THE MOLECULAR BASIS OF OENETICS wy Presented at Ameria sooosiation seam a Omtasans Tkineday 2 May 2930 X an gaye that moet mental Gissase is chemioal in origin, end thet the ehunieal slmoranlities that are involved are neually the remit of aimorualities in the genetic constitution of the iniividual. I think that it te probable that mental ilinsss often results fren « quentisehs Mochesieal abnormality « the presense in the individual of molecules of a mabetanee that is normally present, but in concentrations semevhat Jevger then newsal er senevhat maller then moruel. Presumably, the masafecteure of the retention of the sbetance in too large aunt or too quell eneunt ia often the result of the genetic constitution of the ine @ividual, although in some cases it nay be attributed to his environment « for ammple, to the nature of the food tint he eats. On the other huni, mantel defistenay seems to be often the remilt of « gulitetive atnormalitys the presence in the patient of solesules that differ in their structure fren thoes that are present in « normal Inman beings The namufasture of shacrual wsiewles of this sort is determined by the genetie constitution of the patients the disease is inherited. A dinease of this sort, omsed ly malewules of shmoraal structure present in the patient in place of the molecules of normal structure that are present in norea) tnmen beings, is called a nalecular diseases The expression molecular disease is here used in a special way. All oie tamen beings are made up of molegules, ani in a sense one night say that all diseases invelve thes molecules, ani perhaps alec the uoleewles that make wp bacteria ami viruses, ani that assorfingly all diseases are neleo~ Wher diseases. The restvietion of the expressien noleqdlar disease to Giszeases that are due te abnormal molecules, differing smevhat in struse ture fren related meleeules that are presext in normal taman beings, seens to me to be « useftd one. Siekle-ecell anexia wae the first disease to be sham to be «a nelecular digense.? In this disease the red calls of the blood are tuisted out of shape, vhen they ave in the wenous elroulation = they regain their normal shape in the arteries, The tisted red cells besone stleky, they clap together, ani semstines interfere with the flow of bleod to some parte of the body and canes damge by anoxia they are aleo rapidly reneved fren the cireulation, causing the patient to beecms anemios The disease seens to be & disease ef the red cell, o cellular diseases hovever, it was found that in fast the hemeglobin molecules mamfnctured ty the patient are abnormal, differing signifieetly in their strustere and properties fren the heno~ Giobin moleeules sumfaetured by nemal individvals, ani it is elear that the disease is a dissase of the hemoglobin moleeuls. Although the eanplete molecular structure of siekle-eell-anenia hene- globin is not knows, nor, in feet, the structure of normal hemogickin or any ether protein, the way in whieh the aoleeular slnormlity causes the manifestations ef the disease siskie-cell anemia is elearly unierstoc?. The molecules of the siskle-~cell~eneia hemoglobin heve awh a structure that they clamp on to one anetiwr easily, to form long reda, vbieh line up aide ty side to produes a liquid erystal of the nmatde type. Aa thie liquid erystel «ojo grewa ineide of the red e012, it beceme longer than the diameter of the eell, end in ite comtimed growth it twiste the eel] out of shape. Theve is etweng evidenee that the eanbination of the maleoules of stekle-eull-anenia hemoglobin with ene another is censed by a detalled complementariness in etrueture of one part of the molecule and ancther past of the malesnle, sc that the weak intermolecular foreses that operate between protein molequles in general are able to collaborute when the sure feme region of one moleeule cones into juctapesition vith the complementary wurkaee region of « sseend molecule, forming a bend that helde the molecules tightly tegether. This eauplenentarinsss in structure is destroyed when the heneglobin molesule eambines vith exygeniy and accordingly the process of sickling of the eelis is reversed when the blood ia agensted. Gne ef the properties in vhieh slskle-cell-anmia hemeglobin differs frou normal adult laman hemoglobin is the electric charget molecules of the two kinds of hemoglobin differ fren one another by abut three elestrenta charges. The way in whieh sickle~-celie~anania hemoglobin