The DiGeorge anomaly

The DiGeorge Anomaly 43

Clinical Reviews in Allergy and Immunology Volume 20, 2001

The DiGeorge Anomaly

Richard Hong

Professor of Pediatrics, University of Vermont, Genetic ToxicologyLaboratory, 32 North Prospect Street, Burlington, VT 05401

IntroductionIn the 1960s, the incisive studies of Cooper et al. (1), based on the

chicken model, established two major systems of immunity, one medi-ated by T cells and the other by B cells. Studies by Miller in the mouse(2), and Archer and Pierce in the rabbit (3), established an essential rolefor the thymus gland in the development of T-cell immunity. DiGeorgeprovided the first description of susceptibility to infection associatedwith thymus hypoplasia in humans (4). The original patients also mani-fested hypoparathyroidism and the combined disorder was known asDiGeorge syndrome, today referred to as DiGeorge anomaly (DGA).Subsequently, congenital heart disease, particularly conotruncal de-fects, facial dysmorphism, and other abnormalities, were reported ascommon features (5–7). In 1981, de la Chapelle (8), reported a chromo-some 22 translocation, t(20;22)(q11;q11), in four affected members of afamily, providing the first clue to its genetic origin, and subsequentinvestigation explained the relationship of DGA to a number of clini-cally related diseases, velocardiofacial syndrome (VCFS)(9), andconotruncal anomalies face syndrome (10), which showed some clinicaloverlap with features of DGA (9,10). In all of these conditions, dele-tions within chromosome 22q11 were found. Thus, in the modern view,DGA is considered the severe end of a clinical spectrum that has beendubbed CATCH 22 (Cardiac defects, Abnormal facies, Thymic hypo-plasia, Cleft palate, and Hypocalcemia resulting from 22q11 deletions),by Wilson et al. (11). It is important to emphasize that DGA is part of acontinuum of clinical manifestations as the extreme heterogeneity ofclinical presentation, and variability in severity of each symptom mustbe appreciated by the clinician in order to consider appropriately theindications for and success of immune system reconstitution therapy.

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Clinical Reviews in Allergy and Immunology© Copyright 2001 by Humana Press Inc.1080-0549/00/43–60/$14.25

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This review will deal primarily with the thymus defect of DGA, al-though the reader should appreciate that thymus deficiency is a rarecomplication,. More general treatments of DGA and CATCH 22 havebeen published (11–16). The original term, DiGeorge syndrome, shouldno longer be used, as the constellation of defects is not a syndromeowing to a single cause, but a disorder resulting from failure of an em-bryological field to develop normally. The dependence of the clinicalpicture on an embryological field failure defines this entity as ananomaly rather than a syndrome, and explains why the disorder can bea result of diverse unrelated etiologies (17).

EtiologyThe embryological fault that gives rise to the clinical manifesta-

tions of DGA is an inadequate contribution of the neural crest tissues tothe 3rd and 4th pharyngeal pouches owing to genetic and extrageneticcauses. Using the quail–chick model, Le Douarin (18), showed thatcephalic neural crest tissue contributed to the mesenchyme of facial andcardiac tissue. Extirpation of the neural crest in chicken embryos causedeither marked diminution in size or absence of the thymus and par-athyroids (19).

A number of environmental events also can affect embryogenesisof these pharyngeal pouch derivatives. Fetal exposure to retinoids, al-cohol, bis-dichloroacetylamine, or maternal diabetes have been associ-ated with DGA (17,20–24).

The finding of de la Chapelle et al. (8), focused attempts to definethe genetic cause for DGA. Early studies defined chromosomal region22q11.2 as the affected area, as monosomy occurs in over 90% of DGApatients and the other related disorders of CATCH 22 (12,25–27). Theseattempts were hindered by the large size of the deletions originally de-scribed (approx 2 megabases). More recently, a shorter region of dele-tion overlap has been defined as approx 250 kilobases and contains abreakpoint, shown to be present in a patient with DGA and her mother,who had the related VCFS (28,29). Most of the deletions appear to be denovo lesions, but inheritance can occur in approx 15–28% (16,17,30). Theseverity of the phenotype does not depend on the size of the deletion,so contiguous genes do not appear to be involved. Budarf and Emanuel(31), and Dallipicola et al. (32), have proposed that two genes mappingto the same 22q region may play the major roles.

