Down syndrome--a disruption of homeostasis

Hypothesis: association of the critical region of trisomy 18 and 18q2--syndrome with dermatoglyphic findings and a growth suppressor (deleted in colon cancer) locus

Evidence from the literature is reviewed to suggest that when fingertip dermal ridge patterns in chromosomal deletion syndromes are characteristic of the opposite spectrum of the developmental scale from patterns found in cases trisomic for the same chromosomal region, the association may be a consequence of loci with growth regulatory functions. Evidence is presented that DNA markers at 18q21 should be the first candidate sequences to be used to test this hypothesis in families with fingertip arches segregating in an apparent autosomal dominant fashion.

Clinical characteristics and treatment outcome of children with acute lymphocytic leukemia and Down's syndrome. A Pediatric Oncology Group study

Of 2947 children with acute lymphocytic leukemia (ALL), treated during three consecutive studies of the Pediatric Oncology Group (1974-1986), 52 (1.8%) had Down's Syndrome (DS). Comparison of clinical and laboratory characteristics showed no significant differences in leukocyte count, racial distribution, sex ratio, platelet count, incidence of mediastinal mass, lymphadenopathy or hepatosplenomegaly, or percentage of blood or bone marrow blasts for children with ALL with or without Down's Syndrome (DS-ALL or NDS-ALL, respectively). However, children with DS-ALL were slightly older at the time of presentation and had higher hemoglobin values. The relative frequency of each major immunophenotype (early pre-B, pre-B, T, or B) was also comparable for patients with or without DS. For this report, treatment regimens were categorized as either conventional (no consolidation therapy) or intensive. Cox regression analysis revealed that the presence of DS, a higher leukocyte count, black race, or age older than 10 years was independently associated with a poorer event-free survival (EFS) for children treated with conventional chemotherapy. However, for the cohort of children who received intensive chemotherapy, DS was no longer an independent risk factor. In fact, event-free survival (EFS) was markedly improved to a level comparable with that observed in the children diagnosed as having NDS-ALL. On the other hand, serious toxicity, requiring interruption of treatment, was significantly more frequent in the intensively treated children with DS compared with similarly treated patients with NDS-ALL, although deaths resulting from toxicity occurred infrequently.

Analysis of gene-dosage effects on the expression of CD18 by trisomy 21 lymphoblastoid cell-lines using a statistical model to fit flow cytometry profiles

It is not clear whether Down syndrome, the phenotypic expression of constitutional trisomy for chromosome 21 (T21), is the result of generalised disruption of homeostasis resulting from genetic imbalance, or the over-expression of specific genes on chromosome 21. In order to understand the effect of gene dosage more clearly, we have analysed the predicted and actual levels of expression of the leucocyte integrin beta subunit CD 18 on the surface of T21 leucocytes. Previous studies showed that CD18 expression by T21 lymphoid cell lines (LCL) is greater than on normal LCL. We have now developed a computer model that compares the observed and predicted CD18 flow cytometric profiles for trisomy 21 LCL. Three parameters (alpha, beta and gamma) have been defined that measure different aspects of gene dosage. Using the computer model to calculate these parameters, we have carried out a series of paired comparisons between normal and T21 LCL. The results show that, in some T21 LCL, increased CD18 expression is proportional to the existing gene dosage, in another set the effect is additive, whereas in others there is a combination of proportional and additive effects. The results suggest that gene regulation can exert pleiotropic effects on gene-dosage, and is consistent with a model in which gene dosage itself is the cause of disrupted homeostasis.

Holoprosencephaly: examples of clinical variability and etiologic heterogeneity

Clinical variability and causal heterogeneity of holoprosencephaly is discussed in relation to several newborn infants with cyclopia (cases 4,5,6), cebocephaly (cases 2,3), and premaxillary agenesis (case 1). In subjects with holoprosencephaly, the presence of multiple malformations is an indicator of concomitant chromosome aberrations, as in present case 1 (Down syndrome) and case 3 (trisomy 13). Cases 5 and 6 are two monozygotic twins with the same type of cyclopia and alobar holoprosencephaly recognized by prenatal ultrasonography. The diagnostic importance of ultrasonographic, cytogenetic, and pathological studies is pointed out in view of etiologic evaluation, genetic counseling, and prevention of holoprosencephaly.

Karyotype/phenotype controversy: genetic and molecular implications of alternative hypotheses

Alternative explanations for aneuploid phenotypes are evaluated according to clinical and experimental predictions. The additive hypothesis views the diverse manifestations of aneuploidy as the simple sum of individual gene dosage effects. The interactive hypothesis views aneuploid characteristics as network properties of genes within and outside the aneuploid segment. The importance and historical context of this debate are emphasized, and the 2 hypotheses are contrasted in terms of their experimental predictions and explanatory power. In particular, the concepts of colinearity and continuity derived from single gene action are explicitly examined in human aneuploidy with the conclusion they are likely to be false assumptions.

