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SA MEDICAL JOUR AL 5 February 1977

Leukaemogenesis in Down's SyndromeG. S. GERICKE, P. B. HESSELING, S. BRINK, W. B. BECKER

SUMMARYDue to the fixed karyotype and documented malignancyrisk in patients with Down's syndrome, recently describedaetiological factors can be assigned to their proper placesin a conceptual framework for leukaemogenesis in theseindividuals. This is a more profitable approach thanthose in which various types of karyotypic patterns arematched to different malignancies. It seems that virusesmay play a special role, but they need interaction withother factors, most of which are present in Down's

syndrome. A unifying concept which may be helpful inestablishing research priorities is presented.

S. Afr. med. l., 51, 158 (1977).

Much has been recorded about chromosomal abnormali-ties as a cause or consequence of malignant transformation,and about the correlation of tnmours with specific karyo-typic patterns.

Special circumstances associated with Down's syndromeallow a different approach to the problem. In Down'ssyndrome a fixed karyotype is found and the increasedrisk of different types of malignant change has beendocumented...·14 The various possible aetiological factorscan be assembled in a unified concept to form a workinghypothesis of leukaemogenesis in these individuals. Thepurpose of tills review is to discuss recent clinical andexperimental findings and to present such a unifying con-cept for leukaemogenesis in Down's syndrome.

GENERAL FACTORS IN VIRALONCOGENESIS

Although evidence for a viral aetiology for human leukae-mia has accumulated, it seems highly unlikely that leukae-mia is contagious in the usual sense of the term. It appearsthat tumour initiation may result from a complex inter-

Department of Paediatrics, University of Stellenbosch andTygerberg Hospital, ParowvaUei, CP

G. S. GERICKE, M.B. CH.B., RegistrarP.B. HESSELING, ?!.B. CH.B .. M.MED. (PAED.)

Department of Haematology, University of StelIenbosch andTygerberg Hospital, ParowvalIei, CP

S. BRINK, M.B. CH.B., F.F. PATH. (S.A.)

Department of Medical Virology, University of Stellenboschand Tygerberg Hospital, Parowvallei, CP

"',T. B. BECKER, M.B. CH.B., ~U1ED. (PATH.), M.H.C. (PATH.),M.D., Professor

Date received: 2 September 1976.

action of 3 conditions: Ca) genetic predisposition to cancer;Cb) infection by a tumour virus, especially in irnmuno-deficient individuals; and Cc) environmental factors.'" (Thefirst 2 conditions have definitely been proved in mice, whereinfection causes leukaemia or immunisation. Susceptiblemice can then be inoculated against leukaemia.) All ofthese 3 conditions may be evident in Down's syndrome.

Genetic Predisposition

Krivit and Good" first reported the aSSOCIatIOn betweenDown's syndrome and leukaemia in 1957. Holland et al."followed 2033 probands with Down's syndrome for periodsranging from 1 to 14 years. During this period 7 indexpatients died of leukaemia and 7 from other forms ofcancer. The death rate for leukaemia was 18 times greaterin those with Down's syndrome than in the general popu-lation, while the mortality from other types of cancer was2,6 times above the national rate for the USA. The severalstudies which support the association between Down'ssyndrome and leukaemia have recently been reviewed byRosner and Lee:' Acute myelofibrosis may also occurwith unusual frequency in patients with Down's syndrome."

Tumour VirusesThere is evidence that leukaemic children who have

received whole-body irradiation and bone marrow fromtheir siblings develop leukaemia of donor cell type."

The oncornaviruses represent the leukaemogenic R Aviruses. Human cells can be infected with these viruses andcan be transformed by viral gene products obtained fromanimal cells.'"

A type cR A virus was recently isolated from culturedcells of a patient with myeloblastic leukaemia, but whodid not have Down's syndrome.'"

