Mosaic trisomy 8, also called Warkany syndrome, is a chromosomal abnormality in which there is an extra 8 chromosome in certain cells of the body. Symptoms, causes, incidence, screening… Everything you need to know about trisomy 8.

Trisomy is a chromosomal abnormality characterized by the presence of an extra chromosome in a pair of chromosomes. Indeed, in humans, a normal karyotype (all the chromosomes of a cell) consists of 23 pairs of chromosomes: 22 pairs of chromosomes and one pair of sex chromosomes (XX in girls and XY in boys ).

Chromosomal abnormalities form at the time of fertilization. Most of them result in spontaneous abortion during pregnancy because the fetus is not viable. But in some trisomies, the fetus is viable and the pregnancy continues until the baby is born. The most common trisomies at birth are trisomies 21, 18 and 13 and mosaic trisomy 8. Sex chromosome trisomies are also very common with several possible combinations:

• Trisomy X or triple X syndrome (XXX);
• Le syndrome de Klinefelter (XXY) ;
• Jacob’s syndrome (XYY).

Mosaic trisomy 8 affects between 1 in 25 and 000 in 1 births. It affects boys more than girls (50 times more). This chromosomal abnormality is manifested in children by moderate mental retardation (in some cases) associated with deformities in the face (facial dysmorphia) and osteoarticular abnormalities.

Mental retardation is manifested by sluggish behavior in children with mosaic trisomy 8.

Facial dysmorphia is characterized by:

• a high and prominent forehead;
• the elongated face;
• a broad, upturned nose;
• a large mouth with a peculiar lower lip, fleshy and curved outwards;
• droopy eyelids and eye strabismus;
• a small recessed chin marked by a horizontal dimple;
• ears with a large pavilion;
• a wide neck and narrow shoulders.

Anomalies of the extremities are also frequent in these children (club feet, hallux valgus, flexion contractures, deep palmar and plantar folds). In 40% of cases, abnormalities of the urinary tract are observed and in 25% of cases of abnormalities of the heart and large vessels.

People with mosaic trisomy 8 have a normal life expectancy in the absence of severe deformities. However, this chromosomal abnormality appears to predispose carriers to Wilms tumors (malignant kidney tumor in children), myelodysplasias (bone marrow disease) and myeloid leukemias (blood cancers).

The care is multidisciplinary, each child having specific problems. Cardiac surgery may be considered in the presence of operable cardiac abnormalities.

Apart from trisomy 21, prenatal screening for trisomies is possible by performing a fetal karyotype. This must always be done in agreement with the parents after a medical consultation for genetic counseling. This test is offered to couples at high risk for these abnormalities:

• either the risk is foreseeable before the onset of pregnancy because there is a family history of chromosomal anomaly;
• either the risk is unpredictable but prenatal chromosome screening (offered to all pregnant women) revealed that the pregnancy was in a risk group or else abnormalities were detected on ultrasound.

The realization of the fetal karyotype can be done:

• or by taking amniotic fluid via amniocentesis from 15 weeks of pregnancy;
• or by performing a choriocentesis also called a trophoblast biopsy (removal of the precursor tissue of the placenta) between 13 and 15 weeks of pregnancy.