Am. J. Hum. Genet. 57:743-74 7, 1995

INVITED EDITORIAL

Pigmentation, Pleiotropy, and Genetic Pathways in Humans and Mice

Gregory S. Barsh

Departments of Pediatrics and Genetics and the Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford

Some of the most striking polymorphisms in human groundwork that proved crucial in identifying a genetic- populations affect the color of our eyes, hair, or skin. map position for HPS was laid by the late Carl Witkop, Despite some simple lessons from high school biology who carried out an extensive characterization of HPS in (blue eyes are recessive; brown are dominant), the ge- Puerto Rico, where the gene frequency is -1/21 (Witkop netic basis of such phenotypic variability has, for the et al. 1990a, 1990b). most part, eluded Mendelian description. A logical place Wildenberg et al. (1995) hypothesized that HPS was to search for the keys to understanding common varia- introduced into Puerto Rico by a single mutation-bear- tion in human pigmentation are genes in which defects ing chromosome, and therefore they asked whether 16 cause uncommon conditions such as (Oetting unrelated HPS patients shared alleles for polymorphic and King 1994; King et al. 1995) or (Spritz molecular markers distributed throughout the genome. et al. 1992; Spritz 1994). The area under this lamppost To increase the efficiency of their search, they focused has recently gotten larger, with two articles (Fukai et al. on chromosomes that represented the likely locations of 1995; Wildenberg et al. 1995), one in this issue of the mouse mutations with phenotypic similarity to HPS, and journal, that describe the map position for Hermansky- they used pooled DNA samples, a simple and effective Pudlak syndrome (HPS) and with the recent cloning of approach that has also been applied to mouse and plant a gene that causes X-linked (QA1). In genetics. A strong association was found between HPS addition, a series of three recent articles in Cell (Baynash and an allele for the marker D10S677. Subsequent test- et al. 1994; Hosoda et al. 1994; Puffenberger et al. ing of additional markers in this region in 12 Puerto 1994a) demonstrate (1) that defects in the gene encoding Rican kindreds by conventional linkage analysis pro- the endothelin B (ETB) receptor cause duced LOD scores of 5.07 and 4.18 for two markers, and Hirschsprung disease in a Mennonite population Dl0S298 and D101239, that were nonrecombinant and the mouse mutation piebald (s) and (2) that a defect with each other and with HPS. in the edn3 gene, which encodes one of the ligands for Fukai et al. (1995) also examined unrelated Puerto the ETB receptor, causes the lethal spotting (ls) mouse Rican HPS patients for shared alleles and identified a mutation. nonrandom association with D10S677. Additional The phenotypic abnormalities caused by defects in markers and the power of linkage disequilibrium were endothelin signaling once again illustrate the importance used to identify chromosomes likely to represent histori- of embryology in understanding genetic conditions, for cal recombinants, suggesting that HPS in Puerto Rico it is only in the context of common cell ancestry-the lies within a 0.6-cM region between Dl0S577 and neural crest-that enteric ganglia and are D10S198, on 10q23. Fukai et al. (1995) also found a so obviously related. Pleiotropy looms even larger in nonrandom association between different alleles of the HPS .(Hermansky and Pudlak 1959), an autosomal re- same markers and HPS in an isolated Swiss village (Lat- cessive condition characterized by albinism, platelet- tion et al. 1983; Schallreuter et al. 1993). Neither ceroid storage-pool deficiency, and an accumulation of ceroid- storage nor the systemic manifestations that compro- lipofuscin in reticuloendothelial cells that leads to pul- mise life expectancy in Puerto Rican HPS are found in monary fibrosis and granulomatous colitis (Witkop et the Swiss families. Allelic heterogeneity is a logical ex- al. 1989; also see Pearson et al. 1994). Much of the planation for these differences, but the authors note that the genetics of HPS-like conditions in mice makes such a