PTPS-07COMPETING MOLECULAR GENETIC FORCES: TRISOMY 21(DOWN SYNDROME, DS) AND PTCH1 MUTATION (GORLIN SYNDROME) IN A 21 MONTH-OLD WITH SHH DESMOPLASTIC/NODULAR MEDULLOBLASTOMA

Abstract

BACKGROUND: Individuals with Trisomy 21 have a striking cancer distribution, with a 10- to 20- fold increased risk of acute leukemia compared to a markedly decreased incidence of solid tumors. Medulloblastoma, the most common malignant brain tumor of childhood, is particularly rare in the DS population, with only one published case. As demonstrated in a mouse model of DS, the supernumerary chromosome 21 is associated with cerebellar hypoplasia and a decreased number of external granule cell layer progenitor cells (EGLs). Treatment of these mice with sonic hedgehog (Shh) signaling pathway agonists promotes normalization of EGLs and improved cognitive functioning. PATIENT: We describe a 21 month-old male with DS with desmoplastic/nodular medulloblastoma (DNMB) - a tumor derived from Shh dysregulation and over-activation of EGLs. Immunohistochemistry revealed patchy cytoplasmic GAB-1 staining and patchy strong YAP-1 nuclear and cytoplasmic staining, further placing this DNMB into the new consensus molecular classification Shh subgroup. Additional testing revealed a de novo heterozygous germ line mutation in the PTCH1 gene encoding a tumor suppressor protein in the Shh pathway; both parents tested negative for this mutation. CONCLUSIONS: The reduced presence of EGLs in DS patients offers a plausible explanation for the disparity of medulloblastoma in this population. Conversely, patients with PTCH1 germ line mutations experience Shh pathway overstimulation resulting in Gorlin (Nevoid Basal Cell Carcinoma) Syndrome and an increased incidence of malignant transformation of EGLs leading to medulloblastoma formation. This represents the first documented report of an individual with DS (developmental failure of EGLs, decreased incidence of medulloblastoma) simultaneously carrying PTCH1 germ line mutation (overstimulated EGLs, increased incidence of medulloblastoma). Thus, we observe a highly unusual circumstance in which the PTCH1 mutation appears to ‘trump’ the effects of trisomy 21 in causation of Shh-activated medulloblastoma. Additional genomic studies are ongoing and will be presented.