American Journal of Medical Genetics Supplement 2:65-72 (1986) Developmental Field Concept:275-282

Occurrence of Holoprosencephaly in Chromosome 13 Disorders Cannot Be Explained by Duplication/Def iciency of a Single Locus

Golder N. Wilson, Majed Dasouki, and Mason Barr, Jr.

Centre for Human Genetics, Department of Medical Genetics, and Montreal Children 's Hospital Research Institute, McGill University, Montreal, Quebec (G. N. N), and the Section of Pediatric Genetics, 01225 MPB, University Hospitals, Ann Arbor, Michigan (M. D., M. B.)

Four cases of holoprosencephaly with duplicationldeletion involving chromosome 13 are presented and additional cases are summarized from the literature. When examined as a series, the duplications (trisomy 13, trisomy 13pter -+ q14) and deletions (deletion 13q12 + qter, deletion 13q31 -+ qter, ring 13 with deletion 13q14 --f qter) exclude deletion or duplication of single chromosome 13 bands as the cause for holoprosencephaly. Increased dosage of the 13pter + q14 region relative to the 13q14 .+ qter region as the cause is also ruled out by the duplication 13q21 + qter cases reported in the literature. Altered timing of forebrain devel- opment, causing reversion to a more primitive embryonic and phylogenetic brain structure, is related to dosage imbalance of at least two chromosome 13 regions.

Key words: holoprosencephaly , chromosome 13, MCA/malformation syndromes, heterochrony

I did not know how to express the feeling of admiration that I felt for the grandeur of the creation in general, and for that of man in particular, when I saw that, at a first stage (of ontogeny), the human brain resembled that of a fish; that at a second stage, it resembled that of reptiles; at a third, that of birds; and at a fourth, that of mammals, in order finally to elevate itself to that sublime organization that dominates all nature.

Etienne Serres as quoted in Gould [1977].

Received for publication October 29, 1985; revision received February 24, 1986.

Address reprint requests to Golder N. Wilson, M.D., Ph.D., Room A717, Montreal Children's Hospital, 2300 rue Tupper, Montreal, Quebec, Canada H3H 1P3.

0 1986 Alan R. Liss, Inc.

66 Wilson, Dasouki, and Barr

INTRODUCTION

Explanations for the way in which aneuploidy causes a specific malformation syndrome have often focused upon altered expression of single loci within the triplicated or monosomic chromosome segment. Examples of this reductionist ap- proach include the chromosome “maps” relating malformation to specific bands on chromosome 13 [Noel et al, 19761 or 18 [Turleau et al, 19801. While selected genes on aneuploid chromosomal segments must be responsible for the phenotypic specific- ity of chromosomal syndromes, their relation to individual anomalies is more diffuse and variable [Wilson et al, 1985al than usual models for single gene action would predict. We test this hypothesis by examining the occurrence of holoprosencephaly, a malformation reflecting abnormal forebrain development, in five categories of chro- mosome 13 aneuploidy . We conclude that causation of holoprosencephaly cannot be ascribed to altered dosage of a single chromosome 13 locus.

CLINICAL REPORTS Patient 1

A female infant weighing 2960 g at 38 weeks gestation was born to 22-year-old parents. Anomalies noted at birth (Fig. la) included severe microcephaly (head circumference (HC) was 3 1 cm), alobar holoprosencephaly (by cranial ultrasound), hypotelorism (interpupillary distance of 2.9 cm), bilateral microphthalmia with poorly differentiated globes, antimongoloid slant to the eyes, fused imperforate nares, mal- formed ears, bilateral postaxial polydactyly of the hands, hypoplastic labia majora, and prominent heels. Death occurred at age 3 h and a chromosome analysis (Fig. 2a,b) demonstrated 47,XX, + 13(pter + q14:).