conclusion premature, since there are no fewer than 13 Received August 8, 1995; accepted for publication August 8, 1995. distinct loci that produce pigment dilution and platelet- Address for correspondence and reprints: Dr. Gregory S. Barsh, storage-pool deficiency in mice (Novak et al. 1984; Department of Pediatrics, Stanford University Medical Center, Beck- Swank et al. 1991). HPS has been described in diverse man Center 271A, Stanford, CA 94305-5428. © 1995 by The American Society of Human Genetics. All rights reserved. ethnic populations (reviewed in Witkop et al. 1989), 0002-9297/95/5704-0001$02.00 and it would be surprising if locus heterogeneity did not 743 744 Am. J. Hum. Genet. 57:743-747, 1995 apply, in this situation, to humans as well as to mice. novel gene products. The reciprocal situation applies in Notably, two of the mouse mutations, ruby eye (ru) and the case of gene targeting with mouse embryonic stem pale ears (ep), are closely linked (6 recombinants among cells, as exemplified by the surprising phenotype of mu- 457 animals tested) and are predicted to have human tations in endothelin 3 (ET-3) and the ETBreceptor (Bay- homologues on 10q2 (O'Brien et al. 1994). nash et al. 1994; Hosoda et al. 1994). Endothelins and In both HPS and the ru-like group of mutations, a their G protein-coupled receptors were originally iden- common cell lineage cannot explain abnormalities of tified by Yanagisawa and colleagues because of their melanocytes, platelets, and macrophages. Instead, it potent vasopressor activities (Yanagisawa et al. 1988; seems likely that biogenesis of their intracellular organ- Masaki and Yanagisawa 1992; Sakurai et al. 1992). elles depends on a set of common genes. Cloning of the However, knockout mutations of ET-3 and of the ETB HPS gene (or 1 of the 13 mouse mutations that produce receptor, described by Baynash et al. (1995) and Hosoda an HPS-like phenotype) is likely to reveal more about et al. (1994), respectively, produced developmental ab- pathways of subcellular morphogenesis shared in com- normalities of the neural crest, with a phenotype that mon by melanosomes, platelet granules, and lysosomes. was not predicted but was recognized as similar to that Of course, some gene products required for melanosome produced by the lethal spotting (Is) or the piebald-lethal biogenesis will not be used in other cell types; in fact, one (sl) mutations, which are genetically distinct but produce of these is likely to have been identified with the recent identical phenotypes of piebald spotting and intestinal cloning of the X-linked OA1 gene, also known as the aganglionosis (reviewed in Silvers 1979). The proof of "Nettleship-Falls" type (Nettleship 1909; Falls 1951). As allelism between edn3 and lethal spotting (ls) and be- is true in other forms of albinism, males affected with OA1 tween ednrb and piebald (s) was provided by phenotypic have very poor visual acuity and optic-tract misrouting noncomplementation and by molecular analyses and (Creel et al. 1978). Melanocytes in affected individuals demonstrated that endothelin-signaling pathways are usually contain giant pigment-filled organelles referred to crucial for several different aspects of neural crest devel- as "macromelanosomes" or " macroglobules," opment. which are thought to represent an underlying defect in In work that dramatically illustrates the utility of our melanosome biogenesis (O'Donnell et al. 1976; Garner small furry friends for readers of the Journal, Puffen- and Jay 1980; Cortin et al. 1981; Schnur et al. 1994). berger et al. (1994a) recently showed that abnormalities Macromelanosomes have also been described in Swiss, but in endothelin signaling produce pigmentary abnormalit- not Puerto Rican, HPS patients (see Witkop et al. 1989). ies and intestinal aganglionosis in humans as well as in Thus, the gene products that are defective in OA1 and mice. Absence of intestinal autonomic innervation in HPS may have similar functions, but action of the OA1 humans, or Hirschsprung syndrome (HSCR), is both gene is probably limited to pigment cells. genetically heterogeneous and incompletely penetrant, Rearrangements