Health In Focus
  • Current prenatal testing includes testing for chromosome abnormalities mostly limited to chromosomes 13, 18, 21, X and Y.
  • Any normal cell in the body has 23 pairs of chromosomes and any deviation results in diseases like Downs Syndrome and the like
  • Scientists have shown that genetic tests aimed at testing all the chromosomes detect rare trisomies may be suggestive of miscarriages and other pregnancy related diseases.

Prenatal testing is used to determine if the unborn baby is healthy inside the womb. It helps detect birth defects and genetic diseases in the baby before birth. Most pregnant mothers prefer non-invasive testing like blood tests rather than the invasive procedures like amniocentesis where fluid is taken from the amniotic sac.
Genetic Testing During Non-invasive Prenatal Testing Uncovers Rare Autosomal Trisomies

However, in a small number of patients the non-invasive prenatal testing can be complicated, as they are either not in accord with the fetal karyotype or the test fails. Scientists have discovered that instead of testing for the common genetic defects in specific chromosomes, if all chromosomes are tested rare abnormalities may be identified that are suggestive of feto-placental diseases. The study is published in the journal Science Translational Medicine.

Non-invasive prenatal testing (NIPT)

Prenatal testing is done to check the health of the unborn baby. Prenatal genetic testing is usually only done in case there is a prior issue related to the pregnancy. Prenatal genetic testing is recommended when there is any of the following;
  • Previous child with a genetic defect
  • Increased maternal age
  • Family history of genetic disease or affected children
  • Miscarriages and abortions
While there are a number of methods for genetic testing including amniocentesis and chorionic villus sampling, these methods are invasive. They require that the sample, either amniotic fluid or the chorionic villi be taken from the pregnant mother. These procedures do have a minimal risk of miscarriage or abortion.

Most women today prefer non-invasive prenatal testing (NIPT) to avoid these risks. NIPT is the genetic analysis of fetal DNA by collecting maternal blood. The analysis checks if there are any deviations in the fetal DNA from the normal.

Genetic Analysis:

Genetic tests include whole genome sequencing, targeted sequencing and microarray check for deletions, duplications, repeats among others. However, the most common is the count of the chromosomes which is determined by a test called karyotyping. A normal cell contains 22 pairs of somatic chromosomes and two X chromosomes in case of a girl and X and Y in case of a boy. Genetic defects are caused due to any deviation from this number. Trisomies are very common; in this case instead of a pair of chromosomes, there is three.

The Problem with NIPT:

Despite the increased accuracy of genetic testing when compared to the non-genetic assays, a small percentage of the fetal DNA from maternal blood tests can be complicated. This is either due to atypical findings or discordance with the fetal karyotype.

Standard NIPT typically considers only the chromosomes that are most frequently found to be abnormal, for example trisomic (triple) - chromosome 13, 18, 21, X and Y.

The current study conducted by the National Institute of Health and National Human Genome Research Institute have showed that extending the NIPT to all the chromosomes in the cell can detect genetic disorders that may explain miscarriage and abnormalities during pregnancy.

Latest Study Findings:

The research team studied two independent cohorts. 89,817 maternal plasma samples were analyzed; of these 72,972 came from the U.S cohort and 16,885 came from the Australian cohort.

While genetic testing of all chromosomes showed several abnormalities, including copy number variants and deletions, trisomy of the non-target chromosomes was the single most common finding between both cohorts. Trisomies that were not associated with the typical target chromosomes including chromosomes 13, 18, 21, X and Y, were called rare autosomal trisomy.

Trisomy 7 was the most frequently observed rare autosomal trisomy in both cohorts followed by 15, 16 and 22. Clinical data indicated that rare autosomal trisomies were associated with early miscarriage, in-utero fetal demise, intrauterine growth restriction etc.

Reference :
  1. Pertile,M. D., Halks-Miller, M., Flowers, N., Barbacioru, C., Kinnings, S. L., Vavrek, D.,Bianchi, D. W. (2017). Rare autosomal trisomies, revealed by maternal plasma DNA sequencing, suggest increased risk of feto-placental disease. Science Translational Medicine,9(405). doi:10.1126/scitranslmed.aan1240

Source: Medindia

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