was recognised as a wabstance vith different molecular structure fron normal adult humen heme globin was the meagurement of the nobility of the tve hexeglobins in an elestric field, weing the Tisslius electrophoresis apparatus. The genetie origin of the meleoular almormality was clearly indicated when the heme- globin from the parents of « siekleqwell-anenia patient vas stulied. The hemegiobin of each parent ves found to consist of a mixture of appremimately equal ancunta of the two kinks of hemegicbin. Acconlingly, the parents vere identified as hetercaygotes, containing two allelomorphic genes at seme jewel in a pair af chromosomes. One of these genes maumfactures normal etult human hemeglobin, ami the cther one mamafactares sickle-vcell-anenia »miw hewaglobin. The parents are in good health, eo far as the anenta is con eorned @ the dilution of the abnormal hemoglobin by the normal hemeglobin prevents the siekling process fron oecurring in the hetereaygetes, except under wagual conditions, as at very high altitudes, where the partial pressure of oxygen is lew. When two of the heteronygetes, the carriers of the siekle-cell~anenia gene, marry, one-quarter of their ehildren my be expected to have the disease sickle-eell amnesia, one-quarter to be normal, and one~half te be carriers, like the paventa. The question ef the eentinued high ineidense of the slekle-cell-enenia gene, despite its continued loss because of the lethal character of the homomygeus condition, has been raised by Keels* who suggested three alter native explanations: (1) eentiaued produstion of the siekle-cell-aneia allele though uutations (2) the existence of an abnormal genetic mechanisn that favers the heteromygene eentition over the nermmal coniitien; (3) a positive selection of the hetereaygotes, perhaps through insreased fertility. The first explanstion must be rejected becamee the rete of mutation that would be required is fer greater than any that has ever been checrved for any orgenim., There now existe evidence indicating that the third alter- native provides the ecrrest explanation, and that malaria is involwed. It was firet suggested ty Brain? that the mature of the red calls in the sickle-~sell-qnenia carriers might give protection agsinst malaria parasites, and thus confer an advantage that would balance the disetventage of the lethal homonygesity. 4 test of the hypothesis was carried out by ALlieon,* vbo infected fifteen healthy afult Africans vith siskle-cell-anmia heteroaygeaity ani fifteen sinilar healthy edult Africans vith nora] hesoglobin with Plagotiue faleiparys ty subincowlation with 15 ml. of a Sa iced containing a large number of tropbeseites or by allowing them to be bitten by heavily infested anopheles mosquitoes, in uhieh the presence of eporescites waa confirmed ty dissection of the mosquitoes. The infeo- tien wae established in fourteen out of the fifteen Africans without the elekle-cell~anesia hetereaygoaity, anid in oly two of the fifteen neal Afvieans. It was coneluded by Allieen that the almorsal erythreeytes of the hetercaygous individuals are less easily parasitioe by F. faleiparm than are nermal erythroaytes, and that aevordingly those individuals whe are hateroaygous fer the siekle-cell-anmile allele have a selective eiveantage over normal individuals in regions viere malaria is typerendenics It is, of eoures, not unreasonable that the ahnernal henoglobin might be less effective than normal hemoglobin in nourishing the parasites; moreover, it is knam that the parasitised erythrocyte uses up oxygen 100 tines as fast as a nomal erythrocyte, ani it night be expected, as aaggested by Allison, that the de-cxygenated erythroeyte vould aickle, ani thae crash the parasite. Ascordingly, ve have a molecular meehanian not only for the dissase siekle-eell-anenia, but also for the protection thet the heterenygous condition provides against malarial infection. Sinee the dissovery was mate of the first abnormal hemoglebin, about ten more have been diesevered, anit about a dosen diseases, kimis of heweditary hemelytic anemia, have been reeognined as caused by these ab- normal henogichins. I think that it is likely that many kinds of mental retardation are moleqular diseases, caused by the gene-controlled mamfacture by the patient of abnorwal melewales in plase of normal ones that are nanufactured hy normal individuals. There is strong iniieation thet phenylpyruvie oia eligephrenta is a molecular