Chromosome 18 Syndromes
18번 염색체에 관해서 아래 5가지의 증후군이 있지만 각 증후군 내에서도 임상 소견이 매우 다양하다. 일부 환자에서는 특징적인 소견이 이들 증후군에 정확하게 일치할 수 있지만, 이들 임상정보가 환자의 수수께끼를 풀어가는 기나긴 과정의 출발점으로 작용할 수도 있다.
단완의 끝 부분이 떨어져 나간 형태
장완 끝 일부가 떨어져 나간 형태
단완과 장완의 끝 부분이 떨어져 나가 없어지고, 결실 부위끼리 결합하여 고리 모양(환상)을 만듬.
염색체 한개가 추가된 양상인데, 추가염색체(extrachromosome)는 단완 한쌍이 중심절로 연결된 등완염색체(isochromosme) 형태임.
한쌍의 18번 염색체에 1개가 더 추가되어 3개의 염색체를 보임.
|Chromosome 18 Registry and Research Society|
The Deletion Syndromes of Chromosome 18
This information is intended to help parents understand chromosomal abnormalities in general and those relating to chromosome 18 specifically. It is hoped this information will help to clarify the basic principles of genetics and give a new perspective to parents about the information they may have already received.
These syndrome descriptions are the result of a search of the medical literature. They were derived from medical case reports written by physicians to inform other physicians. These reports are intended to aid in the recognition and diagnosis of the syndromes. A suspected diagnosis of a chromosomal abnormality involves the overall picture of the combination of characteristics. For this reason many seemingly inconsequential factors are mentioned such as an increased number of whorl patterns on the fingertips. It is the combination of characteristics and not so much the magnitude of each of them which is important in suspecting a chromosomal abnormality.
What is known about these syndromes is written primarily by diagnosticians,so there is very little information about the outcomes for the affected individual. What kind of speech problems do they have and how are they best dealt with? What kind of behavioral problems are seen and what treatments have worked best? What are their educational strengths and weaknesses?These are the kinds of questions that are important to parents, and these are the kind of questions that future editions of these syndrome descriptions will address as the Registry collects more of this kind of information.
To help understand these syndromes, we need to begin with some background information. Every cell in the human body contains within it, a complete set of instructions for carrying out every function that every kind of cell will ever need. Although a brain cell does not utilize the same instructions that a liver cell uses, they still contain the entire set of instructions. Just as a cookbook contains many recipes,some of which you rarely use. These instructions are the hereditary material and are packaged within each cell in 23 pairs of chromosomes. The complete set of instructions is called the genome. It is estimated that there are50,000-100,000 genes, or instructions, in the entire human genome. Chromosome18 is a small chromosome and contains only 1,000-2,000 genes. Thus far,only a handful have been identified.
There is one exception
to the rule that every cell contains 23 pairs of chromosomes. These are the
reproductive cells, or more specifically the egg and sperm cells. They each
contain one-half of the normal number, or 23 chromosomes instead of 23 pairs.
This means that the union of an egg and sperm cell will create a new cell with
just the right amount of genetic material for a new person. You have inherited
one chromosome of each pair,or half of your genetic material from each of your
parents. Each of your children will inherit one of each pair from you.
Chromosomes can be looked at microscopically after
they have been chemically stained. This procedure is called a karotype, and
cells used in this test are usually obtained from a blood sample. When the
chromosomes are stained, they show a pattern of light and dark bands which is
characteristic of each chromosome pair. Each chromosome pair also has a
characteristic length, and each pair is assigned a number, with #1 being the
longest and with #18 being one of the shortest. Each chromosome pair also has a
characteristic constriction somewhere along its length,called the centromere.
Since the centromere is not usually exactly in the middle, the chromosome looks
as if the top part is shorter than the bottom.The top portion, or short arm is
called "p" for petite and the bottom portion, or long arm is called "q". Using
these three distinctions (relative length, banding patterns and location of the
centromere),each pair of chromosomes can be reliably distinguished. The banding
pattern is also utilized to provide reference landmarks along each
chromosome.Below is a diagrammatic representation of the banding and therefore
the numerical references of chromosome 18.
The deletion syndromes of chromosome 18 include 18q-, 18p- and ring18.
This abnormalities of chromosome 18 are some of the most
common of the autosomal deletion syndromes. Their combined frequency is
estimated to be 1 in every 46,000 births. The deletion syndromes fall into 3
classes. First, 18p- (eighteen p minus), is any missing piece of the short arm
of chromosome 18. Second, 18q- (eighteen q minus), is any missing piece of the
long arm of chromosome 18. Third, Ring 18 is the joining of ends of the
chromosome into a ring and results in missing material at each end of the
Most deletions arise as de novo events. Meaning that the deletion is anew one and occurred during the formation of the sperm or egg or very early in embryonic development. The parents of such a person have normal chromosomes and have a low probability of having another child with a chromosome abnormality.