Patient 2

A stillborn male fetus weighing 621 g at 22 weeks gestation was born to a 24- year-old primagravida. Fetal anomalies (Fig. lb,c) included microcephaly, posterior encephalocele, semilobar holoprosencephaly, microphthalmia, short and webbed neck, pulmonary hypoplasia, tetralogy of Fallot, aberrant right subclavian artery, renal hypoplasia, asplenia, malrotation of the gut, hemiuterus, anal atresia with rectova- ginal fistula, multiple joint contractures with dislocated hips, absent thumbs, bilateral simian creases, bilateral syndactyly of toes 4 and 5 , and single umbilical artery. Chromosome analysis (Fig. 2c) showed 46,XY,del( 13)(pter + q13:).

Patient 3

This 23908 male infant was born at term to a 31-year-old mother and a 33-year- old father. Death occurred at age 10 weeks due to respiratory insufficiency. Autopsy showed multiple anomalies (Fig. ld,e) including microcephaly (HC 28 cm), arhinen- cephaly with alobar holoprosencephaly, hydrocephalus, cystic fourth ventricle, broad nasal root, upslanting papebral fissures, bifid uvula, short upper lip, downturned corners of the mouth, cupped ears with poorly formed helices, short and webbed neck, bifid scrotum, atretic anus with anterior perineal fistula, bilateral simian creases, hypoplastic right thumb, absent left thumb, syndactyly of the right fourth and fifth digits, hypoplastic left fifth fingernail, absent left fifth toe, and syndactyly of toes 4 and 5 on the right. A white light reflex was noted in the right pupil during life, revealing hyperplastic vitreous, retinal detachment, and cataract but no retino-

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blastoma upon autopsy. Chromosomes were (Fig. 2d) 46,XY,de1(13)(pter -+ q31:); parental chromosomes were normal.

Patient 4

This was a 1370g 33-week-old, breech female infant of a 21-year-old mother and 25-year-old father. Multiple anomalies (Fig. le,Q included microcephaly (HC 27 cm), semilobar holoprosencephaly with cystic dilation of the third and fourth ventri- cles and cerebellar hypoplasia (by CT scan), left microphthalmia with microcornea and a small left palpebral fissure, right optic nerve hypoplasia, micrognathia, hypo- plastic nipples, stenotic anus with a rectovaginal fistula, sacral dimple, absent thumbs and first metacarpals, bilateral simian creases, and bilateral syndactyly between the third and fourth toes with left equinovams. Following a turbulent neonatal course

Fig. la,b. See page 69 for legend.

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Fig. lc-e. See page 69 for legend.

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Fig. 1. Cases 1-4. a) Case 1 demonstrates a cebocephalic face with used nares, hypotelorism, and downslanting palpebral fissues. b,c) Case 2 with absent thumb and microcephaly and unilobar forebrain. d,e) Case 3 with broad nasal root, epicanthal folds, downturned corners of the mouth, malformed ears, and micrognathia. f,g) Case 4 with broad nasal root, left rnicrophthalrnia, and absent thumb with prehensile first finger.

including necrotizing enterocolitis, congestive heart failure, and microhematuria with normal IVP, the patient died of respiratory insufficiency at age 5 months. Chromosome analysis (Fig. 2e,Q showed ring 13 with variable breakpoints 46,XX,r13 (p13:q22 + q33). Parental chromosomes were normal.

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Fig. 2. Partial karyotypes of cases 1-4 beside chromosome 13 duplicationideletion categories. Giemsa- trypsin banded karyotypes from case 1 (a), case 2 (c), case 3 (d), and case 4 (e,f). A silver-stained partial karyotype from case 1 (b) is also shown to support identification of the marker chromosome as 13 q - . Categories of patients having the indicated duplications (+) or deficiencies (-) of chromosome 13 are represented by the bars beside the standard idiogram. Each category has been associated with a significant incidence of holoprosencephaly [Schinzel, 19841.