The leading candidates are GSCL, (Goosecoid like), a homeobox-coding gene that in the mouse appears to play a role in developing neu-ral tissues (33), and HIRA, expressed in high amounts in heart, cranialneural folds, pharyngeal arches, and circumpharyngeal neural crest ofmurines (34). GSCL is found in the 250kb minimal DiGeorge critical



region and shows homology to homeodomain family of transcriptionfactors that are important in organogenesis (35). HIRA has recently beenshown to bind to core histones and is thus involved in chromatin andhistone metabolism (36).

Specific genetic defects may be expressed as different clinical phe-notypes. Tsui et al. (37), describe a family in which two brothers andtheir mother had the same submicroscopic deletion of chromosome 22,but one sibling was diagnosed as having DGA, whereas the other hadVCFS. The mother was asymptomatic. In a monozygotic twin pair con-cordant for the 22q11.2 deletion, only one had a heart defect, althoughboth showed the same dysmorphic features (38). These observationsdemonstrate the role of extragenetic influences on the clinical manifes-tations of various CATCH-22 phenotypes.

Deletions of 10p and other chromosome anomalies have been de-scribed in a subset of DGA patients (39–41). At the present time, candi-date genes have not been mapped to this region. Examination of 10pdeletions suggest that two nonoverlapping regions are involved (42).

At the present, the pathogenesis of DGA can be summarized asfollows. The proper development of the 3rd and 4th pharyngeal pouchderivatives requires the appropriate contribution of cephalic neural-crest cells during embryogenesis. There are many genetic and extra-genetic factors that can interfere with neural-crest development,migration to the pharyngeal pouches, and subsequent integration andgrowth. The net result is a heterogeneous clinical entity, CATCH 22, ofwhich the DGA represents the most severely affected phenotype.

The nature of the immune defect is worthy of additional comment.When DiGeorge originally presented his findings of thymus lack andinfectious susceptibility, a novel concept of compartmentalization ofthe immune system into two independent arms, the antibody mediated(B cell), and the cell mediated (T cell or thymus-dependent), was beingpromulgated (1). In that view, patients with an absent thymus wouldhave a normal immunoglobulin system, and early investigation of DGApatients supported that hypothesis. However, the discrimination of theinvestigation tools available at that time was insufficient to define theexact degree of deficiency of either system. Also, the dependence of B-cell immunity on T cells was not known at this time. In many patients,a small (approx 1 g), thymus with normal architecture was found. Thiswas the same mass of tissue found in children with severe combinedimmunodeficiency (SCID), who had uniformly fatal disease, althoughthe thymus histology was quite different in SCID patients. Not knownthen was the fact that a very small thymus could provide completelynormal T-cell function. Originally, the most widely held opinion re-garding the T-cell status in DGA was that much more than 1 g of thy-mus mass was necessary to provide sufficient T-cell function to live a

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normal life. It was thought that most DGA patients had diminishedT-cell function, to a degree that led to increased susceptibility from in-fections. DGA patients were also thought to possess partial T-cell im-munity, in the sense of having T cells capable of all functions, buthaving a diminished capacity of those functions, a notion untenabletoday. The concept of partial T-cell defect was also used in the sense ofan immature immune system, one that would grow up to a normal“size” and capability with time. These concepts, which are still widelyheld, have greatly clouded the delineation of the immune defect in DGAand also confused the interpretation of therapeutic attempts.

Today, the ability to evaluate T-cell function by in vitro prolifera-tion assays and the more discriminate definition of lymphocyte subsetsby monoclonal antibodies (MAbs) have permitted quantitative andqualitative descriptions of the T-cell immunity in DGA patients, whichmake untenable the concept of T-cell deficiency caused by less than acritical number of grams of tissue, or an immature system that will ulti-mately grow to competence with time. There are no documented ex-amples of this type of thymic defect.

