Since 18p- was first described in 1963, much progress has been made in our understanding of this classic deletion condition. We have been able to establish a fairly complete picture of the phenotype when the deletion breakpoint occurs at the centromere, and we are working to establish the phenotypic effects when each gene on 18p is hemizygous. Our aim is to provide genotype-specific anticipatory guidance and recommendations to families with an 18p- diagnosis. In addition, establishing the molecular underpinnings of the condition will potentially suggest targets for molecular treatments. Thus, the next step is to establish the precise effects of specific gene deletions. As we look forward to deepening our understanding of 18p-, our focus will continue to be on the establishment of robust genotype-phenotype correlations and the penetrance of these phenotypes. We will continue to follow our 18p- cohort closely as they age to determine the presence or absence of some of these diagnoses, including spinocerebellar ataxia (SCA), facioscapulohumeral muscular dystrophy (FSHD), and dystonia. We will also continue to refine the critical regions for other phenotypes as we enroll additional (hopefully informative) participants into the research study and as the mechanisms of the genes in these regions are elucidated. Mouse models will also be developed to further our understanding of the effects of hemizygosity as well as to serve as models for treatment development.

Thus far, the phenotype of tetrasomy 18p has been primarily delineated by published case series and reports. Findings reported in more than 25% of these cases include neonatal feeding problems, growth retardation, microcephaly, strabismus, muscle tone abnormalities, scoliosis/kyphosis, and variants on brain MRI. Developmental delays and cognitive impairment are universally present. The purpose of this study was to more fully describe tetrasomy 18p at both the genotypic and the phenotypic levels. Array CGH was performed on 43 samples from individuals with tetrasomy 18p diagnosed via routine karyotype. The medical records of 42 of these 43 individuals were reviewed. In order to gain additional phenotypic data, 31 individuals with tetrasomy 18p underwent a series of clinical evaluations at the Chromosome 18 Clinical Research Center. Results from the molecular analysis indicated that 42 of 43 samples analyzed had 4 copies of the entire p arm of chromosome 18; one individual was also trisomic for a section of proximal 18q. The results of the medical records review and clinical evaluations expand the phenotypic description of tetrasomy 18p to include neonatal jaundice and respiratory distress; recurrent otitis media; hearing loss; seizures; refractive errors; constipation and gastroesophageal reflux; cryptorchidism; heart defects; and foot anomalies. Additional findings identified in a small number of individuals include hernias, myelomeningocele, kidney defects, short stature, and failure to respond to growth hormone stimulation testing. Additionally, a profile of dysmorphic features is described. Lastly, a series of clinical evaluations to be considered for individuals with tetrasomy 18p is suggested.

C-4.

One of our primary goals is to help families who have a child with an 18q deletion anticipate medical issues in order to optimize their child’s medical care. To this end we have narrowed the critical regions for four phenotypic features and determined the penetrance for each of those phenotypes when the critical region for that feature is hemizygous. We completed molecular analysis using oligo-array CGH and clinical assessments on 151 individuals with deletions of 18q and made genotype–phenotype correlations defining or narrowing critical regions. These nested regions, all within 18q22.3 to q23, were for kidney malformations, dysmyelination of the brain, growth hormone stimulation response failure, and aural atresia. The region for dysmyelination and growth hormone stimulation response failure were identical and was narrowed to 1.62 Mb, a region containing five known genes. The region for aural atresia was 2.3 Mb and includes an additional three genes. The region for kidney malformations was 3.21 Mb and includes an additional four genes. Penetrance rates were calculated by comparing the number of individuals hemizygous for a critical region with the phenotype to those without the phenotype. The kidney malformations region was 25% penetrant, the dysmyelination region was 100% penetrant, the growth hormone stimulant response failure region was 90% penetrant with variable expressivity, and the aural atresia region was 78% penetrant. Identification of these critical regions suggest possible candidate genes, while penetrance calculations begin to create a predictive phenotypic description based on genotype.