Abstract: Molecular Testing in Fetuses with Holoprosencephaly and Normal Karyotype
Meeting: ISPD 2012; Miami, FL
Presenter(s): Jeanne Meck, Daniel Pineda Alvarez, Swaroop Aradhya, Rachel Klein, Carlos Benito, Gabriele Richard
Objectives: Our objective was to determine the frequency, type, and spectrum of pathogenic mutations in fetuses with sonographic features of holoprosencephaly (HPE) and normal karyotype by multigene testing. HPE, the most common malformation of the forebrain with a frequency of 1/250 conceptuses, can be readily detected by prenatal ultrasound. While chromosome abnormalities are found in 25-50% of postnatal cases, that number is likely higher in severe cases with prenatal presentation potentially leading to miscarriage. Point mutations and microdeletions in the four major HPE-associated genes (SHH, ZIC2, SIX3, TGIF) have been reported in 23% of prenatally detected HPE (Bendavid et al., 2006).
Method: We reviewed the results of 75 karyotypically normal fetuses with sonographic features of HPE referred for molecular testing 2003 – 2011. Initially, only sequence analysis of the SHH, ZIC2, SIX3, and TGIF genes was performed (n=33). More recently, samples (n=42) were also tested by MLPA for deletions or duplications of these four genes as well as PTCH1, FBXW11, and TMEM1.
Results: A pathogenic mutation was identified in 28/75 (37%) prenatal HPE cases. Only one copy number mutation, a de novo duplication of SHH, was found. Testing revealed mutations in SHH (11), ZIC2 (9), SIX3 (4), and TGIF(3), six of which had occurred de novo. 43% of mutations were inherited (8 maternal, 4 paternal), including 7 of 11 SHH mutations. In the remaining 10 cases, parental testing was not performed.
Conclusions: After exclusion of chromosome abnormalities, sequencing and deletion/duplication analysis of the four major HPE genes can determine the cause of HPE in approximately 1/3 of cases. Parental testing is important as illustrated by the high frequency of inherited mutations, consistent with the variable HPE phenotype that may go undetected. This information is critical for genetic counseling, recurrence risk determination, and future testing. A recent NICHD multicenter study recommended targeted chromosomal microarray replace chromosome analysis in abnormal fetuses since it has a higher detection rate of chromosome abnormalities. Therefore, we suggest that the combination of targeted microarray and the HPE genetic testing panel described here will rapidly detect the causative genetic abnormality in approximately 75% of prenatal HPE cases.