Syndromology: an updated conceptual overview. X. References

Part X is the reference section for Parts I through VIII which appeared in the four previous issues of the journal and Part IX which appears in this issue. The reference section itself is divided into two parts. The first part provides general references of selected topics for the interested reader. The second part consists of an alphabetical listing of all references cited in Parts I through IX.

Electrical membrane properties of cultured dorsal root ganglion neurons from trisomy 19 mouse fetuses: a comparison with the trisomy 16 mouse fetus, a model for Down syndrome

Because of synteny between mouse chromosome 16 and human chromosome 21, murine trisomy 16 (Ts16) has been considered an animal model for Down syndrome. Indeed, previous investigations have demonstrated that action potentials of cultured dorsal root ganglion (DRG) neurons from human trisomy 21 (Down syndrome) or mouse Ts16 fetuses show increased depolarization and repolarization rates, and decreased spike duration, compared to control neurons. In order to determine the specificity of these changes, we studied the electrical membrane properties of DRG neurons in culture from trisomy 19 (Ts19) and control fetal mice, using the whole cell patch-pipette recording technique. We found no significant differences in action potential parameters and passive membrane properties between Ts19 and control neurons. These findings support the notion that the alterations previously reported in Ts16 DRG neurons are specific, and not a general consequence of genetic imbalance imposed by autosomal trisomies.

The induction of tail malformations in trisomy 16 mouse fetuses heterozygous for the curly tail recessive gene

The mouse mutant curly tail is thought to be inherited as an autosomal recessive (ct/ct) with incomplete penetrance so that approximately 60% of ct/ct individuals exhibit the curly tail (CT) phenotype. By outcrossing ct/ct with mouse stock carrying specific heterozygous combinations of Robertsonian (Rb) chromosomes, trisomy 16 (Ts16) and Ts19 mouse fetuses (and their chromosomally balanced littermates) were derived which were heterozygous for the ct gene. All of the Ts16 (ct/Rb;Rb) fetuses, studied between days 14-19 gestation had tail malformations, 86% of which were tail flexion defects (TFD) apparently very similar to the curly tail phenotype. Neither Ts19 nor any of the chromosomally balanced (ct/Rb) littermates from both experimental crosses showed any type of tail or other spinal malformation. At the 27-29 somite stage of development, Ts16 (ct/Rb;Rb) fetuses did not show any significant delay in the closure of the posterior neuropore (PNP) compared with their littermate controls, suggesting that the tail malformation observed in Ts16 (ct/Rb;Rb) occur as a result of mechanisms which differ significantly from those thought to be responsible to causing the curly tail malformation.

Defects of skeletal morphology, density, and structure in mouse fetuses with trisomy 16

Skeletal anomalies present in trisomy 16 in the mouse--an animal model of human trisomy 21--are described. Altogether 27 fetuses with trisomy 16 and 118 chromosomally balanced siblings were examined radiographically and by alizarin staining on day 20 of gestation; the radiographs were analyzed by computer-aided densitometry and structural differentiation. Extensive asymmetry or abnormal fusion of the vertebral centers and alterations of the vertebral arches were observed along with rib malformations (rib-vertebra syndrome). The skull primarily exhibited anomalies of the occipital bone. Ossification of the humerus, femur, and tibia was characterized by reduced mineralization. Typical, fracture-like alterations affecting only the tibia were also observed. Measurement of the lengths of the humeri of fetuses of comparable weight revealed a growth retardation not correlatable with the degree of mineralization. The significance of these skeletal abnormalities with regard to the trisomy 21 syndrome is discussed.

Anorectal malformations and Down syndrome

During 1980 to 1986, 89 children with Down syndrome and 42 with imperforate anus were diagnosed among 64,870 liveborn infants in the Jewish population of Jerusalem. Two of the children had both Down syndrome and imperforate anus. This indicates a high incidence of imperforate anus among children with Down syndrome (2.2%).

The effect of a pattern in palmar interdigital II on a-b ridge count in black and white Down syndrome cases and controls

An unconfirmed study by Fang (Ph.D. thesis, Univ. of London, 1950) in Britain showed that individuals with Down syndrome had lower total a-b ridge counts in palmar Interdigital area II (ID II) than a group of controls. This study compares 603 white Down syndrome cases and 93 black Down syndrome cases with 668 white and 402 black controls. Our results confirm those of Fang in that the Down syndrome cases in both racial groups had lower total a-b ridge counts than their respective controls. In addition, the black controls and Down syndrome cases had lower a-b ridge counts than their white counterparts. The mean a-b ridge count was significantly lower in individuals with a pattern in ID II compared to individuals without a pattern in ID II in both the Down syndrome and control groups. Some of the lower a-b ridge counts in the Down syndrome samples can be accounted for by the fact that there is an increased frequency of a pattern in ID II in Down syndrome cases. Both Down syndrome and normal individuals who had a pattern unilaterally had a lower than expected a-b ridge count on the contralateral hand that did not have a pattern. There was a tendency also for increased asymmetry in Down syndrome cases with a pattern in ID II.

