Molecular Brain Research, 4 (1988) 283-292 Elsevier 283 BRM 70104 Genetic mapping and analysis of somatostatin expression in Snell dwarf mice Bruce F. O’Hara’, Caterina BendottilY4, Roger H. Reeveb2, Mary Lou 0ster-Granite’32 Josepth T. Coyle2 and John D. Gearhart’ ‘Developmental Genetics Laboratory, Department of Physiology, ‘Department of Neuroscience, ‘Department of Cell Biology and Anatomy and of Gynecology and Obstetrics, Johns Hopkins University School of Medicine, Baltimore, MD 21205 (U.S.A.) and 4Mario Negri Institute, Milan (Italy) (Accepted 26 July 1988) Key words: Snell’s dwarf; Somatostatin; Gene mapping; In situ hybridization; Somatostatin analog Mice homozygous for the gene dwarf (dw ) have elevated levels of somatostatin (SS) in extra-hypothalamic brain regions. By in situ hybridization, increased levels of SS mRNA were observed in regions shown previously to contain higher levels of the SS peptide. Thus, the rate of transcription and/or the stability of SS mRNA are affected by the dw mutation. Since both dw and the gene encoding SS, Smst, are located on mouse chromosome 16, two backcrosses segregating dw and Smst were used to determine whether dw is an al- lele of Smst. In one backcross, an inbred strain derived from the subspecies Mus musculus molossinus was used to provide a high de- gree of DNA sequence polymorphism. The gene order and map distances determined on this backcross were: (centromere) - Prm-I - 7- &l-I - 3 - Smst - 29 - dw - 15 - Sod-l - 4 - Ets-2, demonstrating clearly that Smst and dw are distinct genes. Additional evidence against a primary role for SS excess in the pathogenesis of dw ldw mice was obtained by injecting normal newborn mice with a potent SS analog (cycle II). In contrast to the pattern of cell loss observed in the dwarf anterior pituitary, the pituitaries of injected mice were in- distinguishable from normal controls, further suggesting that the Smst locus is not the primary site of dw gene action. INTRODUCTION The dwarf gene (dw) was originally described by Snell in 1929 (ref. 43). Mice of the DW/J strain ho- mozygous for the dw mutation (dwldw) have severely diminished anterior pituitary function with low, or no production of growth hormone, prolactin, or thyroid stimulating hormone2.6,36,41,42. Such deficiencies re- sult in severe growth retardation; adult dwarf mice are about one quarter the size of their normal litter- mates. While the dwarf brain is partially spared from this growth retardation24,47, several neurochemical ab- normalities do occur15’20.26. Notably, a striking alter- ation in SS expression is observed in adult dwarf mice, with excess levels in extrahypothalamic brain regions and reduced levels in the hypothalamusr4. Presum- ably, the reduced levels of SS in the hypothalamus are secondary to growth hormone deficiency, since they are also observed after hypophysectomy’9,35. This pattern of SS expression led Fuhrmann et al. l4 to suggest that SS excess during critical developmental periods could be the primary molecular defect in dwarf mice. This hypothesis seemed plausible based on ontogenetic studies indicating a developmental role for SS’8*39.48. In this context, abnormal levels of SS in the developing brain might affect adversely the differentiation of those anterior pituitary cells (soma- totrophs, lactotrophs, and thyrotrophs) whose func- tion is modulated by SS2’*44, cells which in dwarf mice are lost. The mapping of the gene encoding preprosomatos- tatin (Smst) to mouse chromosome 16 (MMU 16)22, the same chromosome on which dw is located9,10*23, Correspondence: J.D. Gearhart, Dept. of Physiology, P202, Johns Hopkins University School of Medicine, 725 N. Wolfe St., Balti- more, MD 21205, U.S.A. 0169-328X/88/$03.50 0 1988 Elsevier Science Publishers B.V. (Biomedical Division)