Some cases result because one of the parents has a balanced translocation. This means that the parent has a deletion on one chromosome,but the deleted piece is attached to another chromosome. As a result, the parent still has all of his or her genetic information, although it is arranged is a different way. Therefore, this parent has no symptoms and probably had no idea that he or she has a chromosomal abnormality. A parent with a balanced translocation has a significant chance of having a child with missing or extra chromosomal material, and therefore of being an affected child. The reason parents have chromosome studies performed is to determine if one of them has a balanced translocation.
There are also cases
in the literature in which a parent with one of these syndromes has a child also
with the syndrome. Someone with one of these chromosomal abnormalities has a
significant chance of having a child with the same abnormality. There is no
evidence at this time that indicates that any of these syndromes can be caused by
exposure to environmental agents, such as medications, x-rays or chemicals
before or during pregnancy.
It is important to remember that it is usually the
extreme or unusual cases which are reported, so the available information may
produce a more severe picture of the syndrome. A diagnosis is usually only
sought if the person has developmental or medical problems. A person who may
look a little different and made it through school with C's and D's and
actually has a chromosomal abnormality may never be diagnosed. It is also
important to remember that these descriptions do not represent any single
person.In fact, many parents report that their child has fewer than half of
these problems. But a generalized description can still be useful to the
parent who is searching for answers and trying to ask the right
The female to male ratio is 1.7/1. Birth weight averages 2800 gm (6 lb. 2 oz), which is below average but well within the normal range. Increased age of the parents does not appear to be a factor.
* most of the patients who were described as mentally retarded
IQ score given. For those whose IQ was measured, the scores ranged from 5 to
The only progressive problem reported was tapetoretinal degeneration. It was seen in two unrelated patients age 10 and 49. This eye disease was thought to be the result of a small deletion at band 18q21. Therefore a person with a deletion in this region should be followed by an ophthalmologist.
A young child with 18q- will benefit from an early intervention program. Special services are available though the public school system for children at 3 years of age. Check with your local school district's Department of Special Education for details on the programs that they provide.The poor muscle tone will require years of physical and occupational therapy.The muscle tone problems, along with any hearing problems will probably delay speech and will require years of speech therapy. Mental function is highly variable, and if near normal may make services in special programs hard to obtain. It is recommended that a child with 18q- be seen at least once by the following specialists: a pediatric ophthalmologist, a pediatric neurologist, a developmental pediatrician, an otolaryngology, a pediatric endocrinologist and of course, a geneticist or genetic counselor of they have not already done so.
Here are some of our wonderful children who happen to have the 18q- syndrome.
The female to male ratio is 2/1. Birth weight averages 2600 gm (5 lb. 11 oz.), which is the low end of normal. 19% died as newborns,all from severe brain malformations.
(References: de Grouchy, 1969; Faust, et al., 1976; Finley,et al., 1972;Fischer, et al., 1970; Gorlin, et al.,1968; Leisti, et al., 1973; Levenson,et al.,1971; Moedjono, et al., 1979; Ruvalcaba & Thuline, 1969; Sabateret al., 1972; Schinzel et al., 1974; Schnabel & Hansen, 1983; Stofferet al., 1981; Subrt & Berankova, 1972; Taylor, et al., 1975; Uchida,et al., 1965; Vorsanova, et al., 1986; Weiss & Mayeda, 1969)
Of the 6 cases who have had autopsies (5 were newborns with severe holoprosencephaly, and 1 abortus), 3 had fissured or accessory spleens. The belief exists among geneticists that there are people with 18p- who are so mildly affected that they don't know that they have a chromosomal abnormality until they have a child who is also 18p-, and is more severely affected. While this may be true, there are no reports in the literature to substantiate this.
developmental aspects of this disorder are even less well defined than for 18q-.
Speech is often delayed and behavior problems such as inability to form
relationships, have also been reported. Early intervention for young children and
special education programs for those over 3, will probably be beneficial. These
children should also be seen by a developmental pediatrician,pediatric
neurologist, and of course a geneticist and genetic counselor.A person with 18p-
should also be followed closely by a dentist.
Here are some of our wonderful children who happen to have the18p- syndrome.
The Ring 18 Syndrome
This syndrome results when the usually linear chromosome form a ring. Ring formation involves the breakage of the chromosome at both ends and the joining of the ends of the p and q arms back together.The amount of material lost from each arm of the chromosome can vary from so little that no symptoms of the corresponding syndrome are observed,to a significant part of one or both arms. Ring 18 can therefore appear to be like 18q- or 18p- or a combination of both. There are no unique features of this syndrome. The karyotype results should be able to detect a significant loss and therefore make a prediction on which symptoms to expect.