Isodicentric 15
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Isodicentric 15, or idic(15), is a chromosome abnormality in which a child is born with extra genetic material from chromosome 15. People with idic(15) are typically born with 47 chromosomes in each of their body cells, instead of the usual 46 found in most people. In people with idic(15), the extra chromosome is made up a piece of chromosome 15 that has been duplicated end-to-end like a mirror image. It is the presence of this extra genetic material that is thought to account for the symptoms seen in some people with idic(15).
Occasionally, a person may have more than one extra idic(15), resulting in 48 or 49 chromosomes in all or some of their cells. People born without an extra chromosome but who have a segment of duplicated material within chromosome 15 have a similar condition known as an interstitial duplication. Some children with idic(15) are said to have ‘mosaicism’, meaning that their extra 15th chromosome is present in some, but not all of their body cells.
Signs and symptoms
Since chromosomes carry genes that determine how our bodies grow and develop, having extra chromosomal material can alter a person’s physical and mental development. Many individuals with idic(15) have delays in language development and motor skills such as walking or sitting up. Other traits may include low muscle tone (hypotonia), seizures, short stature, and mental retardation. Distinctive facial features associated with idic(15) may include epicanthal folds (skin folds at the inner corners of the eyes), a flattened nasal bridge, button nose, and a high arched palate (roof of the mouth). Some individuals with idic(15) also have features of autism, such as problems with communication, social interactions, and repetitive and stereotyped behaviors (e.g., lining up toys, playing with a toy in the same manner over and over again, hand flapping, rocking back and forth).
Diagnosis/Testing
The extra chromosome in people with idic(15) can be easily detected through a blood test called a chromosome study. An additional genetic test, called FISH (Fluorescent in situ hybridization), confirms the diagnosis by distinguishing idic(15) from other supernumerary marker chromosomes. Interstitial duplications of chromosome 15 can be more difficult to detect on a routine chromosome analysis but are clearly identifiable using a 15q FISH study. Families should always discuss the results of chromosome and FISH studies with a genetic counselor or other genetics professional to ensure accurate interpretation
Isodicentric chromosome 15 and autism
For more than 12 years, scientists have noticed that some individuals with autism also have idic(15). In fact, idic(15) is the most frequently identified chromosome problem in individuals with autism. (A chromosome anomaly involves extra or missing chromosomal material, not changes within the genes such as Fragile X syndrome). It is suggested that the co-occurrence of autism and idic(15) is not by chance. There may be a gene or genes in the 15q11-q13 region that is/are related to the development of autism in some individuals.
Genetic research studies of individuals without chromosome anomalies also support this idea that an autism-related gene may be present in 15q11-q13. Specifically, research studies found that certain DNA markers from the 15q11-q13 region were found more often in individuals with autism than in individuals without autism. Although these DNA markers are too small to be genes, they suggest that researchers may be getting close to finding an autism gene in this region.
Causes
IDIC(15) may develop from (egg) cells. Generally, people with idic(15) do not have family members with the chromosome abnormality. The idic(15) usually forms by chance in one person in the family. There is no coherence between lifestyle of the parents or environmental factors and the development of IDIC(15). Children with IDIC(15) are born in families with all skin colours, socioeconomic and race and ethnical backgrounds. In other words; there is nothing what parents can do to prevent IDIC(15). Nobody can tell which future parents will bring an IDIC(15) child to the world.
People with IDIC(15) have extra genetic material that has developed from chromosome 15. The material usually exists of the normal pair of chromosome 15 and a little extra chromosome 15 that is called the ‘marker’. The marker usually exists of 2 copies of the top part of the chromosome (p) that are mirrored and doubled. Because of this arrangement, idic(15) used to be referred to as "inverted duplication chromosome 15." In other words; the marker-chromosome is existing of 2 identical segments of the short arm (p), and a part of the long arm (q-arm) of the chromosome. Most commonly, the region called 15q11-q13 is the portion of chromosome 15 duplicated.