The evidence of oncogenic viral infections in humans isimportant. in view of accumulating evidence that antiviralimmunodeficiency occurs in individuals with Down'ssyndrome.··"·35,... It is probably significant that the trisomicchromosome in persons with Down's syndrome also carriesthe locus for interferon-induced antiviral activity." Theexistence of 2 types of individuals with Down's syndromewith lesser and greater degrees of cellular immunodeficiencyhas been reported. lO.... The latter type may be more likelyto become chronic hepatitis B surface antigen CHBsAg)carriers. HBsAg is also f.ound in excess in patients withlymphocytic leukaemia.' It has not yet been shown thatHBsAg carrier status indicates an immune deficit whichcould be implicated in leukaemia, but the significance ofHBsAg carrier status in Down's syndrome must be fullyinvestigated.

5 Februarie 1977 s MEDIESE TYDSKRIF 159

Environmental FactorsIn viral leukaemogenesis in animals. venical transmission

of infection has led to the oncogene hypothesis. which statethat transmission via the zygote results in a viral or pro-viral genome being present in a repressed state in everysomatic cell.'· Malignancy can result when de-repres ion isinduced by environmental influences such as irradiation.chemical carcinogens and ageing. Vertical transmission hanot been conclusively demonstrated in humans. but is ug-gested by reports uch as a recent epidemiological investi-gation carried out in California.' which revealed a possibleleukaemia risk to 0 - 4-year-olds due to their mothers havinguffered an influenza infection during the first trimester of

pregnancy. In this case. the virus may also have exenedits oncogenic effect through increased chromosome break-age. This aspect will be dealt with funher on.

ASSOClATIO BETWEEN DOWN'SSYNDROME AND LEUKAEMIA

ince the 1890s chromosomal and mitotic irregularities havebeen recognised as the one common feature in cancer. Thisfact stimulated the mutation theory of cancer at an earlydate.'" In 1947. the higher incidpnce of leukaemia in rela-tives of leukaemia patients led to the consideration ofgenetic leukaemogenic factors. 53 In the early sixties. thechromosome which is present in triplicate in cells ofpatients with Down's syndrome was thought to be the sameone which exhibits a long-arm deletion in chronic myeloidleukaemia (Philadelphia chromosome). Since the leucocytealkaline phosphatase level is generally increased in patientswith Down's syndrome and decreased in patients withchronic myeloid leukaemia. this was thought to prove theexistence of a leucopoiesis locus on this chromosome.

In 1971. O'Riordan et al." showed the Philadelphiachromosome to be a deleted TO. 22. The extra chromo-ome in Down's syndrome shows delayed uptake of radio-

isotopes. is more often atellited and shows more intensestaining in its long arms when examined by means offluorescence techniques. and thus represents a different G-group autosome - TO. 21." In spite of the fact that theassociation between chromosome 21 and leukaemia hasnot been explained in its initial hypothesis. recent findingsemphasise the possibility that chromosome 21 is importantin the onset of the leukaemic process..·.. or is concernedwith abnormal haematopoiesis."'·...,·" The association be-tween Down's syndrome and leukaemia does not seem tobe a direct cause-effect relationship. but involves an inter-action between many factors.

VIRUSES, CHROMOSOME BREAKAGE ANDVIRAL GE OME INCORPORATIO

Evidence of a viral aetiology in human leukaemia wasrecently provided by Gallagher and Gallo,'" who isolated atype of cR A virus from cultured blood cells of a patientwith acute myelocytic leukaemia. Since the investigatorshad only used biological materials of human origin. con-tamination with animal viruses was unlikely. Immunologi-al evidence in this case also indicates that the virus was of

human origin. The major protein were identical to pro-

tein previou Iy isolated from other patients with acutemyelocytic leukaemia.

Proteins from the new viru were immunologicall~clo ely related to proteins in two viru es that cause leukae-mia in subhuman primates. but were more distantly relatedto proteins from a mouse leukaemia viru . The authorpostulate that interaction with other factors may be re-quired to produce myelogenous leukaemia in man.

