Polymicrogyria is a condition characterized by abnormal development of the brain before birth. The surface of the brain normally has many ridges or folds, called gyri. In people with polymicrogyria, the brain develops too many folds, and the folds are unusually small. The name of this condition literally means too many (poly-) small (micro-) folds (-gyria) in the surface of the brain.

Polymicrogyria can affect part of the brain or the whole brain. When the condition affects one side of the brain, researchers describe it as unilateral. When it affects both sides of the brain, it is described as bilateral. The signs and symptoms associated with polymicrogyria depend on how much of the brain, and which particular brain regions, are affected.

Researchers have identified multiple forms of polymicrogyria. The mildest form is known as unilateral focal polymicrogyria. This form of the condition affects a relatively small area on one side of the brain. It may cause minor neurological problems, such as mild seizures that can be easily controlled with medication. Some people with unilateral focal polymicrogyria do not have any problems associated with the condition.

Bilateral forms of polymicrogyria tend to cause more severe neurological problems. Signs and symptoms of these conditions can include recurrent seizures (epilepsy), delayed development, crossed eyes, problems with speech and swallowing, and muscle weakness or paralysis. The most severe form of the disorder, bilateral generalized polymicrogyria, affects the entire brain. This condition causes severe intellectual disability, problems with movement, and seizures that are difficult or impossible to control with medication.

Polymicrogyria most often occurs as an isolated feature, although it can occur with other brain abnormalities. It is also a feature of several genetic syndromes characterized by intellectual disability and multiple birth defects. These include 22q11.2 deletion syndrome, Adams-Oliver syndrome, Aicardi syndrome, Galloway-Mowat syndrome, Joubert syndrome, and Zellweger spectrum disorder.

In most people with polymicrogyria, the cause of the condition is unknown. However, researchers have identified several environmental and genetic factors that can be responsible for the disorder. Environmental causes of polymicrogyria include certain infections during pregnancy and a lack of oxygen to the fetus (intrauterine ischemia).

Researchers are investigating the genetic causes of polymicrogyria. The condition can result from deletions or rearrangements of genetic material from several different chromosomes. Additionally, mutations in one gene, ADGRG1, have been found to cause a severe form of the condition called bilateral frontoparietal polymicrogyria (BFPP). The ADGRG1 gene appears to be critical for the normal development of the outer layer of the brain. Researchers believe that many other genes are probably involved in the different forms of polymicrogyria.

Normal Function

The ADGRG1 gene, formerly known as GPR56, provides instructions for making a protein that is critical for normal brain development. Before birth, the ADGRG1 protein appears to be essential for the normal growth and movement (migration) of nerve cells (neurons) in a part of the brain called the cerebral cortex. This outer layer of the brain carries out many important functions, such as sensation, voluntary muscle movement, thought, planning, and memory.

Although the ADGRG1 protein has been studied most extensively in the brain, it is active in many of the body's tissues. This protein interacts with other proteins on the cell surface to trigger a series of chemical signals within the cell. Studies suggest that ADGRG1 signaling may play an important role in attaching cells to one another (cell adhesion).

Health Conditions Related to Genetic Changes

Polymicrogyria

At least eleven mutations in the ADGRG1 gene have been identified in people with a severe form of polymicrogyria called bilateral frontoparietal polymicrogyria (BFPP). This disorder causes intellectual disability, delayed development, problems with speech and movement, and recurrent seizures (epilepsy). Most of the identified mutations change a single protein building block (amino acid) in the ADGRG1 protein. These mutations interfere with the normal processing of the protein. The abnormal protein becomes trapped within the cell, where it is unable to reach the cell surface to carry out its normal signaling functions.

A loss of ADGRG1 protein function likely disrupts the normal migration of neurons in the developing brain. As a result, certain regions of the cerebral cortex—areas known as the frontal and parietal lobes—develop too many folds (called gyri), and the folds are unusually small. These brain abnormalities cause intellectual disability and the other neurological problems associated with bilateral frontoparietal polymicrogyria.