and deletions of the X chromosome with segregation analysis providing evidence for autoso- provided the crucial tool in narrowing the search for the mal recessive, autosomal dominant, and polygenic in- OA1 gene to a 110-kb critical region within Xp22.3- heritance and an overall sibling recurrence risk of 4% 22.2 (Schaefer et al. 1993; Wapenaar et al. 1993; Bassi (Badner et al. 1990). Some families with dominant et al. 1994). However, Ballabio and colleagues were HSCR show linkage to 10q1l (HSCR1), where loss-of- frustrated during what they describe as an "arduous function mutations in the RET protooncogene are likely three year effort" (Bassi et al. 1995) by a nonpenetrant to be the underlying defects (Edery et al. 1994; also patient (who, in retrospect, redefined the critical region) see Pearson et al. 1994). In addition, Chakravarti and and by an excellent candidate gene, APXL, which, un- colleagues had previously identified HSCR susceptibility fortunately, spans 80 kb and encodes a nearly 8-kb loci at 13q22 (HSCR2) and 21q22 (HSCR3), using iden- mRNA (Schiaffino et al. 1995). The APXL gene was tity-by-descent analyses and linkage-disequilibrium eventually excluded as a candidate by analysis of its mapping to study a large Mennonite kindred (Puffen- structure in 57 OA1 patients, and the real OA1 gene berger et al. 1994b). was recognized as a 1.7-kb mRNA spanning 20 kb, in Notably, many individuals in the Mennonite kindred which nonoverlapping intragenic deletions were identi- also exhibit a white forelock, heterochromia irides, and fied in 5 OA1 patients (Bassi et al. 1995). The OA1 gene sensorineural deafness, otherwise known as "Waarden- predicts a 424-amino-acid protein with six membrane- burg syndrome" (Dow et al. 1994; Puffenberger et al. spanning domains but has no similarity to previously 1994a). Abnormalities in endothelins or their receptors identified sequences, and it is not yet clear why this gene would not have been considered logical candidates to product is required for melanosome formation. explain pigmentary abnormalities and HSCR, but allel- A successful positional cloning effort, as with OA1, ism of lethal spotting (ls) and ednrb in mice provided a offers the opportunity to make unforeseen connections beacon for HSCR in the Mennonite kindred, since the from well-characterized pigmentation phenotypes to human EDNRB gene had previously been mapped to Barsh: Invited Editorial 745

Waardenburg Syndrome (WSI) Hirschuprung Syndrome

Weardenburg Syndrome (WS2) melanoblaast Plebaldism ZZ-

I e- Hermansky- Pudlak Syndrome Ocular Albinlsm Oculocutaneous Albinlem

Figure I Genetic and embryologic pathway for abnormalities in pigmentation. The neural crest provides precursors for both pigment cells and the enteric nervous system, and action of the EDNRB gene prior to divergence of these two cell types is likely to explain the pleiotropy observed in (Pavan and Tilghman 1994; Hosoda et al. 1994). In contrast, mutations that produce HPS, ocular albinism, and act relatively late in pigment-cell differentiation, and the pleiotropy observed in HPS must therefore result from a common process in subcellular morphogenesis used by platelets, macrophages, and melanocytes (for discussion, see Witkop et al. 1989). The double-headed arrows and dotted lines represent candidate genes for the genetic interactions described by Pavan et al. (1995). Potential roles and times of action for two forms of the Mgfgene product are described by Steel et al. (1992) and Wehrle and Weston (1995). Comprehensive references to the involvement of PAX3 and MITF in Waardenburg syndrome can be found in the work of Pearson et al. (1994). Defects in the Mclr gene cause pigmentation abnormalities in mice (Robbins et al. 1993) but have not yet been described in humans. human chromosome 13. After demonstrating a signifi- bility genes may act independently to modify the likeli- cant association of two EDNRB RFLPs with HSCR in hood of enteric and nonenteric abnormalities. the Mennonite kindred, Puffenberger et al. (1994a) iden- As with the HSCR2 locus, these genes may already tified a hypomorphic missense mutation, W276V, that