NODAL Gene

Nonsyndromic holoprosencephaly is an abnormality of brain development that also affects the head and face. Normally, the brain divides into two halves (hemispheres) during early development. Holoprosencephaly occurs when the brain fails to divide properly into the right and left hemispheres. This condition is called nonsyndromic to distinguish it from other types of holoprosencephaly caused by genetic syndromes, chromosome abnormalities, or substances that cause birth defects (teratogens). The severity of nonsyndromic holoprosencephaly varies widely among affected individuals, even within the same family.

Nonsyndromic holoprosencephaly can be grouped into four types according to the degree of brain division. From most to least severe, the types are known as alobar, semi-lobar, lobar, and middle interhemispheric variant (MIHV). In the most severe forms of nonsyndromic holoprosencephaly, the brain does not divide at all. These affected individuals have one central eye (cyclopia) and a tubular nasal structure (proboscis) located above the eye. Most babies with severe nonsyndromic holoprosencephaly die before birth or soon after. In the less severe forms, the brain is partially divided and the eyes are usually set close together (hypotelorism). The life expectancy of these affected individuals varies depending on the severity of symptoms.

People with nonsyndromic holoprosencephaly often have a small head (microcephaly), although they can develop a buildup of fluid in the brain (hydrocephalus) that causes increased head size (macrocephaly). Other features may include an opening in the roof of the mouth (cleft palate) with or without a split in the upper lip (cleft lip), one central front tooth instead of two (a single maxillary central incisor), and a flat nasal bridge. The eyeballs may be abnormally small (microphthalmia) or absent (anophthalmia).

Some individuals with nonsyndromic holoprosencephaly have a distinctive pattern of facial features, including a narrowing of the head at the temples, outside corners of the eyes that point upward (upslanting palpebral fissures), large ears, a short nose with upturned nostrils, and a broad and deep space between the nose and mouth (philtrum). In general, the severity of facial features is directly related to the severity of the brain abnormalities. However, individuals with mildly affected facial features can have severe brain abnormalities. Some people do not have apparent structural brain abnormalities but have some of the facial features associated with this condition. These individuals are considered to have a form of the disorder known as microform holoprosencephaly and are typically identified after the birth of a severely affected family member.

Most people with nonsyndromic holoprosencephaly have developmental delay and intellectual disability. Affected individuals also frequently have a malfunctioning pituitary gland, which is a gland located at the base of the brain that produces several hormones. Because pituitary dysfunction leads to the partial or complete absence of these hormones, it can cause a variety of disorders. Most commonly, people with nonsyndromic holoprosencephaly and pituitary dysfunction develop diabetes insipidus, a condition that disrupts the balance between fluid intake and urine excretion. Dysfunction in other parts of the brain can cause seizures, feeding difficulties, and problems regulating body temperature, heart rate, and breathing. The sense of smell may be diminished (hyposmia) or completely absent (anosmia) if the part of the brain that processes smells is underdeveloped or missing.

Heterotaxy syndrome is a condition in which the internal organs are abnormally arranged in the chest and abdomen. The term "heterotaxy" is from the Greek words "heteros," meaning "other than," and "taxis," meaning "arrangement." Individuals with this condition have complex birth defects affecting the heart, lungs, liver, spleen, intestines, and other organs.

In the normal body, most of the organs in the chest and abdomen have a particular location on the right or left side. For example, the heart, spleen, and pancreas are on the left side of the body, and most of the liver is on the right. This normal arrangement of the organs is known as "situs solitus." Rarely, the orientation of the internal organs is completely flipped from right to left, a situation known as "situs inversus." This mirror-image orientation usually does not cause any health problems, unless it occurs as part of a syndrome affecting other parts of the body. Heterotaxy syndrome is an arrangement of internal organs somewhere between situs solitus and situs inversus; this condition is also known as "situs ambiguus." Unlike situs inversus, the abnormal arrangement of organs in heterotaxy syndrome often causes serious health problems.

Heterotaxy syndrome can alter the structure of the heart, including the attachment of the large blood vessels that carry blood to and from the rest of the body. It can also affect the structure of the lungs, such as the number of lobes in each lung and the length of the tubes (called bronchi) that lead from the windpipe to the lungs. In the abdomen, the condition can cause a person to have no spleen (asplenia) or multiple small, poorly functioning spleens (polysplenia). The liver may lie across the middle of the body instead of being in its normal position to the right of the stomach. Some affected individuals also have intestinal malrotation, which is an abnormal twisting of the intestines that occurs in the early stages of development before birth.

Depending on the organs involved, signs and symptoms of heterotaxy syndrome can include a bluish appearance of the skin or lips (cyanosis, which is due to a shortage of oxygen), breathing difficulties, an increased risk of infections, and problems with digesting food. The most serious complications are generally caused by critical congenital heart disease, a group of complex heart defects that are present from birth. Biliary atresia, a problem with the bile ducts in the liver, can also cause severe health problems in infancy.

The severity of heterotaxy syndrome varies depending on the specific abnormalities involved. Some affected individuals have only mild health problems related to the condition. At the other end of the spectrum, heterotaxy syndrome can be life-threatening in infancy or childhood, even with treatment.