Chromosome analysis for infertility and miscarriage patients
Some reproductive problems are caused by chromosome abnormalities. Therefore, many reproductive endocrinologists will recommend a chromosome analysis for infertility and miscarriage patients.
Basic information about DNA and chromosomes
Inside of our cells there is a nucleus which contains genetic information. This genetic information is exists as a code determined by the sequence of molecules connected in long stands known as DNA. The DNA strands form a structure known as a double helix. Picture two stairway bannisters wrapping around each other.
The DNA helix is extremely long. It is therefore, usually densely packed and packaged into structures known as chromosomes.
Scientists known as cytogeneticists can use different dyes to “stain” the chromosome. This staining creates a pattern that can be used to identify the chromosome and to determine if there are any abnormalities in the structure of the chromosome. The most common type of staining done by cytogeneticists today uses a chemical called Giemsa. This is known as G-Banding. G-banding produces what look like light and dark stripes along the length of the chromosome. These stripes or “bands” will produce a highly distinctive unique appearance to each chromosome.
Normal and abnormal chromosomes
Normal human beings have 46 chromosomes aligned as 23 pairs. 23 come from the father and 23 from the mother. If you have ever seen a picture of a densely packed chromosome, they look like a capital “X”. The place where the lines cross is called the centromere.
Chromosomes are assigned a long arm and a short arm, based on the position of their centromeres. The shorter arm of the chromosome is known as the p, or petite arm. The longer arm is known as the q, or queue arm.. There are two types of chromosome abnormalities: numeric and structural.
Numeric abnormalities include
Individual extra chromosomes – For example, instead of two copies of chromosome 21 there are three copies. The total number of chromosomes is 47 (instead of 46)
Individual missing chromosomes – for example, instead of two copies of chromosome 21 there is only one copy. The total number of chromosomes is 45 (instead of 46)
Extra sets of chromosomes- for example, instead of two copies of every chromosome, there are three copies of every chromosome. The total number of chromosomes is 69 (instead of 46)
Chromosome structural abnormalities
Duplications / Insertions (Additional genetic material in a chromosome)
Deletions (Missing genetic material in a chromosome)
Inversions (A section of genetic material that has been flipped around within a chromosome)
Translocations (Genetic material that has moved from its usual location on one chromosome to a location on a different chromosome)
Chromosome mapping and resolution
Each arm of the chromosome is then divided into regions which are numbered, and the numbers assigned to each region get larger as the distance from the centromere to the end of the chromosome increases. Regions are identified by the presence of prominent Giemsa-staining bands. The regions are named p1, p2, etc., on the short arm and q1, q2, etc., on the long arm.
Depending on the resolution of the staining procedure, it is possible to detect additional bands within each region, which are designated by adding another digit to the number of the region, once again increasing in value as the distance from the centromere increases.
In the picture, the banding resolution of fthe chromosome on the left is low (350 bands), the middle chromosome is intermediate (550 bands) and the one on the right is high resolution (850 bands).
Chromosome analysis with higher resolution can detect smaller abnormalities (such as deletions, duplications, translocations or inversions). Thus, a chromosome analysis, or karyotype, can be divided into low, medium and high resolution based on the number of bands that can be seen. (picture)The vast majority of karyotypes offered by cytogenetics labs around the world are either low or medium resolution. This is unfortunate because many couples with infertility or recurrent miscarriage may have small abnormalities that will not be detected by low or medium resolution karyotypes.
Who should have a high resolution karyotype?
There is considerable disagreement regarding the answer to this question. First, a group that everyone would agree on are couples with recurrent miscarriage. These couples have an increase in the chance for having translocations. It is estimated that translocations occur in about 1 in every 600 people. In couples with recurrent miscarriage the rate is 18-24 times higher using low resolution techniques. It is likely much higher if high resolution techniques are used.
Other groups that have been suggested as needing a high resolution karyotype:
— Couple undergoing pre-implantation genetic testing of their embryos (PGD, PGS, CCS)
–Couples who have family members with unexplained birth defects
–Couples with multiple IVF failures
What about other technologies?
In recent years, there has been an explosion of new technologies that look for a variety of genetic problems. Each technique has certain advantages and disadvantages. For example, there are two types of microarrays that are popular and widely available now: CGH microarray and SNP microarray. These are even more sensitive than a high resolution karyotype for detecting things like deletions and insertions. However, they are completely unable to detect balanced translocations.
FISH, or Flourescent in-situ hybridization, is very good for confirming chromosome abnormalities once a specific location is suspected, finding very small problems around the centromeres and ends of chromosomes and for clarifying very complex translocations and insertions but FISH is not very good for wide scale screening all areas of all of the chromosomes.
There are also companies which offer screening for large numbers of genetic mutations. This type of testing is different than chromosome testing. These tests look for known specific gene mutations which cause specific diseases such as specific fibrosis. This information derived with these tests is not helpful for identifying couples with fertility or miscarriage issues but rather is of use for couples looking to determine if they carry mutations which might cause specific diseases in their children.
Until further technological developments are available. many experts believe that a combination of technologies may offer the best approach for screening couples with infertility and recurrence miscarriage. IVF1 utilizes labs that perform a 850 band high resolution karyotype as well as multiple types of FISH for unusual abnormalities.