An estimate of how little thymus is necessary to provide adequateT-cell function was provided by Bale and Sotelo-Avila (43), who re-viewed autopsy findings in 34 cases of maldescent of the thymus, 24 ofwhich involved DGA patients. Twenty-two had absent mediastinal thy-mus tissue. Of the remaining 12, in 7, the gland was small or unilateraland in 5, the gland was normal-sized but supramediastinal in location.Cervical nodules weighing as little as 0.7 g and measuring less than 0.5cm in diameter were found. In one case, a small thymus measuringonly 0.1 cm was found, completely contained within the thyroid gland.Although exhaustive immunologic characterization was not per-formed, lymphoid tissue examination did not show abnormalities ofthe thymic dependent T-cell zones, and there was no clinical evidenceof profound T-cell deficiency. Wilson et al. (11), reported that several intheir series of 44 DGA patients had no identifiable thymus at the timeof sternotomy, yet had T-cell numbers only just below the lower limitof normal. Junker and Driscoll (44), reviewed 13 patients with DGAwho were living normal lives at home and not requiring intravenousgammaglobulin (IVIG) or other support for compromised immune sys-tems. Although all patients had CD3+ cell counts over 500/mm3 andCD4+ T cell counts over 350/mm3, 4 of the patients remained below thenormal range even when followed as long as 18 yr. CD8+ T cell countswere below the normal range in 7 of the 13 patients over the same pe-riod. The absolute numbers did not increase with age. Yet, all of thesepatients showed normal ability to make antibodies, which can beviewed as a test of the helper function of their T cells. Another qualita-tive assessment of the T cells, the proliferative response to mitogen,



was completely within the normal range, regardless of the absolutenumber of T cells. Junker and Driscoll’s studies extended a review of 18patients we conducted in 1989 (45), wherein we equated a CD4+ T-cellnumber of over 400/mm3 with effective T-cell immunity. In that study,absence of mediastinal thymus tissue was verified at the time of cardiacsurgery in 11 cases. Eight of these 11 had normal T-cell function. Be-cause so little thymus tissue is needed for freedom from T-cell defi-ciency, I have concluded that those DGA patients who do showimmune defects have essentially no thymus (if any).

Because the cause of DGA is the failure of the cephalic neural crestto make adequate contributions to the pharyngeal pouches from whichthe organ anlage derive, the resulting defects would be expected to bestructural in nature. My view is that structural abnormalities merelyaffect the size of the gland and will not result in global defects involv-ing enzymes, cytokines, or specific cell populations. In my experienceof examining autopsy slides from 20 patients with DGA, I have neverseen abnormal morphology when the thymus could be found. Only thesize was affected. Corticomedullary differentiation and Hassall cor-puscles, the hallmarks of normal thymus morphology, were present inevery case. The results are quite striking compared to the changes seenin any case of primary immunodeficiency involving T cells where eitherthe stem cell is at fault or there is a biochemical lesion. Here, the mor-phology is always abnormal (46). I therefore believe that the cause ofthe T-cell deficiency in CATCH-22 disorders, when present, is absenceof thymus, not merely decrease in mass.

Recent findings may require a modification of that view. Gupta etal. (47), have shown that T-cell subsets from a patient with DGA showedincreased spontaneous apoptosis. This was associated with an increasein Fas and FasL expression. However, under certain circumstances,CD3+CD4+ cells may suddenly increase in number, to almost normalvalues. On the rare occasions when this has occurred, those cells havebeen without adequate function in in vitro tests. This issue will be con-sidered further in the section on diagnosis.

The concomitant deficiency of immunoglobulins is owing to lackof T helper cells. Haire et al. (48), examined the immunoglobulin reper-toire of a patient with complete DGA and another with T-cell numbersapprox one-half those of age-matched controls. The VH repertoire in thecDGA patient was clearly distinct from the fetal repertoire (i.e., had“matured”), and therefore there was no genetic defect of B cells. The VHrepertoire was only lacking in diversity, showing little evidence of so-matic mutation. Thus, the Ig repertoire restriction was owing to lack ofT-cell help to form antibody responses. In the patient whose only mea-surable T-cell defect was a low number, the B-cell repertoire was nor-mal. Of interest, genes associated with autoimmune reactivity were

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expressed in large numbers, which could explain the predisposition toautoimmunity seen in cDGA patients. Thus, the B cells are probablynormal in DGA, but may not be regulated appropriately.

Diagnosis

GeneralThe major clinical features that should suggest the diagnosis of

DGA are the facies, conotruncal heart defect, and neonatal tetany.Typically, the facies is characterized by hypertelorism, microg-

nathia, short philtrum causing a fishmouth appearance, and low setposteriorally rotated ears, often with a defective pinna. However, thedysmorphic features are variable in occurrence and often subtle. Anexcellent catalog is detailed in Wilson’s review (11).

