Array of Testing Characterizes Prenatal Diagnosis of Mosaic Tetrasomy 9p24q22.3 Associated With an Unusually Mild Phenotype and Favourable Outcome

ABSTRACT Background Tetrasomy 9p is a rare chromosomal disorder with distinct clinical features, but wide phenotypic variability. Historically, tetrasomy 9p has been detected by conventional cytogenetic analysis, but newer technologies such as non‐invasive prenatal testing ( NIPT ) and chromosome microarray ( CMA ) allow the identification of cases that would previously have gone undetected. Here we describe a girl with a mosaic isodicentric chromosome 9 (idic(9)), ascertained through NIPT , but with diagnosis not confirmed until after birth. Methods Chromosomal microarray (CMA) was performed on DNA extracted from saliva, buccal and blood samples from a clinically well newborn to confirm abnormal prenatal findings. Subsequent conventional G‐banded analysis was performed on stimulated T‐lymphocyte culture following diagnosis of mild speech delay and hearing loss. Results Copy number gain at 9p24.3q22.32 was identified on CMA in three independent samples. G‐banded analysis subsequently identified a supernumerary isodicentric chromosome 9 in one out of 105 cells examined, with breakpoints at 9p24 and 9q22.3, representing a partial tetrasomy of chromosome 9. Conclusion Tetrasomy 9p has wide phenotypic variability due to mosaicism. This unusually mild case of tetrasomy 9p would have almost certainly gone undetected prior to the implementation of NIPT and CMA. This report highlights the value of reporting cases ascertained through high‐resolution technologies to facilitate early diagnosis and prognosis and enable a better understanding of this condition through precise breakpoint mapping.