have been identified in mice, on the basis of recent work reduced the ability of the ETB receptor to evoke a change by Pavan et al. (1995). Homozygosity for piebald (s), a in calcium flux in response to agonist. The authors' pop- hypomorphic allele associated with an -75% reduction ulation-genetic analyses suggest that this defect accounts in ETB mRNA levels (Hosoda et al. 1994), produces for approximately two-thirds of HSCR in the Mennon- only a white forelock and a white abdominal patch in ite population and exhibits semidominant inheritance the common C3H strain of mice. However, selection and with reduced penetrance: homozygotes and heterozy- inbreeding of highly spotted s/s mice from an unknown gotes for the W276V mutation have a 74% and 21% genetic background by Thomas Mayer produced ani- chance, respectively, of developing HSCR. mals in which melanocytes were missing from virtually It now seems likely that defects in the EDN3 or the the entire midportion of the animal (Mayer 1965). F1 EDNRB gene will be found in patients with the Shah- hybrids between the C3H and Mayer strains are indis- Waardenburg syndrome (Shah et al. 1981), an autoso- tinguishable from parental C3H animals. Pavan et al. mal recessive condition characterized by a white fore- (1995) backcrossed F1 animals to the Mayer strain and lock, heterochromia irides, and HSCR (but not deaf- then determined whether the extent of spotting among ness), and in a Kurdish sibship, recently described by backcross progeny could be correlated with inheritance Gross et al. (1995), in which affected patients have of one or more regions of the genome from the Mayer nearly complete albinism, deafness, and intestinal agan- strain. glionosis. In the Mennonite kindred, some W276V ho- Six modifier loci were identified, of which four ac- mozygotes have HSCR without pigmentary abnormalit- counted for 90% of the increased spotting of Mayer ies, and some W276V homozygotes have pigmentary strain compared with C3H animals. Notably, excellent abnormalities without HSCR. Thus, additional suscepti- candidates for two of these modifier loci are the Domi- 746 Am. J. Hum. Genet. 57:743-747, 1995 nant spotting (W) and Steel (SI) genes, which encode the Hammer RE, Yanagisawa M (1994) Interaction of endo- c- receptor and its ligand, mast cell growth factor thelin-3 with endothelin-B receptor is essential for develop- (Mgf), respectively. Mutations in KIT, the human ho- ment of epidermal melanocytes and enteric neurons. Cell mologue of Dominant spotting (W), are responsible for 79:1277-1285 Cortin P, Tremblay M, Lemagne JM (1981) X-linked ocular autosomal dominant piebald trait (Spritz et al. 1992), albinism: relative value of skin biopsy, iris transillumination and it would not be surprising to find that common and funduscopy in identifying affected males and carriers. allelic variants of KIT (or MGF) influence the likelihood Can J Ophthalmol 16:121-123 of pigmentary abnormalities in the Mennonite kindred Creel D, O'Donnell FJ, Witkop CJ (1978) Visual system anom- described by Chakravarti and colleagues. alies in human ocular albinos. Science 201:931-933 The opportunity in mice to examine mutant gene com- Dow E, Cross S, Wolgemuth DJ, Lyonnet S, Mulligan LM, binations and cell markers at specific times in develop- Mascari M, Ladda R, et al (1994) Second locus for Hirsch- ment allows the construction of genetic and embryologic sprung disease/Waardenburg syndrome in a large Mennon- pathways for pigmentation. Work by Pavan and Tilgh- ite kindred. Am J Med Genet 53:75-80 man (1994) suggests that the ednrb gene is required very Edery P. Lyonnet S, Mulligan LM, Pelet A, Dow E, Abel L, early in neural crest development, whereas work by Steel Holder S, et al (1994) Mutations of the RETproto-oncogene Weston in Hirschsprung's disease. Nature 367:378-380 et al. (1992) and Wehrle and (1995) indicates Falls HF (1951) Sex-linked ocular albinism displaying typical that the Mgf gene is required somewhat later. Attempts fundus changes in the female heterozygote. Am J Ophthal- to construct such pathways (fig. 1) are potentially lim- mol 34:41 ited by their reductionist