The presence of a conotruncal heart anomaly, particularly type Binterrupted aortic arch or truncus arteriosus, are also diagnostic fea-tures of high frequency and uniquely characteristic of neural-crest dis-turbance. Other lesions, including outflow obstructions, tetralogy ofFallot, and right-sided aortic arch are seen (11,16,49).

Hypocalcemia, particularly when owing to hypoparathyroidism,is very suggestive of DGA. Another cause of perinatal hypocalcemia,neonatal hypoparathyroidism is a rare familial disorder. It is an oc-casional, usually transient phenomenon (50). Hypocalcemia usuallyappears within the first 2 wk of life, but delayed onset is occasionallyseen (11,16).

Specific TestsAssessment of the Immune System

Although absence of the thymus shadow in chest X-rays is com-monly taken as an indicator of thymic deficiency, the frequent shrink-age of the thymus owing to stress involution decreases the reliability ofthis sign. A reliable estimate of mediastinal thymus size can be madeby magnetic resonance imaging (MRI) (51,52); however, because inmost DGA patients, the thymus has not descended into the mediasti-num, this expensive procedure is not warranted. The most informativeclinical assessment of thymic aplasia is provided by in vitro tests offunction. On the basis of a preliminary study, we suggested that lowCD4+ (<400/mm3), or poor phytohemagglutinin proliferative re-sponses (stimulation index <10), are reliable indicators of sufficient thy-mic defect to result in persistent immunodeficiency (45). Muller et al.(53), and Junker and Driscoll (44), have reached similar conclusions.On the basis of more recent experience in the past few years (since 1992),I believe that most patients with DGA and clinically significant T-cell



deficiency will have essentially no CD3+/CD4+ cells (unpublisheddata). Our figure of <400 CD4+ cells per µL was a very conservativeestimate based on the small patient number and unavailability of MAbcharacterization in many analyses and should be revised downward inlight of modern information.

We have seen a sudden increase in numbers of CD3+/CD4+ T cellsin patients with DGA who had very low numbers originally. This wasassociated with a modest mitogen response, but proliferative responseto antigens did not occur. Antigen responsivity has been shown to bethe best predictor of the ability of the T cells to protect from infectionand is the most clinically relevant of the in vitro tests of T-cell function(54). In the situations where sudden T-cell increase was observed, thepatients were suffering from a severe eczema that was secondarily in-fected (55). It is possible that T-cell expansion was owing to a super-antigen effect from staphylococcus, but formal proof of this notion wasnot obtained. Markert et al. (56), have shown that defective mitogenproliferation is a reliable predictor of persistence of the immune deficitin DGA and can be used as the indication for intervention to repair theimmune system. In their study of eight patients, occasional responsesto interleukin-2 (IL-2) and allogeneic lymphocytes were seen, but if thepatients’ lymphocytes did not respond to mitogen, death occurred un-less the patient was successfully transplanted (56).

The total lymphocyte count is usually normal; B cells are elevatedand natural killer (NK) cells are normally distributed.

Traditionally, the terms complete and partial DiGeorge anomaly(cDGA and pDGA, respectively), have been used to classify DGA intotwo groups. This classification was proposed by Lischner and DiGeorge(57), on the basis of postmortem studies. The complete form was usedto designate absence of the thymus and parathyroids. It has been help-ful in emphasizing the heterogeneity of the clinical features. However,in order for the designation to be as clinically useful as possible, I sug-gest restricting the term, complete DGA, to the situation of completeabsence of the thymus to imply that therapeutic intervention is indi-cated and that irradiated blood products should be used. I propose us-ing partial DGA (pDGA), to refer to those patients in whom a “waitand see” attitude concerning the immune status is justified. Further,the completeness of the thymic defect needs to be defined on the basisof functional testing of the T-cell system and not some estimate of thy-mus size by physical means. Using the terms “complete DGA” and“partial DGA” in this manner will not confuse the literature with newnomenclature. Furthermore, the terms will have particular relevancefor clinical care.

Durandy et al. (58), found low CD8+ cells at birth with spontane-ous in vitro Ig production in patients with DGA. With time, CD8+ cells

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increased in number and the spontaneous Ig production disappeared.These findings suggest that there may be an element of immune systemdysregulation in DGA patients; Durandy et al. (58), suggest that thereis a delay in maturation of regulatory function in DGA. Similar find-ings were presented by Muller et al. (53), who found abnormally in-creased levels of immunoglobulins and antibodies to bovine serumalbumin (BSA). These observations are consonant with genetic analysisof the B-cell repertoire as described earlier and may relate to autoim-mune problems that are seen in some older DGA patients.