Study of genetic variance in the fluctuating asymmetry of anthropometrical traits

We have studied the fluctuating asymmetry (FA) of 8 bilateral morphometric traits in two-parent families, comprising 216 families with one newborn baby, and 60 families with two children (age range 5-18 years). Heritability was assessed by: (1) multiple regression analyses of the children's measurements on the mother's and father's measurements; (2) midparent-child regressions; and (3) sibling correlations. The extent of genetic determination of individual FA measurements was generally low, albeit statistically significant in some cases. However, even these correlations were inconsistent between samples and relatives. However, the mean FA values for all 8 studied traits showed positive and significant correlation between parents and children in two samples and in total. Additive genetic variance, calculated from multiple regression analyses and midparent-child correlations, was estimated to be between 0.25-0.30. Three multiple regressions (two for the separate group and one for the total sample) yielded a statistically significant value (between 0.21-0.33) also for the non-additive genetic component.

Premature aging changes in neuromuscular junctions of transgenic mice with an extra human CuZnSOD gene: a model for tongue pathology in Down's syndrome

We examined the tongue muscles in several strains of transgenic mice carrying the human Zn-Cu superoxide dismutase (CuZnSOD) gene. The presence of the extra gene was confirmed in mated progeny and the gene product activity was measured in the tongue and found to be much higher than in normal littermate controls. Using electron microscopic morphometry, the neuromuscular junctions of the transgenic mice showed significant changes resembling excessive aging, with atrophy, degeneration, withdrawal, and sometimes destruction of the terminal axons, as well as the development of multiple small terminals. The myofibers showed little change except for slight hypertrophy and an increased variability in size. They also had more megamitochondria, fat droplets and lipofuscin bodies. Excess CuZnSOD generates H2O2 and hydroxyl radicals which affect both NMJ membranes and plasticity, and which may produce premature aging. The findings resemble those observed in tongues of patients with Down's syndrome, in whom an extra CuZnSOD gene is present as part of the trisomy of chromosome 21.

Holoprosencephaly and trisomy 21 in a child born to a nondiabetic mother

This is the second reported case of a child with holoprosencephaly and trisomy 21. The first case was born to a diabetic woman; in our case, there was no evidence of diabetes in the mother. Most of the distinctive facial features of Down syndrome were obscured by the presence of cyclopia and a supraorbital proboscis in this infant. The relevance of chromosome analysis in cases with holoprosencephaly is discussed.

Decreased developmental stability as assessed by fluctuating asymmetry of morphometric traits in preterm infants

Fluctuating asymmetry (FA) of 8 morphometric traits was studied in 113 preterm infants (26-36 wk of gestation), 103 term infants (37-41 wk), and their respective parents. With 3 different measures of FA, the highest values were obtained from extremely preterm infants (26-29 wk), and the lowest from the group of term infants. The estimates of FA values among parents, particularly mothers, showed a similar, albeit less pronounced, trend. Multiple regression analysis of individual mean FA values, calculated in infants for the 8 studied bilateral traits, documented a significant inverse correlation with gestational age and with the health status of the infants and their mothers, as well as a positive correlation with the mothers' mean FA values.

Heterochrony and human malformation

The role of altered developmental timing or heterochrony in morphologic evolution has intrigued classical and modern biologists. Analogous manifestations of developmental asynchrony occur in human dysmorphogenesis where they illustrate the residue and repertoire of phylogenetic change. Certain single malformations such as holoprosencephaly immediately suggest heterochrony by their resemblance to antecedent phylogenetic or embryologic structures. Multiple malformation syndromes of genetic, chromosomal, or teratogenic etiology may have altered developmental timing as an underlying theme. The persisting alpha-fetoprotein synthesis in ataxia-telangiectasia, the morphologic atavisms in Down or trisomy 13 syndromes, and the delayed growth or fetal to adult hemoglobin switch in diabetic embryopathy all exemplify developmental asynchrony. The perspective of heterochrony stresses the molecular history and hierarchy which is recapitulated with each pregnancy, and reconciles apparent discrepancies between the rates of molecular and morphologic evolution. Recognition of heterochrony places isolated anomalies in the context of pattern and suggests monitoring of teratogenesis through altered expression of ontogenetically regulated, phylogenetically relevant molecules.