nature. In fact, if the Kit, Mgf, Fukai K, Oh J, Frenk E, Almodovar C, Spritz RA (1995) Link- or Mclr gene product is shown to interact with the ETB age disequilibrium mapping of the gene for Hermansky- receptor at the molecular level (an interaction denoted Pudlak syndrome to chromosome 10q23.1-q23.3. Hum Mol by double-headed arrows in fig. 1), pigmentation genet- Genet 4:1665-1670 ics may be more aptly represented by a network than Garner A, Jay BS (1980) Macromelanosomes in X-linked ocu- by a pathway. lar albinism. Histopathology 4:243-254 Muddy analogies aside, it is abundantly clear that a Gross A, Kunze J, Maier RF, Stoltenburg-Didinger G, Grim- common cell lineage explains the phenotypic pleiotropy mer I, Obladen M (1995) Autosomal-recessive neural crest syndrome with albinism, black lock, cell migration disorder of Waardenburg syndrome, whereas a common process of the neurocytes of the gut, and deafness: ABCD syndrome. of subcellular morphogenesis explains phenotypic plei- Am J Med Genet 56:322-326 otropy in HPS. These and other abnormalities of pig- Hermansky F, Pudlak P (1959) Albinism associated with hem- mentation in humans and in mice are a rich genetic orrhagic diathesis and unusual pigmented reticular cells in resource that is just beginning to be mined, and they the bone marrow: report of two cases with histochemical promise to offer continued insight into long-standing studies. Blood 14:162 questions of both cell biology and embryology. Hosoda K, Hammer RE, Richardson JA, Baynash AG, Cheung JC, Giaid A, Yanagisawa M (1994) Targeted and natural (piebald-lethal) mutations of endothelin-B receptor gene Acknowledgments produce megacolon associated with spotted coat color in mice. Cell 79:1267-1276 I thank members of my laboratory and my colleagues who King RA, Hearing VJ, Creel DJ, Oetting WS (1995) Albinism. study pigment cells in a variety of experimental organisms, for In: Scriver CR, Beaudet AL, Sly WS, Valle DV (eds) The many helpful discussions. I am also grateful to Carolyn Scha- metabolic basis of inherited disease, 7th ed. McGraw-Hill, nen for bringing the family described by Gross et al. to my New York, pp 4353-4392 attention and for helpful comments on the manuscript. Lattion F, Schneider P, Da PM, Lorez HP, Richards JG, Picotti GB, Frenck E (1983) Hermansky-Pudlak syndrome in a Va- References lais village. Helv Paediatr Acta 38:495-512 Masaki T, Yanagisawa M (1992) Endothelins. Essays Biochem Badner JA, Sieber WK, Garver KL, Chakravarti A (1990) A 27:79-89 genetic study of Hirschsprung disease. Am J Hum Genet Mayer TC (1965) The development of piebald spotting in 46:568-580 mice. Dev Biol 11:319-334 Bassi MT, Bergen AA, Wapenaar MC, Schiaffino MV, van Nettleship E (1909) On some hereditary diseases of the eye. SM, Yates JR, Charles SJ, et al (1994) A submicroscopic Trans Ophthalmol Soc UK 29:59 deletion in a patient with isolated X-linked ocular albinism Novak EK, Hui SW, Swank RT (1984) Platelet storage pool (OA1). Hum Mol Genet 3:647-648 deficiency in mouse pigment mutations associated with Bassi MT, Schiaffino MV, Renieri A, De Nigris F, Galli L, seven distinct genetic loci. Blood 63:536-544 Bruttini M, Gebbia M, et al (1995) Cloning of the gene for O'Brien EP, Novak EK, Keller SA, Poirier C, Guenet JL, Swank ocular albinism type 1 from the distal short arm of the X RT (1994) Molecular map of chromosome 19 including chromosome. Nat Genet 10:13-19 three genes affecting bleeding time: ep, ru, and bm. Mamm Baynash AG, Hosoda K, Giaid A, Richardson JA, Emoto N, Genome 5:356-360 Barsh: Invited Editorial 747