Kourtis et al. (59), found a low number of CD5+ B cells in patientswith DGA, although the exact degree of thymic deficiency was not pre-cisely defined. The significance of this finding is as yet undetermined.On the basis of these and similar findings in patients with rapidly pro-gressive forms of AIDS, they have proposed that CD5+ B-cell numberprovides a marker of decreased thymic function.

Parathyroid AssessmentConfirmation of a parathyroid etiology for the calcium deficiency

is obtained by measurement of parathyroid hormone levels (50). Diso-dium edetate challenge will unmask cases of latent or subclinicalhypoparathyroidism; deficient responses are often observed in asy-mptomatic siblings and parents (51). The parathyroid hormone secre-tory reserve is often diminished in DGA patients in spite of normalcalcium and parathyroid hormone levels; this finding may predict sub-sequent overt clinical hypoparathyroidism (60).

Cardiac AssessmentThe cardiac status is assessed by standard methodology.

Chromosome Assessment.Fluorescence in situ hybridization (FISH), using probes for the

DiGeorge critical region of chromosome 22, is sensitive and readilyavailable for clinical testing and genetic counseling (17). FISH is readilyaccomplished on cultured amniocytes and chorionic villi and is sim-pler and faster than older techniques. Chorionic villi can be sampled asearly as the 10th–12th week of gestation. If a conotruncal heart malfor-mation is detected in the fetus, FISH can be performed to determine ifthe patient has a 22q11 deletion and therefore placed in the DGA/VCFScategory (17).

Monosomy 10p13, monosomy 18q21.33, trisomy 18, as well asother defects have been reported (39–42).

OtherWells et al. (61), found lack of ossification of the thyroid during the

first month of life more common in DGA (75%) than in normals (25%).



Lack of ossification was present in tetralogy of Fallot only when associ-ated with DGA. Also, DGA patients with X-ray evidence of a thymushad normal ossification. The thyroid cartilage is a derivative of the 4thbranchial arch.

A deficiency of neural crest-derived, thyrocalcitonin containingcells in the thyroid was described by Burke et al. (62), a findingthat lends support to the concept of inadequate neural crest migrationin DGA.

Clinical ManifestationsAs mentioned in the introduction, this review deals primarily with

the thymus involvement of DGA. The extreme variability of the clinicalpicture and the small patient size in most reviews provides an inad-equate treatment of the myriad of phenotypes expressed in the clinic.This shortcoming has been largely overcome by the extensive review of558 patients with 22q11 deletion studied throughout Europe (16). Thereader is referred to this outstanding work for an exhaustive treatmentof the clinical manifestations of 22q11 deletion diseases. A summary of

Table 1Incidence of Certain Abnormalities in 22q11 Deletiona

Abnormality Frequency Comment

<50% ht/wt 27/38 (71%)b With normal heart; with severeheart disease, restricted growthseen in 87%

<3% ht/wt 11/38 (29%) With severe heart disease, occurs in38%

Moderate/severe 60/338 (18%)learning difficulty

Behavioral/psychiatric 22/252 (9%) 13 behavioral; 6 attention deficit; 2problem psychosis; 1 mood changes

ENT 242/496 (49%) Velopharyngeal insufficiency 32%;cleft palate 9%; submucous cleft 5%

Renal 49/136 (36%) Absent, dysplastic, or multicystickidneys (17%); obstruction(10%);vesicoureteric reflux (4%)

a Table adapted from data in ref (16).b Numerator, number with abnormality; denominator, number of patients reported.

Not all of the 558 responses contained information in all categories. Only 38 of the 558patients had normal hearts, and they are considered separately from those with severeheart problems to show the effects of the 22q11 deletion on growth in the absence of thisconfounding factor.

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their findings is given in Table 1. These data define the extent of theneurological and renal involvement and suggest a major effect of 22q11deletion on growth.