Taurodontism: an anomaly of teeth reflecting disruptive developmental homeostasis

Two models concerning morphometric traits occurring frequently in aneuploidy states posit, respectively, 1) that they reflect the expression of specific major oligogenes for that trait on the chromosome involved or 2) that they result from a generalized disruption of developmental homeostasis. In contrast to previous studies that have investigated variations in morphometric traits in a single aneuploidy state, this study investigates a single morphometric trait, taurodontism, as it occurs in otherwise normal individuals, in nonchromosomal syndromes, and in aneuploidy syndromes to determine whether the trait best fits the oligogene or the disrupted developmental homeostasis model. Taurodontism is diagnosed from dental radiographs. It is an extreme variation in tooth form seen in multirooted teeth in which the bifurcation or trifurcation of the roots is displaced toward the apex of the root, resulting in increased size of the pulp chamber. The point of furcation, and consequently the size of the pulp chamber, is a quasicontinuously distributed trait. The results indicate that taurodontism most likely is the result of disrupted developmental homeostasis.

Skeletal anomalies in trisomy 21 as an example of amplified developmental instability in chromosome disorders: a histological study of the feet of 21 mid-trimester fetuses with trisomy 21

In a previous radiographic study on the feet of 71 adults with trisomy 21 we found, in comparison to control individuals, an increased prevalence of biphalangeal toes and metatarsophalangeal sesamoid bones. The present histological study on the feet of 21 mid-trimester fetuses with prenatally diagnosed trisomy 21 confirms results of the earlier study. At both stages of development these minor bone anomalies have about the same frequency, thus suggesting 1) that they are selectively neutral, and 2) that they reflect a basic (innate) failure of ordered morphogenesis. Our observation that the normal spatial pattern of skeletal variants is reproduced in trisomy 21 simply on a quantitatively higher level lends sound support to the hypothesis of amplified developmental instability in chromosome trisomies.

Collagen and fibronectin synthesis by trisomic and triploid fibroblasts from human spontaneous abortuses

Collagen and fibronectin synthesis by trisomic and triploid fibroblasts derived from human spontaneous abortuses was studied. It was demonstrated that the level of fibronectin and collagen production in fibroblasts with trisomy 7, trisomy 9, and triploidy was reduced as compared with diploid cells. A correlation between this observation and an increased rate of intracellular 14C-procollagen degradation was also established for the anomalous strains. No difference in hydroxylation of 14C-proline residues in alpha 1(I) and alpha 2(I) collagen chains and no fluctuation in the collagen type (I): type III ratio was found in the strains with the abnormal karyotypes. It was concluded that differentiation of the abnormal fibroblasts was impaired. The data also favour the hypothesis that the deficiency of the fibroblasts in producing proteins may account for a variety of anatomic abnormalities of embryos.

Atypical phenotype associated with deletion (15) (pter----q11::q13----qter)

We report on a 10-year-old boy with an interstitial deletion within the region of bands 15q11----q13. Authors have associated the manifestation of the Prader-Willi syndrome (PWS) with variable deletions involving the bands q11----q13. Our patient had atypical manifestations not usually associated with PWS, ie, normal stature, proportionally sized hands and feet, normal genitalia, and was nonambulatory and severely mentally retarded. This case emphasized the clinical diversity seen in proximal 15q deletions in the region considered to be correlated with the PWS.

Down's syndrome and leukemia: epidemiology, genetics, cytogenetics and mechanisms of leukemogenesis

The association of Down's syndrome and leukemia has been documented for over 50 years. Multiple studies have established the incidence of leukemia in Down's syndrome patients to be 10- to 20-fold higher than that in the general population. The age of onset for leukemia in these children is bimodal, peaking first in the newborn period and again at 3-6 years. This increased risk extends into adulthood. All cytogenetic types of Down's syndrome apparently predispose to leukemia. The proportion of acute lymphoblastic leukemia and acute nonlymphoblastic leukemia in patients with Down's syndrome is similar to non-Down's syndrome leukemia patients matched for age. There are case reports in which leukemia, Down's syndrome, and other chromosomal aberrations cluster within a family. In these kindreds, there may be a familial tendency toward nondisjunction. Congenital leukemia also occurs with increased frequency in Down's syndrome patients, and is characterized by a preponderance of acute nonlymphoblastic leukemia (similar to non-Down's syndrome patients). Transient leukemoid reactions have been observed in Down's syndrome patients, as well as in phenotypically normal children with constitutional trisomy 21 mosaicism. The transient leukemoid reactions are characterized by a high spontaneous remission rate. However, in some Downs syndrome patients with apparent transient leukemoid reaction, leukemia relapse following periods of spontaneous remission have been reported. Cytogenetic studies of leukemic cells in Down's syndrome patients show a tendency toward hyperdiploidy. Besides trisomy 21, there is no other specific cytogenetic abnormality that is characteristic of the leukemia cells in Down's syndrome patients. The possible mechanisms for leukemogenesis in Down's syndrome patients may involve factors at the levels of the organism, the organ/system, the cell, the chromosomes or the DNA.

Skeletal anomalies in a patient with the Pallister/Teschler-Nicola/Killian syndrome

We report on a 3-8/12-year-old boy with the Pallister/Teschler-Nicola/Killian syndrome and previously unreported bilateral skeletal anomalies consisting of small feet and short but otherwise normal humeri, ulnae, femora, and fibulae. His peripheral blood chromosomes were normal; however, 47,XY,+i(12p) was found in 100% of fibroblasts.