Hypothese concerning the infectious transmission ofleukaemia have been tested. but the findings were mostlynegative. Statistical studies of leukaemic clustering have notprovided solid evidence for an :nfectious mode of pread.The incidence of leukaemia among marital partners ofleukaemic persons or in children born of women withleukaemia during pregnancy is not exce sive."" These obser-vations do not exclude the possibility of a viral aetiologyfor leukaemia. but may be due to the complex interplayof many factors.

Chromosome Breakage

The importance of genetic factors in some cases i illu-trated by the pedigree reported by Zuelzer et al.'" The einvestigators did chromosomal studies of relatives of aninfant with myeloblastic leukaemia and with cytogeneticfeatures of partial D trisomy. ;:-hey showed excessivebreakage, chromatid exchange, endoreduplication and frag-ment formation in 3 generations of the affected child'sfamily. This suggested a genetic factor acting in the mannerof a breakage gene and constituted a possible link betweenleukaemogenesis. congenital anomalies and chromosomeaberrations. In this special situation the breakage gene'... could have facilitated the occurrence of exogenoumutagenic events. e.g. the entry of hypothetical leukaemiaviruses into the genome of rapidly replicating cells sucha that of bone marrow'. Very little is known about normof chromosome breakage in the 'average' population.Breakage is the phenomenon which occurs before an al:>-normal chromosome forms. and at least one of these. acentric fusion of the D-D type. is believed to be one of thecommonest forms of autosomal heterozygosity found inthe population at large." These individuals are believed tobe completely normal. Patients with genetic conditions inwhich there is a clearly recognisable increa e in chromo-some breakage. such as Bloom's syndrome. Fanconi'sanaemia and ataxia telangiectasia. have a notable su cepti-bility to cancer. especially of the Iymphoreticular system.Chronic myeloid leukaemia is known to be associated witha specific type of breakage chromosome. the Philadelphiachromo ome.

Viruses have been demonstrated to cause at least 3 typeof chromosome breakage in \·itro'· Chromosome breakageand breaking agents are important, but complex phenomenaand the relationship between breakage and abnormal growthare very difficult to define. Evidence for increased breakagein Down's syndrome has been provided by Higurashiet al.'" who found that cultured cells from patients withDown' syndrome display more chromosome breakageafter virus infection than those of normal ubjects. Wehave found'" nuclear membrane abnormalities in 30 patientswith Down s syndrome. Membrane abnormalities may be

16U SA MEDICAL JOUR 'AL 5 February 1977

Viral Genome Incorporation

an underlying factor in the increa ed chromo ome break-age. ince chromo ome replication i a membran -a 0-

iated phenomenon.

A relationship between experimental immunodeficiency andD A virus-induced tumours was first reported byVandeputte et al.'" Allison et at.' induced tumour byadenovirus in susceptible mi e after using antilymphocyteserum or neonatal thymectomy to produce immunodefi-

Hypothetical incorporation of the viru genome into thehromosome. nucleus or cytoplasm of the ho t cell is theecond major area of importance.'· Early maternal virus

infection may cause non-disjunction. This mayor maynot be the same viru that is subsequently incorporatedinto the genome of the host cell. on-disjunction ma\'occur through persistence of the nucleolus during meiosis.or by an effect of the ovum on the nucleol s. leading to afailure of separation of half bivalents during meiosis.'"

Clinical and laboratory evidence for this concept in-cludes the following: Collmann and Stoller" have shownthat a cyclical incidence of infective hepatitis was followed9 months later by the same cyclical pattern in the incidenceof the birth of children with Down's syndrome. Recentinvestigations have shown that the nucleoli in man areorganised on the hort arms of the acrocentric chromo-ome in groups D and G. and this may account for the