disease, or perhaps « complex of solemular diseases, The investigations of Filling, Jervis, ani others have chom @leavly that phenylpyruvie aligephrenia is the result of a homosygous guratic abnormality that effecte an enaywe that normally catalyses the oxidation of the amino seid phenylalanine te tyrosine. The patients with phenylpyruvia oligophrenia are not able to carry out this oxide tion effestively; they do not manufacture an effective enayne to catalyses the reactions We may infer that the patient has inherited fron each of hia parente an abnormal gene, which leats to the namfacture of an alnorual molequle in place of the normal enayme. The elternetive is that there is a block in the process of aynthesis of the enayne, so that nothing at oll ts wamfactured ~ it may not be important to differentiate between the failure to manufacture the enaywe and the ability to manufacture an abnormal molegule that is not able to perform the catalytic function of the enaymes Le be Paulingy lie Ac Itanoy Se J Singer, and I. Ce Wells. Sickle-~eell Anmis, a Melecular Disease. Selense 130, 543 (1949). 2. de Ve Neel, The Population Geneties of Two Inherited Bleod Dyscrasias in Mane Cald apring Harber Symposia, Guamt. Biol. 15, 14) (1951). 3. Fs Brains Siekle-Cell Anemia in Afriea. Hrit. Med. Je if, 880 (1952). de Ae Ce Alligon. Protection Affortied by Sickle-Cell Trait Against Subtertian Malarial Infection, Brit. Hede Jo §, 296 (1954). the first step in solving a problem is to unterstand it. The discovery eof the abnormal hemoglobine has provided use vith a far decper understanding of the hereditary hemolytic anemias than existed before. In the same way, wo Tw wueh may be dome in increasing our understanding of phenylpyruvie olige- phrenia. It may be fount that there are many different allelenerphie genes that ean comtritute to the produetion of the disease; that ia, that these genen, in haneaygous state oy double heterceygous state, my produss any one of a large master of somewhat different o-nditions that are now grouped together, anf that some of the patients, who nemfacture abnormal enayme nolecules that retain « certain ancunt of ontalytic activity, may be susceptible te treatment. A test that would distinguish the carriers of phenylpyravie oligophrenta vould be useful; in pertieular, the siblings of a phenylpyruvie oligophrenis patient aould learn whether or not he fa a carrier of the gene, and whether or not he should avoid marrying ancther A few monthe ago, when I gave the Rdeel 8. Fort Lecture, I mentioned thet a seore of disenses have eo far been recognized as onuyme dineasse, premmably resulting fren the namfasture of alnornel nelequles in place of the active enayme nolemiles, and that it seens to me to be not unlikely that there ave thousands of sash diseases. I continued by saying that I foresee the day when many of these diseases will be treated hy the use of artificial ensymes. ‘shen cur welerstanding of enayne activity beecues great enough « ani this will require that a determination be male of the detaile? arrangement of the thousanis of atane thet uske up one of the molegules of the engyne ~ it will be possible to synthesise a catalyst for the exidation ef phenylalanine to tyrosine. A mall snust of this catalyst may then be attached to a retieular fremewor:: inside of a mall open-enied polythene tube, which ean be permanently placed in an artery of « nev-bern ehild who has been shown by the presence of phenylpyruvie acid in the urine to have inherited phenylpyruvie oligophrenta; through the action of the catalyst the child should then develop in « normal way, This iten ssens fantastic nowy but the verld of 1955 is a fantastic world from the viewpoint of 1905, ami I hawe little doubt that sy predietion about the vorld of 2005 will turn cut net to be a bold ene, tut rather « tinid and unimaginative one. These hereditary diesases involve the genes ~ abnormal genes, abnormal. molegules that we oan now safely identify as melequles of deoxyribosenuolete acid. Recent afvances in knowledge about the structure of deoxyribossmelete eel’ have provided the basis fer confident speeulation about the noleouler nature of the processes cf heredity. Yor fifteen years there has existed strong support for the belief that biclegieal specificity in general Lavolwes a detailed complenentariness in structure of interacting moleeules,® ant the proposal was made ten years age’ that the mechanimm of self-