Chromosome 18 aneuploidy: anatomical variations observed in cases of full and mosaic trisomy 18 and a case of deletion of the short arm of chromosome 18

Cases of full and mosaic trisomy 18 and a body of an infant with the 18p-syndrome were dissected in detail to compare the anatomical variations associated with these 3 chromosome imbalances involving autosome 18. The types and numbers of morphologic variations present in both the full and mosaic trisomy 18 bodies were similar to the types and numbers of variations seen in all other cases of full trisomy 18 that have been studied by gross dissection. Apart from an atrial septal defect, the body of the infant with the 18p- imbalance showed only 2 striking defects: 1) deficiencies of the levator palpebrae superioris muscle of the upper eyelid, and 2) absence of the ligament of the head of the femur. The first variation provides a morphologic basis to explain the ptosis which is observed frequently in affected individuals. Absence of the ligament of the head of the femur may be a factor contributing to congenital dislocation of the hip, which is reported occasionally in affected individuals. In addition to providing more detailed information about the phenotype of individual aneuploidy syndromes, studies of cases of different imbalances of single autosomes may provide additional insights about the genotype/phenotype relationships of specific chromosome segments.

Partial duplication 8q12----q21.2 in two sibs with maternally derived insertional and reciprocal translocations: case reports and review of partial duplications of chromosome 8

We report on two sibs with duplication of the segment 8q12----8q21.2 resulting from malsegregation of a maternal insertional translocation: [inv ins (5;8)(p13;q12q21.2)]. The mother also carries a reciprocal translocation [t(1;6)(q31;q5)], which was transmitted in the balanced state to the propositi and to a phenotypically normal son and daughter. The literature on two translocations occurring in one individual and on insertional rearrangements is reviewed in terms of reproductive risks to balanced carriers. The two affected infants have a previously undescribed partial duplication of an interstitial segment of 8q and a pattern of abnormalities distinct from those seen in other partial duplications of 8. These infants are reviewed with 78 other cases of partial duplications of chromosome 8 with regard to phenotype-karyotype correlations.

Duplication 3p21----3pter and cyclopia

We report on a patient with an interchromosomal duplication of 3p, from 3p21 to 3pter, which apparently arose de novo. The infant had multiple malformations including holoprosencephaly and cyclopia. It is possible that duplication 3p has a generalized effect on the holoprosencephalon or the cleavage of the embryonic forebrain. Fibroblasts from the patient are available from the NIGMS Human Genetic Mutant Cell Repository (GM 7216).

Does "ring syndrome" exist? An analysis of 207 case reports on patients with a ring autosome

Analysis of 207 case reports on patients with ring autosome showed that: Forty patients, a fifth of the total, had extreme growth failure together with an otherwise almost-normal appearance, viz. no major malformation, no specific deletion syndrome, no or only a few unspecific minor anomalies. This phenotype may be regarded as the "ring syndrome", a term proposed by Cote et al. (1981) since it is independent of what chromosome is involved. Severe growth failure, the sole major physical abnormality in the "ring syndrome", was seen significantly more often among patients with ring of larger chromosomes than among patients with a smaller ring, indicating that the greater the chromosome involved in ring formation, the higher is the probability of severe growth failure. Larger ring chromosomes showed significantly more often instability than smaller rings, suggesting that there may be a correlation between ring instability and the size of the chromosome involved. Growth failure was present in significantly more patients with a "labile" ring than with a "stable" ring, indicating that a correlation may exist between ring instability and growth failure. It is suggested that the "ring syndrome" observed in many cases with ring autosome may result from end-to-end fusion of chromosome ends, an event not involving deletion in the genetic sense. It is also suggested that the "ring syndrome" is caused by a continuous generation of secondary aneuploid cells with increased mortality, i.e. structural ring instability which seems to be a function of the size of the chromosome involved. Thus, formation of a ring chromosome in certain cases might be regarded as a "structural mutation", i.e. an alteration in the structure of the genetic material per se, rather than a loss or gain of genetic dosages.

Dermatoglyphic traits as possible markers of developmental processes in humans

Here we provide data on the relationship in variability between common anthropometric characters (stature, interocular diameter, mesosternal chest circumference, bi-trochanteric diameter, and palm length) and dermatoglyphic traits in groups of young healthy individuals. Our working hypothesis was that quantitative variables of fluctuating asymmetry, diversity, and variability involving dermatoglyphic traits, will be higher in more homozygous groups than in heterozygous ones. It was found that individuals who were in the center (average +/- 0.67 SD) of the morphological trait distribution (and therefore perhaps more heterozygous at loci determining the aforementioned morphological traits), have reduced fluctuating asymmetry in their ridge counts (RC). These heterozygous individuals tended to have lower values of the RC-diversity-indices and the Shanon information-measures of discrete digital patterns. On the basis of the present results and a large body of literature data, the use of dermatoglyphic traits as "markers" of developmental processes are discussed.