DGA was estimated to occur in a German population at a fre-quency of approx 5 per 100,000 children (5). Studies from Australia es-timate the frequency at 1 in 66,000 total population, compared to anoverall incidence of immunodeficiency of 1 in 10,000 (63). With the in-creased sensitivity of detection provided by FISH analysis for mono-somy 22 and the inclusion of the related syndromes, more recentestimates place the incidence of the DGA/VCFS patients in the rangeof 1 in 2000–3000. Over a period of 25 years in the Seattle area, autopsiesfor DGA accounted for 0.7% of a total of 3469 postmortem examinations(6). However, of autopsies performed in patients with congenital heartdisease, 3% had DGA. This agrees with the clinical impression thatheart disease is the major mode of clinical presentation.

In order to determine the frequency of 22q11 deletion in patientswith conotruncal defects, Goldmuntz et al. (65), studied 251 patientsprospectively. Of the interrupted aortic arch patients, deletions werefound in 50.5%. For truncus arteriosus and tetralogy of Fallot, deletionsnumbered 34.5 and 15.9%, respectively. In posterior malalignment-typeventricular septal defect, 2 of 6 patients manifested 22q11 deletions and1 of 20 patients with double-outlet right ventricle. Of the 45 patientswith transposition of the great arteries studied, none displayed thechromosome deletion.

In the European study of Ryan et al. (16), the deletion was reportedusually to occur de novo (204/285 or 72%). Deletions were preferen-tially found in the paternal chromosome when tested (24/37). In thosecases of inherited deletions, the maternal chromosome was involvedpreferentially (61 maternal, 18 paternal) (16). An autosomal dominantform of DGA was first reported by Rohn (66), and later confirmed byothers (67,68). In addition, autosomal recessive and X-linked inherit-ance has been reported (17).

FaciesAn unusual facies is commonly present, but a “typical” facies

should not be sought for. There are many subtle varieties (11,16).

Mental RetardationMental retardation, to a variable degree, is present in many pa-

tients. However, severe hypoxia associated with hypocalcemic convul-sions may be the cause. It is important to have a high index of suspicionfor CATCH 22 disorders so that appropriate monitoring can be accom-plished and injury to the central nervous system (CNS) averted. In theirseries, Ryan et al. (16), found 32% to be developmentally normal except



for speech delay, which occurred in 37 of 338 patients reported. Over-all, 62% were either normal or had only minor learning problems.

ImmunodeficiencyIt is unusual for children with DGA to present initially with infec-

tion as the major complaint. In the perinatal period, hypoparathyroid-ism and cardiac malformations produce most of the problems. A failureto appreciate the association of the cardiac defect with DGA can resultin the initial manifestation of immunodeficiency as graft-vs-host dis-ease (GVHD) should open heart surgery be performed. In the face of asignificant thymic defect, the administration of unirradiated blood canbe fatal.

When the diagnosis of DGA is made early, prophylaxis withtrimethoprim/sulfamethoxazole is usually instituted; thus, pneumo-cystis pneumonia is rarely seen. If the patient does not have a high de-gree of exposure to infectious agents (e.g., from siblings), severalmonths may pass before any significant infection. Not all patients willsuffer from intractable oral thrush, an important sign of T-cell defi-ciency. Thus, in the face of a major deficit of the immune system, theremay be a surprising lack of clinical clues. This grace period is usuallyshort-lived, and sooner or later, the usual litany of recurrent severe in-fections typical of immunodeficiency disorders begins. We have seenone child acquire a B-cell lymphoproliferative syndrome during a pe-riod of observation and prior to any therapy. Another patient with DGAwho was thought to have an intact immune system was free of recur-rent severe infections. However, in adolescence, she acquired anEpstein-Barr virus (EBV)-induced B-cell lymphoproliferative disease(manuscript in preparation).

Aplastic anemia, as an unusual response to adenovirus infection,has been recorded (69).

With more stringent requirements for defining significant immu-nodeficiency, the documented number of patients with significant im-munodeficiency has continued to decrease. Only 3 of the 588 patientsstudied in the European series had severe immunodeficiency (16).Markert et al. (56), estimate that less than 10% of the DGA populationthat they study show a defect that requires therapy, and because theirstudy population is but a subset of all patients with DGA, the numberof all patients with 22q11 deletion diseases who manifest significantT-cell deficiency is also quite low.

AutoimmunityA patient studied at the age of 18 mo in 1968 (70), was reported to

have Graves disease at the age of 16 yr (71). His interval history of in-fectious susceptibility was negative. Increased juvenile rheumatoid arthri-tis has also been observed in association with monosomy 22q11.2 (72).

