Trisomy 13

Excerpt

Trisomy 13 is a chromosomal aneuploidy originally described by Patau et al. in 1960. The occurrence of trisomy 13 is 1 in 10,000 to 20,000 live births with antenatal mortality of over 95% of gestations. It can occur as complete, partial, or mosaic expression. The complete trisomy is the most common presentation representing about 80% of all patients. This expression characteristically demonstrates the presence of three chromosomes 13 copies. The partial expression is characterized by a Robertsonian translocation t(13;14), while only 5% of all cases present with mosaicism. Mosaicism is characterized by a percentage of cells remaining trisomic while others maintain euploidy.

Trisomy 13 arises from the nondisjunction of germ cells during meiosis I or II of either parental cells. Nonetheless, maternal germ cell nondisjunction correlated to the increased age of conception contributes to 91% of cases. The mode of inheritance for the complete trisomy 13 is caused by spontaneous interference in meiosis, while vertical inheritance is hereditary in balanced translocations.

Phenotypic findings in trisomy 13 are associated with patterns of congenital anomalies and mental disabilities incompatible with life. The embryological defects in trisomy 13 develop in the absence of fusion of prechordal mesoderm, which phenotypically presents as midline defects. These midline defects are associated with aberrant SHH genes. Despite the accelerated mortality of trisomy 13, it remains clinically significant due to its variable expressivity in patients with compatible mosaicisms.