Developmental consequences of autosomal aneuploidy in mammals

Autosomal aneuploidy in mammals adversely affects developmental processes. In human beings, for example, trisomy 21 is the most frequent aneuploidy detected among newborns and the most common known genetic cause of mental retardation. In this review, several hypotheses are discussed that have been proposed to explain the mechanisms by which aneuploidy (especially trisomy) disrupts development. These mechanisms included specific gene dosage effects, generalized disruption of genetic homeostasis, and the influence of the parental origin of the duplicated chromosome. The availability of specific chromosomal rearrangements in mice, coupled with selective breeding schemes, permits generation of aneuploidy of specific chromosomes or chromosomal segments on controlled genetic backgrounds, thus enabling the systematic study of the causes and consequences of defined aneuploidy. Phenotypic characteristics associated with a number of specific aneuploidies in the mouse are discussed. Emphasis is placed on the effects of trisomy 16. Genetic homology between mouse chromosome 16 and human chromosome 21 has led investigators to suggest that analogous mechanisms will be responsible for the developmental abnormalities produced in these respective aneuploidies. Analysis of trisomy 16 mice from the organismal to the subcellular level has revealed a number of phenotypic characteristics (particularly neurobiologic ones) shared with human trisomy 21. The dosage effects of shared genes (or their products) may contribute to the development of these features.

Effect of aneuploidy and neoplasia on human ribosomal DNA inheritance

A DNA length polymorphism in the nontranscribed spacer region of the repeating unit that codes for human ribosomal RNA produces a characteristic pattern or restriction fragments in each individual. Quantitative densitometric analysis of ribosomal ribosomal DNA fragment distribution in families demonstrates additive inheritance in those with chromosomally normal or aneuploid offspring. Differences between offspring and parental ribosomal DNA patterns could be explained by a heterogeneous distribution of length variants on the acrocentric chromosomes. New ribosomal DNA length variants of 9.0, 6.7, 5.5, and 5.2 kilobases were observed in normal individuals after BamHI restriction, and the former two were present in multiple copies. A panel of solid tumor specimens exhibited ribosomal DNA patterns that were generally characteristic of the patient rather than tumor type. However, novel ribosomal DNA length variants or changes in length variant proportions were noted in three of the four tumors for which adjacent normal tissue was available for comparison; these alterations occurred in a Burkitt lymphoma, a teratoma, and a Wilms tumor. A consistent karyotype of 50,XY in the Wilms tumor specimen supports previous evidence for increased repetitive DNA variation in aneuploid, neoplastic tissues.

The Farber lecture. Prenatal and perinatal death: the future of developmental pathology

It seems evident that most of "humanity" dies before, not after birth and that perhaps only one-third survive from earliest beginnings until birth or the end of the first year of life. As many as 50% of all human ova may have a chromosome abnormality with over 99% mortality, making this type of genetic defect not just the commonest cause of death prenatally and the cause of a substantial proportion of the malformations of abortuses and fetuses, but the commonest cause of death in humans altogether. This study, along with numerous studies before it, is the most convincing justification for doing autopsies on pre- and perinatally dead embryos and fetuses. With appropriate supervisory collaboration from a university pediatric pathology unit, such studies can be done efficiently at secondary-care centers, including some in rather remote locations. Thus, there exists no excuse anymore for not doing such studies which, in a high proportion of cases, will give the parents and attending physicians an explanation of the events and observed abnormalities with a chance in all cases at correct diagnostic and genetic counseling, appropriate monitoring of the next pregnancy, and an improved possibility of population monitoring for teratogens and substantial increases in the mutation rate. These immediate benefits to patients and the quality of medical care are the major, but not the only, justification for doing fetal pathology. Literally thousands of new discoveries still await the investigator with a prepared mind and will afford many research opportunities to those with an interest in normal and abnormal human development. And if it can be resolved in present-day pathology training programs how to attract greater numbers of interested residents and fellows into the field and to motivate them to take the additional training to become expert in developmental and genetic analysis, then "developmental" pathology will be facing a very bright future indeed.