DISCUSSION

The spectrum of forebrain malformations described by the term holoprosen- cephaly is particularly relevant to understanding the relationship of morphogenesis to genomic change. The derivation of olfactory bulbs and stalks, cerebral hemispheres, optic primordia, and basal ganglia from a single prosencephalic vesicle [Warkany et al, 19811 is highly conserved in vertebrates as indicated by the occurrence of cyclopia in most vertebrate species [Rogers, 19631. Alterations of this developmental field [Opitz, 19851 range from a simple holosphere (alobar holoprosencephaly) to partial (semilobar) or complete (lobar) differentiation of the cerebral lobes [Warkany et al, 198 11 with uniform deficiency of olfactory lobes (arhinencephaly) . The underlying brain malformation is predicted by variable facial consequences which include cyclo- pia, ethmocephaly, cebocephaly, or premaxillary agenesis [Cohen et al, 19711. Etio- logic heterogeneity of this malformation spectrum is shown by the induction of cyclopia by various agents (KC1, low oxygen pressure, ethyl alcohol) in experimental animals [Rogers, 19631, and by sporadic [Lichtenstein and Maloney, 19.541, autosomal recessive or chromosomal [Holmes et al, 19741, and teratogenic [Barr et al, 19831 causes in man.

Figure 2 presents the partial karyotypes of cases 1-4 and illustrates the extent of duplicated/deleted material beside the standard idiogram. Also represented in Figure 2 are duplications of the entire 13 chromosome and of the 13q21 -+ qter region which have well documented association with holoprosencephaly [Schinzel, 19841. When viewed as a group, it is clear that the dosage relationships diagrammed in Figure 2 preclude a single chromosome 13 region being responsible for the genesis of holoprosencephaly malformation sequence. Trisomy for the entire 13 chromosome, trisomy for the pter -+ q14 or q14 + qter segments, and monosomy for the q12 +

qter or q31 + qter segments all are associated with holoprosencephaly. Increased dosage of the 13pter -+ q14 region relative to 421 qter could be invokcd as an

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explanation, except that 2 of the 20 cases of 13q21 + qter trisomy in the literature had holoprosencephaly [Schinzel, 19841. Imbalance of at least two chromosome 13 regions is therefore necessary to cause holoprosencephaly, and this relationship to multiple loci is emphasized by the occasional occurrence of holoprosencephaly in 23 other chromosomal syndromes [Schinzel, 19841.

The multilocus causation and variable incidence of holoprosencephaly relative to aneuploidy is characteristic of most component anomalies in chromosomal syn- dromes. The idea of multiple regions on the chromosome being required for genesis of a malformation is consistent with polygenic models invoked to explain the inci- dence of many isolated defects [Carter, 19651. If stated with modern knowledge of a mobile genome in mind, this concept is reminiscent of Goldschmidt’s idea that major morphologic change derives from altered structure of the chromosome as a whole (macromutations) rather than from gradual accumulation of single gene mutations [Goldschmidt, 19821. Further, the selective retardation of prosencephalon develop- ment in holoprosencephaly is not only analogous to the evolutionary process of heterochrony [Gould, 19771, but recalls Goldschmidt’s emphasis upon rate-determin- ing genes to explain the significance of chromosome pattern. Thus a progression from multilocus duplication/deficiency to altered timing of developmental events, to rapid morphologic change may be hypothesized from the study of human aneuploidy. More rarely, mutations in key rate-determining genes may cause complex malformations to follow Mendelian inheritance.

