Clinical Background

The verifi^® prenatal test now provides you the option to add testing for fetal sex chromosomes in addition to chromosomes 21, 18, and 13. Measurement and reporting of sex chromosomes is optional; the verifi^® prenatal test will continue to report on chromosomes 21, 18, and 13 when this option is not selected.

What are sex chromosome aneuploidies?

Sex chromosome aneuploidies consist of a group of chromosomal abnormalities where there is a variation from the usual two sex chromosomes. The most common sex chromosome aneuploidies include Monosomy X (Turner syndrome), XXY (Klinefelter syndrome), XYY, and XXX. Sex chromosome aneuploidies occur in one in 300-400 live births, making them about twice as common in newborns as trisomy 21.

Monosomy Xⁱ(also referred to as Turner syndrome) is a chromosomal condition in which females are missing all or part of the second “X” chromosome. Roughly half of Turner syndrome cases are due to the complete absence of the second sex chromosome. The other half are due to either mosaicism (“Turner mosaicism”) or a structural abnormality of the X chromosome. Prenatally, fetuses with Monosomy X may have ultrasound findings such as cystic hygroma, congenital heart defects, and renal malformations. Many Turner fetuses, especially those with abnormal ultrasound findings, can end in spontaneous miscarriage or fetal demise. Turner syndrome occurs in approximately 1 in 2500 live female births.

Adults with Monosomy X will likely have short stature and primary ovarian failure resulting in infertility. Learning disabilities are also possible, however most patients have normal intelligence. Individuals with Turner syndrome may be treated with growth hormone and hormone replacement therapy, if indicated after consultation with an endocrinologist.

Klinefelter syndromeⁱ (XXY) is the most common sex chromosome aneuploidy in newborns, occurring in about 1 in 600 live births. Klinefelter syndrome is caused by an extra X chromosome, resulting in a 47, XXY karyotype. Most often, the prenatal ultrasound will be normal.

Adults with Klinefelter syndrome will be phenotypically male and will likely have hypogonadism (most often resulting in infertility). They are also at risk for gynecomastia. Speech, language and motor delays are also common. Advanced maternal age is associated with Klinefelter syndrome, but to a lesser degree than in autosomal aneuploidies.

Males with Klinefelter syndrome can be treated with testosterone replacement therapy, if indicated. Studies suggest that treatment started around the age of puberty can help a patient have more normal body development, but is unlikely to reverse infertility.

XXXⁱ (Triple X syndrome or Trisomy X), is a chromosomal condition due to the addition of an extra X chromosome, resulting in a 47, XXX karyotype. It has an incidence of 1 in 1000 live births. Prenatal ultrasound is usually normal. Advanced maternal age is associated with XXX.

Adults with XXX are phenotypically female, can be tall in stature (taller than average, but not unusually tall), and are at risk for motor and language delays as well as learning difficulties. Fertility is usually normal.

XYYⁱ (Jacob’s syndrome) is a condition caused by an extra Y chromosome, resulting in a 47, XYY karyotype. It occurs in approximately 1 in 1000 live births. Most often, the prenatal ultrasound will be normal.

Patients with XYY are phenotypically male, can be tall in stature (taller than average, but not unusually tall), are at an increased risk for learning difficulties, and behavioral development is variable. Fertility is normal.

Are sex chromosome aneuploidies inherited?

No, they are typically due to non-disjunction during gametogenesis and are a sporadic chromosomal event; but advanced maternal age risks exist for XXY and XXX.

Individuals with a sex chromosome aneuploidy may have a higher risk of having a child with a sex chromosome abnormalityⁱ.

verifi^® prenatal test with Sex Chromosomes Option

What are the benefits of the sex chromosomes option?

• Until now sex chromosome aneuploidies could only be determined invasively. The sex chromosomes option provides a non-invasive alternative which may be preferable to many patients.
• Early diagnosis leads to early intervention and treatment. Due to the latent nature of these syndromes, many individuals may not be diagnosed early in childhood. This may lead to a delay in recognition of learning difficulties among other issues.
• Non-invasive prenatal testing that includes testing for sex chromosomes can provide an alternative method of identifying an underlying etiology in cases when invasive testing may not be possible.

Will there be a change in turnaround time or price with the new addition to the test menu?

No. There is no additional cost for the sex chromosome option. Turnaround time remains the same, 8-10 days from sample receipt.

How can I indicate that I want the sex chromosomes option for my patient?

The original verifi^® test will remain for chromosomes 21, 18, and 13. The sex chromosome option will be an optional second check on the test request form, as depicted below.

Can we add on the sex chromosome option for a patient if it wasn’t ordered initially?

