The hereditary macular dystrophies are progressive degenerations of the central retina and contribute significantly to irreversible visual loss in developed countries. Among these disorders, Sorsby's fundus dystrophy (SFD), an autosomal dominant condition, provides an excellent mendelian model for the study of the genetically complex age-related macular degeneration (AMD), the most common maculopathy in the elderly. Recently, we mapped the SFD locus to 22q13-qter. This same region contains the gene for tissue inhibitor of metalloproteinases-3 (TIMP3), which is known to play a pivotal role in extracellular matrix remodeling. We have now identified point mutations in the TIMP3 gene in affected members of two SFD pedigrees. These mutations are predicted to disrupt the tertiary structure and thus the functional properties of the mature protein.

A mouse somatostatin (SS) receptor cDNA was cloned from neuroblastoma x glioma (NGI08-15) cells. The sequence is almost identical to that of the mouse SgTR2 r~eeptor [(1992) Proc, Nail, Acad, Sci, USA 89, 251)l bat lacks about 300 nu01eotides between transmembran¢ domain VII and the C-terminus. This spliced variant of SSTR2 (designated SSTR2B) encodes a protein which is 23 residues shorter than that predicted from the SSTR2 sequence, and differs in 15 amino acids at the C-terminus. mRNA corresponding to SSTR2B occurs in mouse tissues in higher abundance than that of SSTR2, SSTR2B binds SS peptides with high aflinit), when expressed in mammalian cells,

The somatostatin receptor 2 (mSSTR2) is alternatively spliced into two isoforms (mSSTR2A and mSSTR2B) which differ at the C-terminus. Both receptors bind somatostatin peptides with a similar high affinity when stably expressed in CHO-Kl cells. However, the spliced form (mSSTR2B) mediates a more efficient inhibition of adenylate cyclase and is much more resistant to agonist-induced reduction of binding than the longer form (mSSTR2A). These findings indicate that alternative splicing may be a physiological mechanism to modulate receptor desensitization and G-protein coupling of mSSTR2.

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Sorsby's fundus dystrophy (SFD) is an autosomal dominant macular degeneration developing in the third or fourth decade. Patients lose central vision from subretinal neovascularization and atrophy of the choriocapillaris, pigment epithelium and retina. SFD shares some striking clinical features with age-related macular degeneration (AMD), the most common cause of blindness in western countries thereby providing a valuable genetic model for AMD. To map the SFD locus, we performed linkage analysis in a single large SFD family. After exclusion of approximately 65% of the autosomal genome, we found significant linkage to several markers from chromosome 22. Recombinant chromosomes sublocalize the SFD gene to 22q13-qter between D22S275 and D22S274.