An Overview of Karyotyping

Its Role in Diagnosis and Prenatal and Predictive Screening

Fluorescence light micrograph of the 46 chromosomes from a normal human female. This constitutes the full karyotype, that is the total number of chromosomes found in nearly every cell of the female human body. Each cell contains 22 matched pairs of chromosomes, and one sex-determining pair (bottom right). Female and male karyotypes differ only in the sex pair: male sets would be labeled XY instead of the X pair here.

Dept. Clinical Cytogenics, Addensbrook Hospital/Science Photo Library/Getty Images

A karyotype is, quite literally, a photograph of the chromosomes that exist within a cell. Chromosomes are the thread-like structures in the nucleus of cells which we inherit from our parents and carry our genetic information in the form of genes. Genes direct the synthesis of proteins in our bodies which determine how we look and how our bodies function. Any error in the genetic coding may affect our development and the way we function. In some cases, it can place us at an increased risk of a disease or a physical or intellectual defect.

A doctor may order a karyotype during pregnancy to screen for common congenital defects. It is also sometimes used to help confirm a leukemia diagnosis. Less commonly, a karyotype is used to screen parents before they conceive if they are at risk of passing a genetic disorder to their baby.

Depending on the aim of the testing, the procedure may involve a blood test, bone marrow aspiration, or such common prenatal procedures as amniocentesis or chorionic villus sampling.

What a Karyotype Tells Us

A karyotype is the characterization of chromosomes based on their size, shape, and number. All humans typically have 46 chromosomes, 23 of which we inherit from our mothers and fathers, respectively. The first 22 pairs are called autosomes, which determine our unique biological and physiological features. The 23rd pair is sex chromosomes (known as X or Y) which designate whether we are female or male.

Chromosomal defects occur when a cell divides during fetal development. Any division occurring in the reproductive organs is called meiosis. Any division occurring outside of the reproductive organs is called mitosis.

The type of chromosomal defects can be broadly defined as being either numerical or structural. While numerical abnormalities are those in which you either have too few or too many chromosomes, structural abnormalities can encompass a wide range of chromosomal flaws, including:

  • Deletions, in which a portion of a chromosome is missing
  • Translocations, in which a chromosome is not where it should be
  • Inversions, in which a portion of a chromosome flipped in the opposite direction
  • Duplications, in which part of a chromosome is accidentally copied

Numerical Abnormalities

Some people are born with either an extra or missing chromosome. If there are more than two chromosomes where there should only be two, this is called a trisomy. If there is a missing or damaged chromosome, that is a monosomy.

Among some of the numerical abnormalities a karyotype can detect are:

  • Down syndrome (trisomy 21), in which an extra chromosome 21 causes distinctive facial features and intellectual disabilities
  • Edward syndrome (trisomy 18), in which the extra chromosome 18 translates to a high risk of death before the first birthday
  • Patau syndrome (trisomy 13), in which an extra chromosome 18 increases the likelihood of heart problem, intellectual disability, and death before the first year
  • Turner syndrome (monosomy X), in which a missing or damaged X chromosome in girls translates to a shorter height, intellectual disability, and an increased risk of heart problems
  • Klinefelter syndrome (XXY syndrome), in which an extra X chromosome in boys can cause infertility, learning disabilities, and underdeveloped genitalia

Structural Abnormalities

Structural abnormalities are not as commonly seen or identified as trisomies or monosomies but can be every bit as serious. Examples include;

  • Charcot-Marie-Tooth disease, caused by a duplication of chromosome 17, leading to reduced muscle size, muscle weakness, and motor and balance difficulties
  • Chromosome 9 inversion, associated with intellectual disability, facial and skull malformation, infertility, and recurrent pregnancy loss
  • Cri-du-Chat syndrome, in which the deletion of chromosome 5 causes delayed development, small head size, learning impairment, and distinctive facial features
  • Philadelphia chromosome, caused by the reciprocal translocation of chromosomes 9 and 22, resulting in a high risk of chronic myeloid leukemia
  • Williams syndrome, in which the translocation of chromosome 7 causes intellectual disability, heart problems, distinctive facial features, and outgoing, engaging personalities

The expression of structural chromosomal abnormalities is vast. For example, anywhere from 2 percent to 3 percent of Down syndrome cases are caused by a translocation on chromosome 21. However, not all chromosomal abnormalities confer to illness. Some, in fact, may be beneficial.