O'Donnell FJ, Hambrick GJ, Green WR, Iliff WJ, Stone DL and long segment Hirschsprung disease: possible variant of (1976) X-linked ocular albinism: an oculocutaneous macro- Waardenburg syndrome. J Pediatr 99:432-435 melanosomal disorder. Arch Ophthalmol 94:1883-1892 Silvers WK (1979) White spotting: piebald, lethal spotting, Oetting WS, King RA (1994) Molecular basis of oculocuta- and belted. In: The coat colors of mice. Springer, New York, neous albinism. J Invest Dermatol Suppl 131S-136S pp 185-205 Pavan WJ, Mac S, Cheng M, Tilghman SM (1995) Quantita- Spritz RA (1994) Molecular basis of human piebaldism. J In- tive trait loci that modify the severity of spotting in piebald vest Dermatol Suppl 137S-140S mice. Genome Res 5:29-41 Spritz RA, Holmes SA, Ramesar R, Greenberg J, Curtis D, Pavan WJ, Tilghman SM (1994) Piebald lethal (sl) acts early Beighton P (1992) Mutations of the KIT (mast/stem cell to disrupt the development of neural crest-derived melano- ) proto-oncogene account for a con- cytes. Proc Natl Acad Sci USA 91:7159-7163 tinuous range of phenotypes in human piebaldism. Am J Pearson P, Francomano C, Foster P, Bocchini C, Li P, McKu- Hum Genet 51:1058-1065 sick V (1994) The status of Online Mendelian Inheritance Steel KP, Davidson DR, Jackson IJ (1992) TRP-2/DT, a new in Man (OMIM) media. Nucleic Acids Res 22:3470-3473 early melanoblast marker, shows that steel growth factor Puffenberger EG, Hosoda K, Washington SS, Nakao K, deWit (c-kit ligand) is a survival factor. Development 115:1111- D, Yanagisawa M, Chakravart A (1994a) A missense muta- 1119 tion of the endothelin-B receptor gene in multigenic Hirsch- Swank RT, Reddington M, Howlett 0, Novak EK (1991) sprung's disease. Cell 79:1257-1266 Platelet storage pool deficiency associated with inherited ab- Puffenberger EG, Kauffman ER, Bolk S, Matise TC, Washing- normalities of the inner ear in the mouse pigment mutants ton SS, Angrist M, Weissenbach J, et al (1994b) Identity- muted and mocha. Blood 78:2036-2044 by-descent and association mapping of a recessive gene for Wapenaar MC, Bassi MT, Schaefer L, Grillo A, Ferrero GB, Hirschsprung disease on human chromosome 13q22. Hum Chinault AC, Ballabio A, et al (1993) The genes for X-linked Mol Genet 3:1217-1225 ocular albinism (OA1) and microphthalmia with linear skin Robbins LS, Nadeau JH, Johnson KR, Kelly MA, Rosellireh- defects (MLS): cloning and characterization of the critical fuss L, Baack E, Mountjoy KG, et al (1993) Pigmentation regions. Hum Mol Genet 2:947-952 phenotypes of variant extension locus alleles result from Wehrle HB, Weston JA (1995) Soluble and cell-bound forms point mutations that alter MSH receptor function. Cell of steel factor activity play distinct roles in melanocyte pre- 72:827-834 cursor dispersal and survival on the lateral neural crest mi- Sakurai M, gration pathway. Development 121:731-742 T, Yanagisawa Masaki T (1992) Molecular char- Wildenberg SC, Oetting acterization of endothelin receptors. Trends Pharmacol Sci WS, Almadovar C, Krumwiede M, 13:103-108 White JG, King RA (1995) A gene causing Hermansky- L, Pudlak syndrome in a Puerto Rican population maps to Schaefer Ferrero GB, Grillo A, Bassi MT, Roth EJ, Wapen- chromosome 10q2. Am J Hum Genet 57:000-000 aar MC, van OG, et al (1993) A high resolution deletion Witkop CJ, Almadovar C, Pineiro B, Nunez BM (1990a) Her- map of human chromosome Xp22. Nat Genet 4:272-279 mansky-Pudlak syndrome (HPS): an epidemiologic study. Schallreuter KU, Frenk E, Wolfe LS, Witkop CJ, Wood JM Ophthal Paediatr Genet 11:245-250 (1993) Hermansky-Pudlak syndrome in a Swiss population. Witkop CJ, Nunez BM, Rao GH, Gaudier F, Summers CG, Dermatology 187:248-256 Shanahan F, Harmon KR, et al (1990b) Albinism and Her- Schiaffino MV, Bassi MT, Rugarli EI, Renieri A, Galli L, Bal- mansky-Pudlak syndrome in Puerto Rico. Bol Assoc Med P labio A (1995) Cloning of a human homologue of the Xeno- R 82:333-339 pus laevis APX gene from the ocular albinism type 1 critical Witkop CL, Quevedo WC, Fitzpatrick TB, King RA (1989) region. Hum Mol Genet 4:373-382 Albinism. In: Scriver CR, Beaudet AL, Sly WS, Valle DV Schnur RE, Wick PA, Bailey C, Rebbeck T, Weleber RG, Wag- (eds) The metabolic basis of inherited disease, 6th ed. staff J, Grix AW, et al (1994) Phenotypic variability in X- McGraw-Hill, New York, pp 2905-2947 linked ocular albinism: relationship to linkage genotypes. Yanagisawa M, Kurihara H. Kimura S, Tomobe Y, Kobayashi Am J Hum Genet 55:484-496 M, Mitsui Y, Yazaki Y, et al (1988) A novel potent vasocon- Shah KN, Dalal SJ, Desai MP, Sheth PN, Joshi NC, Ambani strictor peptide produced by vascular endothelial cells. Na- LM (1981) White forelock, pigmentary disorder of irides, ture 332:411-415