apparent non-random involvement of these autosomes intrisomy, primarily by means of 'nucleolus dysfunction'. Ifthe trisomy in Down' yndrome is a consequence of themechanical prevention of disjunction due to nucleolar per-sistence, then this immediately suggests a possible reasonwhy virus infection could be important in relation to theaetiology of this condition. particularly if the viruses areD A viruses that replicate within the host cell nu leus.The nucleolus is assumed to play an important part in thedevelopment of the viru . Polani et at." have suggested in1960 that the persistence of nuclear remnants might be animportant factor in the development of Down's syndrome.Factors that are responsible for the dissolution of thenucleolu at mitosis or meiosis are not well understood.Tt ha been shown that nucleoli can be made to persistat mitosis by the use of cobalt salts and that their pro-pensity for fusion in interphase can be enhanced bytreatment with thyroxin.

Heneen and ichols" have shown that virus infe tioncan increase persisting nucleoli during mitosis in culture.This holds chromatin together and impedes separationduring anaphase. which can lead to non-disjunction andtri omy. Bjonness et al.' have described a neonate \ ithcongenital leukaemia and a G trisomy without phenotypicmanifestations of Down's syndrome. These authors postu-lated that the extra chromosome was of postzygotic origindue to non-di junction or selective endoreduplication andthat a mutagenic factor such as a virus could have beenimportant in the initiation of both conditions.

ciency. McEndy ('£ al.' demon trated that thymectomyoften results in a decrea e of mouse leukaemogenesis. Otherworks suggest that inta t immunological defences are im-portant to control R A virus-induced leukaemias and lym-phomas in the mouse.""""

Immunodeficiency has been studied extensively in graft-versus-host (GVH) reactions in animal. Recent experi-ments have demonstrated that the development of GVH-induced lymphomas is associated with murine leukaemiavirus activation.'· In mixed leucocyte culture systems, blastformation is often associated with leukaemia virus activa-tion."

A finding which may be important in understandingthe relationship between virus immunity and cancer risk.is the localisation by abholz'" and by Tan e£ al.'" of theinterferon-induced antiviral locus on chromosome 21. Tane£ a'."' illustrated a gene dosage effect for this locus bydemonstrating an enhanced antiviral response in trisomy 21fibroblasts in \'i,ro as compared with trisomy 13, trisomy18 and diploid control fibroblasts. It is possibly significantthat the gene which regulates the expression of the anti-viral state is situated on the ame chromosome which isassociated with a high leukaemic risk in the trisomic state.It is not clear how the observations of enhanced antiviralresponse in cultured cell with trisomy 21 can be related tofindings which indicate abnormal virus immunity in Down'syndrome. Knowledge of the effect of regulator genes onseparate chromosomes and of the factors which couldinfluence these regulator genes would be important in thicontext.

Evidence of disturbed virus immunity in Down's syn-drome includes increased susceptibiLity to ill \'itro trans-formation after exposure to oncogenic SV40 virus." It iinteresting that monosomy of G-group chromosomes inprimary human cells transformed by SV-W virus has beenreported by Shein and Ender" in an early study. Possiblythe gene responsible for antiviral activity could exhibitless functional activity in monosomy than when it dis-plays a gene dosage effect in trisomy. We have shown that46°~ of reported patient with the 11 deletion syndromesuffered from repeated infections." Yamada and Furusawa"have found an unusually high figure of involvement ofthe No. 21 chromosome in 8 patients with leukaemia; In5 of these. monosomy for No. 21 was present.

Other important findings are the depressed reactivity to-skin test antigens in \·i\·o." impaired PHA response." andviral-like inclusions found in the blasts of a patient withDown's syndrome and leukaemia.' Such a virus may beof aetiological importance or it may simply represent apassenger unrelated to the leukaemic process. Levin eT al."substantiated previous reports of a deficient T-cell systemin individuals with Down's syndrome. They found the T-cell population in children with Down's syndrome to beonly 60o~ of that in normal children. The infections seenwere not of the type usually found in children with grosslydeficient T-cell systems af)d thymic aplasia. such as moni-liasis or progressive vaccinia. but rather the commonbacterial and viral respiratory infections. In individualswith Down's yndrome who develop leukaemia it may bethat transformation frequency of target cells i of uchmagnitude that other immunological surveillance mecha-

CERPRIMARY TIVIRALODEFICIE CY AND CIMM

5 Februarie 1977 SA MEDIESE TYDSKRIF 161

unifying concept from the different factor which maycause leukaemia in individuals with Down's syndrome(Fig. 1).