A relationship of increased developmental variability to the extent of chromatin excess has been hypothesized in a review of dup(9) disorders [Wilson et al, 19851, and altered embryologic timing can be viewed as one manifestation of disordered developmental homeostasis [Shapiro, 19831. Heterochronic changes have previously been documented in trisomy 21 [Hall, 1965; Aziz, 19811, trisomy 13 [Huehns et al, 19641, trisomy 18 [Barash et al, 19701, and in nonchromosomal disorders such as diabetic embryopathy in man [Perrin et al, 19851 or rats Wilson et al, 1985bl. While animal models [Rogers, 19631 should allow delineation of altered timing in the ontogenesis of multiple RNA or protein molecules, comparative anatomic study of human malformation is important to place such information in the context of morpho- logic evolution. The opportunity to relate vertebrate forebrain ontogeny to its phylo- geny, a parallel so vividly described by Serres in 1860, emphasizes the value of continued attention to holoprosencephaly in man and other vertebrates.

ACKNOWLEDGMENTS

This is publication no. #86005 from the Montreal Children’s Research Institute.

REFERENCES

Aziz MS (1981): Possible “atavistic structures” in human aneuploids. Am J Phys Anthropol 54: 347-353.

Barr M, Jr., Hanson JW, Curry K, Sharp S, Toriello H, Schmickel RD, Wilson GN (1983): Holoprosen- cephaly in infants in diabetic mothers. J Pediatr 102565-568.

Barash BA, Freedman L, Opitz JM (1970): Studies of malformation syndromes XXIII: Anatomical studies in the 18-trisomy syndrome. New York: Alan R. Liss, Inc., for the National Foundation- March of Dimes. BD:OAS VI (4): 3-15.

Carter CO (1965): The inheritance of common congenital malformations. Progr Med Genet 4: 54-75. Cohen MM, Jr., Jirasrk JE, Guzman RT, Gorliii RJ, Petersen MQ (1971): Holoprosenceplialy and facial

DFC:281

72 Wilson, Dasouki, and Barr

dysmorphia: nosology, etiology, and pathogenesis. Birth Defects 7: 125-135. Goldschmidt R (1982): “The Material Basis of Evolution.” New Haven: Yale University Press. Gould SJ (1977): “Ontogeny and Phylogeny.” Cambridge: Harvard University Press, p 47. Hall B (1965): Delayed ontogenesis in human trisomy syndromes. Hereditas 52:334-344. Holmes LB, Driscoll S, Atkins L (1974): Genetic heterogeneity of cebocephaly. J Med Genet 11: 35-

Lichtenstein BW, Maloney JE (1954): Malformation of the forebrain. J Neuropathol Exp Neurol 13:

Noel B, Quack B, Rethore MO (1976): Partial deletions and trisomies of chromosome 13; mapping of

Opitz JM (1985): The Developmental Field Concept. Am J Med Genet 21: 1-11. Perrin SP, Greene MF, Faller DV (1985): Delay in the fetal globin switch in infants of diabetic mothers.

New Engl J Med 312:334-338. Rogers KT (1963): Experimental production of perfect cyclopia in the chick by means of KCl, with a

survey of the literature on cyclopia produced experimentally by various means. Dev Biol 8: 129- 150.

40.

117-128.

bands associated with particular malformations. Clin Genet 9593-602.

Schinzel A (1984): “Catalogue of Unbalanced Chromosome Aberrations in Man.” Berlin: de Gruyter. Shapiro B (1983): Down syndrome-a disruption of homeostasis. Am J Med Genet 14:241-269. Turleau C, Chavin-Colin F, Narbouton R, Asensi D, Grouchy J de (1980): Trisomy 18q. Trisomy

Warkany J, Lemire RJ, Cohen MM Jr. (1981): “Mental Retardation and Congenital Malformations of

Wilson GN, Raj A, Baker D (1985a): The phenotypic and cytogenetic spectrum of partial trisomy 9.

Wilson GN, Howe M, Stover JM (1985b): Delayed developmental sequences in rodent diabetic embry-

mapping of chromosome 18 revisited. Clin Genet 18:20-26.

the Central Nervous System.” Chicago: Year Book Medical Publishers, pp 176-190.

Am J Med Genet 20:277-282.

opathy. Pediatr Res 19: 1337-1340.

Edited by John M. Opitz and James F. Reynolds

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