occurs during allergic reactions. The most common translocation that causes PDGFRB-associated chronic eosinophilic leukemia fuses part of the PDGFRB gene from chromosome 5 with part of the ETV6 gene from chromosome 12. Translocations that fuse the PDGFRB gene with other genes have also been found to cause PDGFRB-associated chronic eosinophilic leukemia, but these genetic changes are relatively uncommon. The translocations that are associated with PDGFRB-associated chronic eosinophilic leukemia are somatic.

The protein produced from the ETV6-PDGFRB fusion gene, called ETV6/PDGFRβ, functions differently than the proteins produced from the individual genes. The ETV6 protein normally represses gene activity and the PDGFRβ protein plays a role in turning on signaling pathways. The ETV6/PDGFRβ protein is always turned on, activating signaling pathways and increasing gene activity. When the ETV6-PDGFRB fusion gene occurs in cells that develop into blood cells, the growth of eosinophils (and occasionally other white blood cells) is poorly controlled, leading to PDGFRB-associated chronic eosinophilic leukemia. It is unclear why this genetic change mostly affects eosinophils.

Periventricular heterotopia

Abnormalities in chromosome 5 have occasionally been associated with periventricular heterotopia, a disorder that is characterized by abnormal clumps of neurons around fluid-filled cavities (ventricles) near the center of the brain. In some cases, affected individuals had extra genetic material caused by an abnormal duplication of part of this chromosome. A deletion of material on the long arm of chromosome 5 has also been found to cause periventricular heterotopia.

Other chromosomal conditions

Other changes in chromosome 5 can have a variety of effects, including delayed growth and development, distinctive facial features, birth defects, and other health problems. Changes to chromosome 5 include an extra segment of the short (p) or long (q) arm of the chromosome in each cell (partial trisomy 5p or 5q), a missing segment of the long arm of the chromosome in each cell (partial monosomy 5q), and a circular structure called ring chromosome 5. Ring chromosomes occur when a chromosome breaks in two places and the ends of the chromosome arms fuse together to form a circular structure.

Other cancers

Deletions that involve the long arm of chromosome 5 or a complete loss of one copy of chromosome 5 (monosomy 5) can cause AML and MDS. While deletions in a specific segment of chromosome 5 are associated with 5q- syndrome, other deletions are related to other forms of these blood disorders. These changes are typically somatic.

Studies suggest that some genes on chromosome 5 play critical roles in the growth and division of cells. When segments of the chromosome are deleted, as in some cases of AML and MDS, these important genes are missing. Without these genes, cells can grow and divide too quickly and in an uncontrolled way. Researchers are working to identify the specific genes on chromosome 5 that are related to AML and MDS.