This region, also referred to as the critical PWS/AS-region named after the Prader-Willi and/or Angelman-syndrome, is held responsible for the symptoms of IDIC(15). The longer the part of the extra genetic material, the more difficult the consequences for the one involved. It is possible that the marker is that small that there is no (important) heritable information on the marker. Bearers of such a marker feel no consequences and can pass on the marker to their children
IDIC(15) is also called partial tetrasomy because parts of chromosome 15 appear more in parts of some body cells. The names that have been used in various researches are different but all mean the same; IDIC(15), extra Marker, Inv dup 15, inverted duplication 15, isodicentric 15, partial tetrasomy 15
Pathophysiology
Isodicentric chromosome 15 is the scientific name for a specific type of chromosome abnormality. Individuals with isodicentric chromosome 15, or "idic(15)", have 47 chromosomes instead of the typical 46 chromosomes. The extra chromosome is made up of a portion of chromosome 15 that has been duplicated and "inverted," so that there are two identical copies attached to one another that appear to be mirror images. Because of this arrangement, idic(15) used to be referred to as "inverted duplication chromosome 15." Most commonly, the region called 15q11-q13 is the portion of chromosome 15 duplicated. Sometimes the duplicated region is larger. The size of the idic(15) varies depending on the size of the region of chromosome 15 that is duplicated.
Individuals with idic(15) have a total of four copies of this chromosome 15 region instead of the typical two copies (1 copy each on the maternal and paternal chromosomes and 2 copies on the idic(15).
Treatment/Management
At the present time, there is no specific treatment that can undo the genetic pattern seen in people with idic(15). Although the fundamental genetic differences cannot be reversed, therapies are available to help address many of the symptoms associated with idic(15). Physical, occupational, and speech therapies along with special education techniques can stimulate children with idic(15) to develop to their full potential.
In terms of medical management of the symptoms associated with dup15q syndrome, families should be aware that individuals with chromosome 15 duplications may tolerate medications differently and may be more sensitive to side effects for some classes of medications, such at the serotonin reuptake inhibitor type medications (SSRI).[1] Thus, these should be used with caution and any new medication should be instituted in a controlled setting, with slow titration of levels and with a clear endpoint as to what the expected outcome for treatment is. This includes supplements
Prognosis
At the present time, there is no specific treatment which can undo the genetic pattern seen in people with idic(15). The extra chromosomal material in those affected was present at or shortly after conception, and its effects on brain development began taking place long before the child was born. Although the fundamental genetic difference cannot be reversed, therapies are available to help address many of the symptoms associated with idic(15). Physical, occupational, and speech therapies along with special education techniques can stimulate children with idic(15) to develop to their full potential.
Prevention/Screening
In general IDIC(15) occurs ‘spontaneously’. This means that both parents do not bear the marker chromosome. Research can be done through a blood test with both parents. When there is no marker chromosome the chances to have another child with IDIC(15) is not bigger than with other parents.
Occasionally, a parent of a child with idic(15) is found to have a rearrangement or other difference in one of his or her number 15 chromosomes. Although the parent is not affected by idic(15), the presence of an unusual chromosome 15 pattern predisposes him or her to conceiving children with idic(15). For such parents, there is an increased chance of having a second child with idic(15). There is also a possibility that other members of the parent's extended family could carry the same predisposing chromosome rearrangement. Chromosome studies are recommended for parents of all children with idic(15) to help identify those with an increased chance for recurrence. Each family is unique, and questions about recurrence should be discussed with a genetic counselor or other genetics specialist.
Epidemiology
IDIC(15) is one of the rare chromosome abnormalities that occurs more often. It is estimated that 18 out of 100.000 babies are affected. This number includes the children with a large IDIC (15) marker as well as children with a small marker (without negative phenomena). This means that each year in Germany 135, in the Netherlands 36, in the U.S. 720, and 45 babies in Australia with IDIC(15) are born. That appears to be a lot but reality learns that IDIC(15) is very unknown and that most doctors or pediatricians yet never examined a child with IDIC(15). 'IDEAS' and others self help groups s such as ‘Unique’ and ‘Leona’ know many families that have an IDIC(15) child. They offer information and support and mediate between parents.
Social impact
There is no clear picture of the future over the development of a child with IDIC(15). The children we know are younger and all are individuals, each with its own singularities. Yet there are a number of characterizations for people with IDIC(15) which vary a lot and can be very conspicuous. These characterizations are however also exemplary for many other chromosome deviations. There is no unambiguously specific profile that makes a diagnosis easier. None of the characterizations describes the symptoms of this syndrome exclusively. Added to that, the external appearance of IDIC(15) children, in general, is not different from other children. Because it is generally accepted that genetic abnormalities lead to specific external appearances this can lead to a delay in the diagnosis.
Notes
References
- ↑ Schanen, C: Research update on chromosome 15 duplications – idic(15) and interstitial duplications: The duplication 15q syndrome. Presentation at 2005 International Conference on Isodicentric 15 and Related Disorders.
External links
- Ideas is a non- profit organization providing support and advocacy for people with chromosome 15q duplications.
- Unique is a UK based source of information and support to families and individuals affected by any rare chromosome disorder and to the professionals who work with them.
- Pan-European self-help Yahoo group