Other Names for This Gene

• 7-transmembrane protein with no EGF-like N-terminal domains-1
• DKFZp781L1398
• EGF-TM7-like
• G protein-coupled receptor 56
• GPR56
• GPR56_HUMAN
• TM7LN4
• TM7XN1

Genomic Location

Normal Function

The TUBB2B gene provides instructions for making one version of a protein called beta-tubulin (β-tubulin). This protein is part of the tubulin family of proteins that form and organize cell structures called microtubules. Microtubules are rigid, hollow fibers that make up the cell's structural framework (the cytoskeleton). They are composed of β-tubulin and a similar protein called alpha-tubulin (α-tubulin) that is produced from a different gene. Microtubules grow and shrink as tubulin proteins are added to and removed from the ends of fibers. This process allows cells to move and change shape.

β-tubulin produced from the TUBB2B gene is found primarily in the brain and in nerve cells (neurons). In neurons, microtubules are integral for the cells' movement to the proper location in the brain and for a process called axon guidance, by which specialized extensions of neurons (axons) reach their correct positions. Once in the right location, axons relay messages to and from the brain to control muscle movement and detect sensations such as touch, pain, heat, and sound.

Health Conditions Related to Genetic Changes

Congenital fibrosis of the extraocular muscles

At least one mutation in the TUBB2B gene has been found to cause a rare form of congenital fibrosis of the extraocular muscles (CFEOM) called CFEOM3 with polymicrogyria. Individuals with this condition are unable to move their eyes normally; they have difficulty looking up, and they have droopy eyelids (ptosis). In addition, affected individuals have a brain malformation called polymicrogyria, in which the surface of the brain develops too many folds, and the folds are unusually small. Individuals with this form of CFEOM3 typically have intellectual disability.

The TUBB2B gene mutation that causes CFEOM3 with polymicrogyria changes a single protein building block (amino acid) in the β-tubulin protein. It replaces the amino acid glutamate with the amino acid lysine at protein position 421 (written as Glu421Lys or E421K). Microtubules that contain the altered β-tubulin protein do not grow and shrink as they should, which prevents axons from reaching their proper location. Nerves in the head and face (cranial nerves) that control muscles that surround the eyes (extraocular muscles) are particularly affected.

Abnormal development of cranial nerves impairs the function of extraocular muscles, leading to the characteristic features of CFEOM such as restricted eye movement and droopy eyelids. It is unclear how the CFEOM-related change in the TUBB2B gene results in polymicrogyria.

Isolated lissencephaly sequence

MedlinePlus Genetics provides information about Isolated lissencephaly sequence

Polymicrogyria

MedlinePlus Genetics provides information about Polymicrogyria

Other disorders

Mutations in the TUBB2B gene have been identified in people with brain abnormalities affecting the surface of the brain (the cortex), which are classified as malformations of cortical development. These individuals do not have problems with extraocular muscles (described above). The brain abnormalities typically lead to intellectual disability in affected individuals.

Brain malformations commonly associated with TUBB2B gene mutations result from abnormal development of the cortex and can include polymicrogyria (linked above); reduced folding (simplified gyration); an abnormally smooth surface (lissencephaly), sometimes with an abnormally small head size (microlissencephaly); slits or clefts in one or both halves of the brain (schizencephaly); or other cortical abnormalities. In some affected individuals, a region of the brain called the cerebellum is particularly affected (cerebellar dysplasia).

It is thought that mutations in the TUBB2B gene disrupt the movement of neurons and axons to their correct locations, altering brain development and leading to brain malformations.

Other Names for This Gene

• class IIb beta-tubulin
• TUBULIN, BETA, CLASS IIB
• TUBULIN, BETA-2B

Genomic Location

Isolated polymicrogyria can have different inheritance patterns. Several forms of the condition, including bilateral frontoparietal polymicrogyria (which is associated with mutations in the ADGRG1 gene), have an autosomal recessive pattern of inheritance. In autosomal recessive inheritance, both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Polymicrogyria can also have an autosomal dominant inheritance pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. Other forms of polymicrogyria appear to have an X-linked pattern of inheritance. Genes associated with X-linked conditions are located on the X chromosome, which is one of the two sex chromosomes. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Some people with polymicrogyria have relatives with the disorder, while other affected individuals have no family history of the condition. When an individual is the only affected person in his or her family, it can be difficult to determine the cause and possible inheritance pattern of the disorder.

• Developmental delay
• Crossed eyes
• Epilepsy
• Paralysis of the face, throat, and tongue
• Difficulty with speech and swallowing
• Drooling

The prevalence of isolated polymicrogyria is unknown. Researchers believe that it may be relatively common overall, although the individual forms of the disorder (such as bilateral generalized polymicrogyria) are probably rare.