Neurochemical characterization of embryonic brain development in trisomy 19 (Ts19) mice: implications of selective deficits observed for abnormal neural development in aneuploidy

In this study, we examined the neurochemical profiles of selected brain regions (cerebral hemispheres, diencephalon/brainstem) in fetal (day 14 to 18 gestation) trisomy 19 (Ts19) mice. The neurochemical characteristics we observed in Ts19 mice were quite different from those we observed previously in Ts16 mice. Choline acetyltransferase (ChAT) activity was reduced significantly in the cerebral hemispheres, but not in the brainstem/diencephalon, of the fetal Ts19 mouse brain, suggesting a selective vulnerability of telencephalic cholinergic neurons. Additionally, the activity of glutamic acid decarboxylase (GAD) was reduced significantly in both hemispheres and diencephalon/brainstem of late gestation Ts19 fetuses, suggesting a selective vulnerability of GABAergic neurons as well. While the levels of catecholaminergic and dopaminergic markers were reduced significantly at late gestational ages, the relative rate of turnover of dopamine (DA), measured by the ratio of DOPAC/DA, was elevated significantly in Ts19 mice. Neither reduction in the thickness of various cellular zones of the cerebral cortex nor reduced cell density of the cerebral cortex accounts for the alterations in neurochemical parameters observed in Ts19 mice. These results suggest that the effects of the triplication of specific genes on the respective chromosomes, rather than a generalized disruption of developmental homeostasis resulting from extra chromosomal material, may produce selective alterations in neurochemical and neuroanatomical markers observed in these two mouse trisomies.

Developmental genetics

Of particular concern to the human geneticist are the effects of genetic abnormalities on development. To gain an understanding of these effects it is necessary to engage in a reciprocal process of using knowledge of normal developmental events to elucidate the mechanisms operative in abnormal situations and then of using what is learned about these abnormal situations to expand our understanding of the normal. True developmental genes have not been described in man, although it is likely that they exist, but many developmental abnormalities are ascribable to mutations in genes coding for enzymes and structural proteins. Some of these even produce multiple malformation syndromes with dysmorphic features. These situations provide a precedent for asserting that not only monogenic developmental abnormalities, but also abnormalities resulting from chromosome imbalance must ultimately be explicable in molecular terms. However, the major problem confronted by the investigator interested in the pathogenesis of any of the chromosome anomaly syndromes is to understand how the presence of an extra set of normal genes or the loss of one of two sets of genes has an adverse effect on development. Several molecular mechanisms for which limited precedents exist may be considered on theoretical grounds. Because of the difficulties in studying developmental disorders in man, a variety of experimental systems have been employed. Particularly useful has been the mouse, which provides models for both monogenic and aneuploidy produced abnormalities of development. An example of the former is the mutation oligosyndactylism which in the heterozygous state causes oligosyndactyly and in the homozygous state causes early embryonic mitotic arrest. All whole arm trisomies and monosomies of the mouse can be produced experimentally, and of special interest is mouse trisomy 16 which has been developed as an animal model of human trisomy 21 (Down syndrome). In the long run, the most direct approach to elucidating the genetic problems of human development will involve not only the study of man himself but also of the appropriate experimental models in other species.

The neurobiologic consequences of Down syndrome

Trisomy of the whole or distal part of human chromosome 21 (HSA 21) (Ts21) results in Down Syndrome (DS), which is characterized in part by mental retardation and associated neurological abnormalities. Structural abnormalities observed frequently include reduced brain weight, decreased number and depth of sulci in the cerebral cortices, neuronal heterotopias, and reduced numbers of specific populations of neurons, such as granule cells, in the cerebral cortices. Abnormalities in the structure of cells, primarily of the dendrites, are observed in portions of the neuraxis, such as the hippocampus, cerebellum, and cerebral cortices. Functional abnormalities in membrane properties in peripheral structures and in neurotransmitter enzyme systems in both peripheral and central structures are observed also. Brains of DS individuals over the age of 40 exhibit the characteristic neuropathologic and neurochemical stigmata of Alzheimer's disease (AD). The cholinergic and noradrenergic systems appear to be particularly vulnerable. To elucidate the mechanisms responsible for these abnormalities, identification of the genes located in the distal part of HSA 21 and the systematic study of animal model systems with close genetic homology are essential.

Abnormal neural fold development in mouse trisomy 12 and trisomy 14. II. LM and TEM

LM and TEM observations of embryonic tissue during the period of cranial neurulation are described in trisomic mouse embryos known to develop exencephaly, and are compared with tissue from normal mouse embryos. The earliest regularly observed differences were visible from the late presomite stage onwards, in the extracellular matrix of the cranial region. These were local defects of the basement membrane of the neural epithelium and enlarged areas of mesenchymal extracellular matrix, with associated abnormalities of mesenchymal cell distribution, cell number and cell contacts, and deficiency of alcian blue staining. Apical neuroepithelial microfilament bundles were observed at later somite-stages in trisomic embryos c.f. = compared with controls, and development of the concave neuroepithelial curvature was correspondingly retarded. Apposition of the neural folds at the forebrain/midbrain junction was never made, even though late neural fold fusion occurred in the hindbrain and ventral forebrain. At later stages (9-20 somites) the neuroepithelial cells showed pyknotic nuclei and dense intracellular inclusions. These are interpreted as secondary effects.

