Holoprosencephaly (HPE) covers a nearly continuous spectrum of midline abnormalities ranging from unmistakable cyclopia with absence of forebrain separation to mild microforms, such a single central incisor.
The objective of these studies is to identify genetic factors (coding and non-coding) that contribute to the pathogenesis of holoprosencephaly (HPE) or related brain malformations. Our approach involves common genetic strategies including mutational analysis of candidate genes. All individuals with overt or subtle clinical findings consistent with the HPE spectrum are eligible to participate. Mutational analysis of our entire coded collection of HPE probands (approximately 600 cases) in selected genes is the principal research method used to determine that a given candidate gene is commonly mutated in HPE. This approach pertains to an individual gene(s) or genetic element(s) as well as to targeted capture panels that study gene sets of hundreds of developmental genes whose involvement in brain development is supported by the basic research literature. We are also asking to include the option of investigating the entire set of genetic factors present in the DNA or RNA of patients who have HPE through a proposed change in the consent documents. Whenever a sequence variant is identified, that is not present in a commercially available control set of samples, attempts are made to test the functional significance of this change on the protein itself, or its expression. Sequence changes with a strong probability of being medically significant will be verified in a CLIA-approved lab (e.g. Muneke lab, for selected genes, or GeneDx) at our expense, before any results are given to the family through genetic counseling. Parental DNA (and rarely that of siblings) is usually obtained at the same time that a proband is enrolled. Typically, these samples are studied only to perform limited family studies once a sequence variant of potential medical significance has already been determined. Participation by direct blood relatives is encouraged since virtually all bone fide mutations are either family-specific or de novo. Such family information is critical for the research determination of genetic risk factors and accurate genetic counseling.
The majority of subjects enrolled in this study will continue under the care of their local physician or genetic counselor with limited contact with the NIH investigators. Only rarely will families be seen at the NIH CC. These visits will involve face-to-face genetic counseling of medically significant results, following verification in a CLIA approved lab. This is not a treatment protocol. Our empiric ability to generate medically significant research results is limited by the extensive genetic and other etiologic heterogeneity. Therefore, for most participants this research is not a diagnostic study.
We have modified our procedures to test all new probands for mutations in the four HPE genes (SHH, ZIC2, SIX3 and TGIF). As new genetic elements that confer a risk for HPE are identified, we intend to add additional tests to this panel. Our lab is now certified to receive and test new samples according to CLIA guidelines. However, all previously collected samples will not be considered suitable for diagnostic purposes; hence, a second sample will need to be requested in these cases for CLIA confirmation.