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TreatmentHypoparathyroidism is managed by calcium supplements and vi-

tamin D administration. It may be necessary to decrease the phospho-rus in the diet to gain maximal control.

The cardiac malformation should be corrected without requiringprior correction of the immune deficit. It is important to stress that thepulmonary congestion is much more likely to be secondary to heartfailure or vascular shunting, not the immune deficiency. Major cardiacsurgery can be performed in these children and in our experience, theyhave not had overwhelming septic or infectious complications as a re-sult. Irradiated, cytomegalovirus (CMV)-negative blood products mustbe employed, however.

When an immune deficiency is documented by appropriate invitro proliferation tests, the usual prophylactic regimens for patientswith combined T- and B-cell deficiency disease are indicated. The pa-tient should receive trimethoprim-sulfamethoxazole prophylaxis, andno live virus vaccines should be given either to the patient or any mem-ber of the family. IVIG replacement therapy is indicated.

As mentioned previously, severe intractable eczema may occur.Aggressive therapy is necessary to control the lesions (55).

Thymic hormones and thymic transplants have been used to cor-rect the defect (73–76). In assessing the benefit of procedures performedmany years ago, it is necessary to remember that approx 95% of thepatients do not have any clinically significant immunodeficiency. Inparticular, beneficial responses occurring within hours or minutes ofthe treatment must be suspect. We have performed five transplantswith thymus tissue obtained at the time of cardiac surgery and grownas explant cultures for 7–15 d. All patients are well with completeT- and B-cell recovery for periods of follow-up from 1–11 yr. All hadprofound B-cell deficiency as well as absence of T-cell function. Prior to1993, the tempo of improvement in T-cell responses was slow. Increasesin numbers of cells positive for pan T cell (CD2 or CD5), or CD4+ cellsappeared within weeks, but normal mitogen responses required sev-eral months. The CD4+ T cells have stabilized at levels of approx 400/µL with completely normal in vitro proliferative responses. Since 1993,improved techniques of culture have markedly improved the trans-plant results. Measurable in vitro proliferative responses are now seenin the first few months and the CD4+ T-cell levels are near normal(~700/µL), (Hong, manuscript submitted). Thong et al. (77), used cul-tured fetal thymic tissue and his patient is healthy 20 yr after transplant(Thong, personal communication).

In an exhaustive review of therapeutic attempts, Goldsobel et al.(78), (Stiehm, personal communication), concluded that only 7 of 20



children transplanted with fetal (17), or cultured mature (3), thymustissue were alive and clinically well, with a maximum follow up of 15yr. However, 9 of that group were lost to follow up. Thus, only 11 of theoriginal group were evaluable. Two died of their heart disease. Infec-tion may have contributed to the death of the other two. AlthoughGoldsobel et al. (78), viewed the results of thymus transplant to be dis-appointing, if their original assessment of benefit is corrected for thoselost to follow-up, the benefits of thymus transplantation seem moresalutary (7/11).

Nevertheless, on the basis of their original assessment, Goldsobelet al. (78), felt that thymus transplantation was of questionable benefitand accordingly, performed a bone marrow transplant (BMT) from anhuman leukocyte antigen (HLA) identical sibling. The child is fully re-constituted for both T- and B-cell systems and has remained well for 12yr with CD4+ T cells stabilizing at 400/µL (Stiehm, personal communi-cation). Borzy et al. (79), and Matsumoto et al. (80), also performedBMTs using a matched sibling donor with similar results. Matsumotoet al. (80), had to use busulfan and cyclophosphamide to obtain en-graftment. A first attempt, using only antithymocyte globulin to con-trol rejection, was unsuccessful. The need for conditioning is difficultto explain and reconcile with the postulated defect in DGA.

Recently, Bowers et al. (81), accomplished successful reconstitu-tion in a DGA patient using only mononuclear cells obtained from pe-ripheral blood, obtained from a normal HLA-identical 7–yr-old malesibling. The first dose employed was 4 × 106 CD3+ cell/kg body weightof the recipient. The peripheral blood CD3+ cell count fell after a fewweeks and another infusion was necessary. Using an increased dose of2 × 107 CD3+ cells/kg, a stable count of 315 CD3+ cells/µL was ulti-mately obtained. Normal immune responses, including antibodyresponses to diphtheria and tetanus toxoid immunization, and in vitroproliferative responses to antigen were obtained. This experience sup-ports the notion that it is the post-thymic T cells in the BMT that ac-complish the reconstitution, and offers a simple form of reconstitutionwhen HLA-identical siblings are available.