Comparative anatomical analysis of human trisomies 13, 18, and 21: I. The forelimb

Human trisomies 13, 18, and 21 exhibit specific neuromuscular phenotypes (Pettersen and Bersu, '82) which include a high proportion of neuromuscular forelimb variations, many of which are atavistic in nature (de Beer, '58; Barash et al., '79; Aziz, '81a). In order to test the neuromuscular phenotype, examine the atavistic nature, and analyze the developmental delay of the trisomy forearm musculature, we dissected the forelimbs of five trisomy 13, ten trisomy 18, and two trisomy 21 cases. Our dissections compare favorably with the existing published trisomy cases (Opitz et al., '79; Pettersen and Bersu, '82). Additionally, we found significant differences in the stage at which developmental arrest occurred in trisomies 13 and 18 for the pectoral complex, extensor digitorum profundus, and intrinsic hand musculature. Some of these muscles, which occur normally in nonhuman primates (Cihak, '67, '69; Dunlap et al., '85), also appear briefly in normal human ontogeny (Cihak, '72), constituting further evidence for developmental delay in aneuploids. The disproportionately effected limb tissues also lend support to the evidence for some degree of autonomy in their development in normal individuals. Our observations are consistent with Shapiro's amplified developmental instability model ('83). Aneuploids may be viewed as genetic variants from which much may be learned about normal limb development, how aneuploidy affects dysmorphogenesis, and the kind of information which exists on the duplicated (or monosomic) chromosome.

Neural tube defects in trisomy 18

In a retrospective survey, the incidence of neural tube defects in liveborn trimsomy 18 was found to be 6.2 per cent. Based on these data one would expect to find trisomy 18 in 1 of the 117 patients with myeloidysplasia; the incidence of trisomy 18 in dysraphic fetuses would be anticipated to be higher. These observations underscore the need for amniocentesis karyotyping of fetuses with neural tube defects, and the importance of careful examination of infants born with neural tube defects.

On the pathogenesis and therapy of dementia of the Alzheimer type: some neuropathological, biochemical, genetic and pharmacotherapeutic considerations

The extensive literature on dementia of Alzheimer type (DAT) testifies to the enormous progress achieved in the clinical and biochemical delineation of this disease. Newly developed laboratory and imaging techniques are also being applied to the diagnosis of DAT. Nevertheless, unequivoval diagnosis still relies primarily on morphological data from biopsy or autopsy. An overview is presented of major morphological changes occurring at different levels of organization in the central nervous system (CNS) in DAT. Currently formulated etiopathogenic hypotheses of DAT are reviewed and discussed in the context of morphological alterations. Some of the recombinant DNA methods, that are currently available for gene analysis, are described. Some approaches for studying Alzheimer specific genes using the above methods have been suggested. Finally, a critical overview of the current pharmacotherapeutic armamentarium used in DAT and senile dementia is presented. The efficacy, side effects, and the main mechanisms of action of the two categories of drug therapy -supposed etiopathogenic and symptomatic- are presented.

Occurrence of holoprosencephaly in chromosome 13 disorders cannot be explained by duplication/deficiency of a single locus

Four cases of holoprosencephaly with duplication/deletion 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----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 development, causing reversion to a more primitive embryonic and phylogenetic brain structure, is related to dosage imbalance of at least two chromosome 13 regions.

Forelimb anatomy of New World monkeys: myology and the interpretation of primitive anthropoid models

The forelimbs of 12 genera of New World monkeys, two genera of Old World monkeys, and a gibbon were dissected. Of the 54 muscles examined, 19 exhibited significant intergeneric variation. We present arguments for which morphologies are primitive and which are derived within platyrrhines and within anthropoids. We conclude that the forelimbs of Cebus apella and Callicebus moloch represent good models of the ancestral anthropoid morphology. Thus among living anthropoids they are most appropriate for comparisons with early fossil anthropoids. They are also useful for determining whether myological anomalies of human aneuploids are atavistic. Wagner tree analyses were conducted to assess the value of these myological characters in phylogenetic studies of platyrrhines. In most respects the Wagner trees were consonant with phylogenies previously proposed, although some hypothesized trees are less parsimonious than others in explaining our data. There is an unexpected number of derived features shared by Aotus and the Atelines. There are marked dissimilarities in forelimb musculature between Aotus and Callicebus.

Genetics of congenital heart malformations: a stochastic model

A stochastic model is proposed to explain how alterations in the properties of developing endocardial cells could control the outgrowth of endocardial cushions in normal persons, in subjects from families with a predisposition to congenital heart defects, and in subjects with trisomy 21. Normal and abnormal outgrowth of the endocardial cushions of the atrioventricular (AV) canal were modeled by computer simulations. Computer simulations depicted not only the sequence of normal AV valve development, but also illustrated how increased cellular adhesiveness of fibroblasts from the endocardial cushions of the AV canal--which we have observed in vitro among cultured cells from Down syndrome abortuses--may result in AV canal defects. The stochastic model so elaborated demonstrates how single gene changes may result in abnormalities in only a proportion of subjects carrying mutant alleles, yielding inheritance patterns characterized previously as being "multifactorial" in origin.