One such example is sickle cell disease (SCD) caused by a defect on chromosome 11. While inheriting two of these chromosomes will lead to SCD, having just one can protect you against malaria. Other defects are believed to provide protection against HIV, stimulating the production of broadly neutralizing HIV antibodies (BnAbs) in a rare subset of infected people.

Indications

When used for prenatal screening, karyotypes are typically performed during the first trimester and again in the second trimester. The standard panel tests for 19 different congenital diseases, including Down syndrome and cystic fibrosis.

Karyotypes are sometimes used for preconception screening under specific conditions, namely:

  • For couples with a shared ancestral history of a genetic disease
  • For couples in whom one partner has a genetic disease
  • For couples in whom one partner is known to have an autosomal recessive mutation (one that can only cause disease if both partners contribute the same mutation)

Karyotyping is not used for routine preconception screening but rather for couples whose risk is considered high. Examples include Ashkanzi Jewish couples who are at high risk of Tay-Sachs disease or African American couples with a family history of sickle cell disease. Couples who are either unable to conceive or experience recurrent miscarriage may also undergo parental karyotyping if all other causes have been explored and excluded.

Finally, a karyotype may be used to confirm chronic myeloid leukemia in association with other tests. (The presence of the Philadelphia chromosome on its own cannot confirm the cancer diagnosis.)

How Karyotypes Are Performed

A karyotype can theoretically be performed on any body fluid or tissue, but, in clinical practice, samples are obtained in four ways:

  • Amniocentesis involves in the insertion of a needle into the abdomen to obtain a small amount of amniotic fluid from the womb. The procedure is performed between weeks 15 and 20 of the pregnancy and guided by an ultrasound to avoid harm to the fetus. While relatively safe, amniocentesis is associated with a one-in-200 risk of miscarriage.
  • Chorionic villus sampling (CVS) also uses an abdominal needle to extract a sample of cells from placental tissues. Typically performed between weeks 10 and 13, CVS carries a one-in-100 risk of miscarriage.
  • Phlebotomy is the medical term for a blood draw. The blood sample is usually obtained from a vein in your arm, which is then exposed to ammonia chloride to isolate leukocytes (white blood cells) for karyotyping. Injection site pain, swelling, and infection are possible.
  • Bone marrow aspiration may be used to aid in the diagnosis of chronic myeloid leukemia. It either performed by inserting a needle into the center of the hip bone. It is performed under local anesthesia as an in-office procedure. Pain, bleeding, and infection are among the possible side effects.

    After the sample is collected, it will be analyzed in a lab by a specialist known as a cytogeneticist. The process begins by growing the collected cells in a nutrient-enriched media. Doing so helps pinpoint the stage of mitosis in which the chromosomes are most distinguishable.

    The cells are then placed on a slide, stained with a fluorescent dye, and positioned under the lens of an electron microscope. The cytogeneticist will then take microphotographs of the chromosomes and re-arrange the images like a jigsaw puzzle to correctly match the 22 pairs of autosomal chromosomes and two pairs of sex chromosomes.

    Interpreting the Results

    Once the images are correctly positioned, they will be evaluated to determine whether any chromosomes are missing or added. The staining can also help reveal structural abnormalities, either because the banding patterns on the chromosomes are mismatched or missing or because the length of a chromosomal "arm" is longer or shorter than the other.

    Any abnormality will be listed on the karyotype report by the chromosome involved and the characteristics of the abnormality. These findings will be accompanied by "possible," "likely," or "definitive" interpretations. Some conditions can be definitively diagnosed with a karyotype; others cannot.

    Results from a prenatal karyotype take between 10 and 14 days. Others are usually ready within three to seven days. While your doctor will usually review the results with you, a genetic counselor may be on-hand to help you better understand what the results mean and do not mean. This is especially important if a congenital disorder is detected or preconception screening reveals an increased risk of an inheritable disease if you have a baby.

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