The use of bone marrow or peripheral blood mononuclear cells areexciting therapeutic alternatives and raise a number of interesting theo-retical considerations. The fact that the T cells still show good functionat least four years after the transplant by Goldsobel et al. (75), (Stiehm,personal communication), suggests that T-cell function is being main-tained by a dividing population of mature postthymic T cells. The issueis of major import in consideration of bone marrow strategies whenHLA-identical donors are not available. In this case, transplantationwith T cell-depleted marrow from a parent or nonmatched sibling hasbeen used with great success in other primary immunodeficiency dis-

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orders (82–84). However, if there is no thymus, one could questionwhether there are sufficient numbers of replicating mature T cells in Tcell-depleted bone marrows to provide long-lasting benefit. Indeed,repeated attempts to reconstitute a DGA patient with T cell-depletedpaternal marrow were unsuccessful. In the situation whereHLA-matched siblings are not available, the use of non-T-cell-depletedmatched nonrelated donors, particularly using umbilical cord blood,might represent a useful option (85,86).

Genetic CounselingA number of questions arise when the clinician encounters a case

of DGA. Although these issues are more likely within the purviewof the geneticist, I have included some information that may be helpfulto immunologists so they have a sense of some of the genetic issuesinvolved.

The most important issue relates to the possibility of recurrence.Most genetic lesions occur de novo or as a result of intrauterine toxinexposure and so should not recur, to the extent that the mother canavoid recurrent exposure. The parents need to be screened for the pos-sibility of transmitting a defective gene, in which case the likelihood ofanother affected child is dictated by the laws of inheritance. In this re-gard, various modes of inheritance have been described (49).

It is also important to note that because of the mildness of clinicalexpression, in some cases a parent may indeed have a mild form ofDGA and further evaluation of the endocrine, cardiac, or immune sys-tems may be indicated for the parent.

Goldmuntz et al. (65), prospectively analyzed 251 patients withvarious conotruncal heart lesions to ascertain the incidence of 22q11deletions. The goal of the study was to define guidelines for deletionscreening in patients with conotruncal defects. On the basis of theirfindings, they recommend that all newly diagnosed infants with inter-rupted aortic arch type B, truncus arteriosus, tetralogy of Fallot, andposterior malalignment type ventricular septal defect without interrup-tion of the aortic arch be evaluated for 22q11 deletion. Bristow andBernstein (84), however, take an opposing view and point out adverseeffects of deletion screening. The reader is urged to read both of thesearticles and arrive at his or her own decision about the risks and ben-efits of deletion screening in these situations.

SummaryThe DiGeorge anomaly, originally considered a clinical paradigm

for isolated thymus deficiency, has now been redefined as a member ofa group of disorders that share in common a chromosome deletion



resulting in monosomy 22q11 (CATCH-22 or DGA/VCFS). In additionto the thymus defect, conotruncal heart anomalies, dysmorphism, hy-poparathyroidism, and cleft palate are prominent features. Despite theemphasis on thymus involvement in DGA, a clinically significant thy-mus defect is found only in a small percentage of these patients prob-ably occurring in less than 5% of the cases. Maldescent of the thymus,however, is extremely common, leading to an absence of mediastinalthymic tissue in nearly all cases.

The basic embryological fault in these disorders is an inadequatedevelopment of the facial neural-crest tissues, resulting in defective or-ganogenesis of pharyngeal pouch derivatives that receive cephalicneural-crest contribution to the mesenchmyme. The causes for thismaldevelopment are both genetic and extragenetic in origin and thegenetic lesions act in concert with random environmental events to pro-duce the ultimate clinical picture.

The modern research approaches now available have cleared awaymost of the confusion clouding the clinical and theoretical aspects ofDGA and related disorders, providing the clinician with useful land-marks to assess and treat these intriguing clinical challenges.

AcknowledgmentI gratefully acknowledge support for my studies from the Univer-

sity of Vermont Immunomodulation Fund, which has particularly ben-efited from continuous generous support of Greg and Tully Raetz.

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