Trisomy 5 Mosaicism
Posted October 30, 2008on:
TRISOMY 5 MOSAICISM
Complete trisomy 5 is a rare and lethal abnormality. Trisomy 5 mosaicism detected on chorionic villus sampling of a normally developed embryo is usually confined to the placenta. Confined placental mosaicism for trisomy 5 has been associated with normal outcome with no evidence of the trisomy in newborn blood. Diagnosis of trisomy 5 on CVS should be followed up with amniocentesis and serial ultrasound.
Trisomy 5 mosaicism has occasionally been detected on amniocentesis and has been associated with both normal outcome and with children born with multiple congenital anomalies. Fetal blood sampling may not be helpful for detecting true trisomy 5 mosaicism, since in a past case trisomy 5 was detected in fibroblasts and not in peripheral blood (Hsu, 1997).
Trisomy 5 detected on CVS
Three cases of confined placental mosaicism for trisomy 5 were reported in a large study of chromosomal mosaicism detected on chorionic villus sampling. The findings were not confirmed in any of the fetuses (Hahnemann and Vejerslev, 1997).
Fryburg et al (1993) described a case of trisomy 5 detected on cultured CVS. Amniocentesis was normal and low level mosaic trisomy was seen in the fetus. In most caseswhere trisomy is found on CVS but not on amniocentesis, the outcome is normal. However, an abnormal outcome can also occur.
Trisomy 5 detected on amniocentesis
Hsu et al (1997) reviewed 5 reported cases of trisomy 5 mosaicism detected at amniocentesis. Two cases were born with health concerns. One child had was small, had a heart murmur and an ear pit. The other child also was born small with facial dysmorphism and a congenital heart defect. In both of the abnormal cases trisomy 5 was confirmed in fibroblasts. The percentage of trisomy cells in the amniocytes did not seem to predict pregnancy outcome. In one of these cases 80% of amniocytes showed trisomy 5 cells and the pregnancy resulted in a normal liveborn (Hsu et al, 1997).
Uniparental Disomy (UPD 5)
There are no reports of imprinted genes on chromosome 5 (Ledbetter & Engel, 1995).
Paternal isodisomy for chromosome 5 was detected in a 2-year-old boy with Type III Spinal Muscular Atrophy (SMA), an autosomal recessive degenerative disorder of the motor neurons. No developmental abnormalities, other than those attributable to classical childhood-onset SMA, were present (Brzustowicz et al, 1994) .