En~,ronmenl

·.·~~'d Le..Ruc(oO"

---- .. 1,

7

"'S5Ag Camef Slill"'.O'ug$ ,AgeIng

~:~:,~I~;Ple!101'l ~ 8Iu~~e

A"t,~"a 1",-n,J'l:)jeICOlI'll:f Eh,.

h.gtl Ce T.,~IQf....aI'C"

nisms (T- and B-cell systems) may be inadequate to pre-vent malignancy. On the other hand, whether it is atransient or a permanent phenomenon, all children withDown's syndrome may not exhibit the same degree ofcellular deficiency.

A high prevalence of Australian antigenaemia in indi-viduals with Down's syndrome has been established.Sutnick et al'" found 2 groups of patients with Down'ssyndrome with greater and lesser degrees of cellularimmunodeficiency. The former may be more prone tobecome persistent HBsAg carriers (and to develop leukae-mia?). Campion and Wangel" attempted to define therelative importance of HBsAg carrier status and cell-mediated immune response in Down's syndrome. Theydemonstrated an association, but the correlation was notcomplete and it was thought unlikely that defective cell-mediated immunity could be the only reason for theHBsAg carrier state. Geographical factors, age and thesize of the institution seem to influence the prevalence ofHBsAg among groups of people with Down's syndrome.

An increased prevalence of antithyroid" and smoothmuscle auto-antibodies" has been found in the serum ofindividuals with Down's syndrome. According to Schaller"one could postulate that viruses play a major role in theaetiology and pathogenesis of human auto-immunediseases. Viruses appear to be capable of exerting signifi-cant effects on the host immune system, including depres-sion of T-cell function and stimulation of B-cell function.Individuals with some underlying immune deficiency mightbe predisposed to harbour the viruses in the first place.This concept appears to fit in very well with what is knownabout auto-antibodies, depressed T-cell function and anti-viral immunodeficiency in Down's syndrome.

CANCER RISK IN DOWN'S SYNDROME Fig. 1. Proposed multifactorial concept of leukaemo-genesis in Down's syndrome.Three surveillance mechanisms seem to protect normalindividuals from cancer, according to a scheme proposedby Kersey and Good."

Target cells must be relatively resistant to transformationby exogenous or endogenous agents. The increased suscepti-bility to oncogenic SV40 virus transformation indicates anintracellular or first-component defect in patients withDown's syndrome.

Both cellular and humoral mediated immune responsesare important in the subsequent control of a transformedcell. In addition to the deficiency of T cells, which hasbeen mentioned, B-cell defects also exist in Down's syn-drome. These second-component defects would allowuncontrolled proliferation of transformed target cells.Variation in the degree of T- and B-cell defects may partlyexplain why only some children with Down's syndromedevelop leukaemia. Chronic antigenic stimulation, lack ofimmunoregulatory feedback mechanisms and undefinedgraft-versus-host reactions also feature in the scheme ofKersey and Good. The significance of these factors -inDown's syndrome is unknown.

CONCLUSIOPresent knowledge of genetics, immunology, virology,haematology and cytology makes it possible to construct a

We wish to repeat the idea expressed by Burkitt' thatcancer research is '... sometimes too "blinkered" in thatit often tends to be focused only on malignant lesions tothe exclusion of other conditions, the investigation of whichmight point the way to a better understanding of certainforms of cancer'.

In view of the special circumstances associated withDown's syndrome, the individuals with this condition pro-vide a suitable model for the investigation of the complexinteraction between genetics, viruses and the environment.

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