Identification of mosaic partial trisomy 12p (Pallister-Killian Syndrome) by FISH analysis. C.W. Yu1, O.B. Evans2, H. Huang1, C. Thompson1, O.A. Abdul-Rahman1 1) Dept Preventive Medicine; 2) Dept Pediatrics, Univ of Mississippi Med Ctr, Jackson, MS.

   Pallister-Killian Syndrome (PKS) was first reported in 1977 with mosaicism in fibroblasts for an extra chromosome composed of 12p. PKS was characterized by profound psychomotor retardation, inability to sit or speak, seizures, and joint contractures. We report a case of partial duplication of 12p in an infant with unusual chromosome rearrangements identified from blood lymphocytes. A Caucasian female infant was born at 39 weeks gestation to a 22 year-old primigravida mother and 22 year-old father. The mother experienced tonsillitis during the pregnancy, but was otherwise uncomplicated. The infant could not sit or crawl at nine months and was hospitalized for seizures at ten months. On physical examination at 11 months, the infant had a head circumference 45.3 cm, height of 74.6 cm, and weight of 12.6 kg. The dysmorphic features included a board forehead with high anterior hairline, hypertelorism, flat nasal bridge, short upturned nose, down turned corners of the mouth, short fingers, single transverse crease in the right hand, and hypotonia. Chromosomes from PHA stimulated blood cultures were analyzed. Five of the eighty G-banded metaphases examined had extra material on the terminal short arm of chromosome 12. G-banding suggested that it might be a translocation from chromosome 21 or a duplication of 12p. Both parental chromosomes were normal. Tissue chromosome study was not available. FISH studies were done using locus specific probes for loci TEL (12p13.1) and AML1 (21q22), telomere probes for 12pter and 12qter, and whole chromosome panting probes for 12. FISH and cytogenetic studies suggest that the derivative 12 possibly has double interstitial duplications from 12p12.2 to 12p13.31, which might have resulted from repeated DNA replications within a crossing-over loop in a somatic cell. The infant has mosaic tetrasomy of segment 12p12.2 to 12p13.31 (three on the derivative chromosome 12 and one on the normal chromosome 12) and has clinical features of PKS, further demonstrating the critical genomic regions and the phenotypic expressions.