Add-ons are not typically possible, unless requested very early in sample processing. Please contact Client Services to discuss any urgent requests regarding add-ons, or cancellations of samples.

Why are the sensitivity and specificity for the sex chromosomes option different from the study in Obstetrics & Gynecology?

For comprehensive performance data for the verifi^® prenatal test and sex chromosomes option specifically, please see Verinata publication Analytical Validation of the verifi^® prenatal test: Enhanced Test Performance for Detecting Trisomies 21, 18, and 13 and the Option for Classification of Sex Chromosome Status for details.

Since publication of the clinical validation results, Verinata’s research team has analyzed and implemented several changes to the testing procedure that yield enhanced test performance. These changes include:

• Incorporating new DNA sequencing chemistry (number of sequence tags is 2.4x greater)
• Further optimizing NCV calculations through increased counting statistics
• Expanding the verifi^® test based on adjustment of sex chromosomes classification to reflect laboratory enhancements, allowing for a clear 6-zone classification

What are the other options to detect sex chromosome aneuploidies during pregnancy?

The verifi^® test with the sex chromosome option is the first non-invasive test to detect fetal sex chromosome aneuploidies. Presently, the only definitive tests to detect fetal sex chromosome aneuploidies are invasive tests (amniocentesis and CVS), which can pose a risk to the fetus and mother. (Monosomy X can present with ultrasound abnormalities such as a cystic hygroma, however, in other sex chromosome aneuploidies, ultrasound findings are usually normal.)

Does the verifi^® prenatal test report fetal sex?

Fetal sex will only be reported if the sex chromosomes option is ordered. If the sex chromosomes option is ordered, and no sex chromosome aneuploidies are detected, the result is reported as either XX or XY. It is for the provider and patient to decide if the fetal sex information is to be revealed to the patient.

Knowing fetal sex can assist with prenatal testing decisions when a pregnancy is at-risk for an X-linked disease. Determination of the fetal sex can also potentially — assist in pregnancy management in cases of disorders of sexual development such as ambiguous genitalia.

Can you report the fetal sex without reporting sex chromosome aneuploidies?

No. Fetal sex is only analyzed when the sex chromosomes option is ordered. This option includes six analysis categories (XX, XY, XXY, XXX, XYY, and Monosomy X). There is no option to request testing for only fetal sex analysis on the verifi^® prenatal test.

How are results reported for the verifi^® prenatal test?

For chromosomes 21, 18, and 13, the results will state either “No aneuploidy detected”, “ANEUPLOIDY SUSPECTED (Borderline Value)”, or “ANEUPLOIDY DETECTED”. For the sex chromosomes, the results will state either “No aneuploidy detected” or “ANEUPLOIDY DETECTED”.

What is an ‘ANEUPLOIDY SUSPECTED (Borderline Value)’ result?

For chromosomes 21, 18, and 13, there is a small chance that the verifi^® test could report a particular chromosome as ‘ANEUPLOIDY SUSPECTED (Borderline Value)’, indicating that the result for that chromosome was a borderline value between ‘No aneuploidy detected’ and ‘ANEUPLOIDY DETECTED’. This result gives an indication to the provider and patient that although there is a higher risk for fetal aneuploidy than a ‘No aneuploidy detected’ result, the chances of a false positive are slightly greater than results in the ‘ANEUPLOIDY DETECTED’ zone. Both ‘ANEUPLOIDY SUSPECTED (Borderline Value)’ and ‘ANEUPLOIDY DETECTED’ results should be confirmed by an invasive procedure if a more definitive diagnosis is desired. In validation studies, ‘ANEUPLOIDY SUSPECTED (Borderline Value)’ as a result occurred in 0.2% to 0.6% per chromosome.

How results are reported for the sex chromosomes option?

The sex chromosomes option will be reported in one of the six categories — Monosomy X, XXX, XXY, XYY, or in the absence of a sex chromosome aneuploidy, XX or XY.

If a sex chromosome aneuploidy is detected, does a patient still need to have a follow-up invasive procedure?

If a definitive diagnosis is desired, Verinata recommends confirmation of a positive result with an invasive procedure such as amniocentesis or CVS.

Will there still be the option to add in testing for Monosomy X only, without getting information about the other sex chromosome abnormalities?

Monosomy X by itself is no longer an option; it is only available as part of the sex chromosomes option.

Why do you no longer require the presence of a fetal cystic hygroma to test for Monosomy X?

The algorithm now includes identification of all sex chromosomes. Most other sex chromosome aneuploidies are not associated with a cystic hygroma, and therefore it is no longer a requirement.

ⁱGenetic Disorders and the Fetus: Diagnosis, Prevention, and Treatment. Sixth Edition, ^©2010; Milunsky and Milunsky; pp. 273-300