Down syndrome--a disruption of homeostasis

The Down syndrome critical region

Since the early 1970's numerous attempts have been made to learn whether specific segments of chromosome 21, when triplicated, are responsible for the clinical condition Down syndrome (DS). Studies were reported in which positive or negative clinical diagnoses of DS were made in the presence of partial trisomy of one or another segment of the chromosome. The distal half of the long arm of 21 (21q22) possesses most of the gene transcribing sites of the chromosome. It was this region that was thought to contain loci essential to production of the clinical syndrome. Subsequent studies identified subregions of this band as "minimal" or "critical" sites necessary and sufficient to produce the clinical condition. A major problem with these assignments was that different investigators defined different critical/minimal regions. In 1994 evidence was presented in which regions of most of the long arm of chromosome 21 were said to contribute to the DS phenotype. Soon after, a report described a child with DS and partial tetrasomy of the short arm and proximal long arm of 21, segments clearly distinct from the previously identified critical areas. Thus the clinical diagnosis of DS can be made in the presence of partial aneuploidy of nearly all segments of chromosome 21. It must be concluded that no evidence exists that individual loci on 21 are singularly responsible for specific phenotypic abnormalities in DS. Without exception, each of the clinical findings associated with DS is a multifactorial trait. The analysis of each trait in DS should thus be similar to analyses of the same traits in the general population with a focus on the way aneuploidy affects expression of multifactorial characteristics.

Genetic and environmental influence on the asymmetry of dermatoglyphic traits

Fluctuating asymmetry (FA) is defined as random deviations from bilateral symmetry of the body. Thus, its magnitude is often used to evaluate developmental homeostasis. In this study we evaluate the following hypotheses: 1) FA of dermatoglyphic traits has a significant genetic component; 2) prenatal maternal environment (PME) has a significant effect on the FA of dermatoglyphic traits in developmentally healthy individuals; and 3) genetic or environmental factors affect FA on organismal or systemic levels. Therefore, their effect is better seen in composite scores of FA rather than in FA indices for single traits. We analyzed 15 dermatoglyphic traits from 140 pairs of monozygous twins, 120 pairs of dizygous twins, and 106 pairs of mothers and daughters. All individuals were developmentally healthy. The influence of genetic and environmental factors on FA was evaluated by analysis of variance and regression analysis. For a majority of the traits in our study, FA showed significant but weak heritabilities, with values falling within the 0.20-0.35 range. None of the traits taken separately demonstrated the effect of PME on FA to be significantly greater than zero. The composite score of FA tended to have greater heritability values than individual traits. One of them, obtained in principal components analysis, showed a significant PME effect, supporting the hypothesis that FA is a systemic property.

Aneuploidy vs. gene mutation hypothesis of cancer: recent study claims mutation but is found to support aneuploidy

For nearly a century, cancer has been blamed on somatic mutation. But it is still unclear whether this mutation is aneuploidy, an abnormal balance of chromosomes, or gene mutation. Despite enormous efforts, the currently popular gene mutation hypothesis has failed to identify cancer-specific mutations with transforming function and cannot explain why cancer occurs only many months to decades after mutation by carcinogens and why solid cancers are aneuploid, although conventional mutation does not depend on karyotype alteration. A recent high-profile publication now claims to have solved these discrepancies with a set of three synthetic mutant genes that "suffices to convert normal human cells into tumorigenic cells." However, we show here that even this study failed to explain why it took more than "60 population doublings" from the introduction of the first of these genes, a derivative of the tumor antigen of simian virus 40 tumor virus, to generate tumor cells, why the tumor cells were clonal although gene transfer was polyclonal, and above all, why the tumor cells were aneuploid. If aneuploidy is assumed to be the somatic mutation that causes cancer, all these results can be explained. The aneuploidy hypothesis predicts the long latent periods and the clonality on the basis of the following two-stage mechanism: stage one, a carcinogen (or mutant gene) generates aneuploidy; stage two, aneuploidy destabilizes the karyotype and thus initiates an autocatalytic karyotype evolution generating preneoplastic and eventually neoplastic karyotypes. Because the odds are very low that an abnormal karyotype will surpass the viability of a normal diploid cell, the evolution of a neoplastic cell species is slow and thus clonal, which is comparable to conventional evolution of new species.

Aneuploidy vs. gene mutation hypothesis of cancer: Recent study claims mutation but is found to support aneuploidy

For nearly a century, cancer has been blamed on somatic mutation. But it is still unclear whether this mutation is aneuploidy, an abnormal balance of chromosomes, or gene mutation. Despite enormous efforts, the currently popular gene mutation hypothesis has failed to identify cancer-specific mutations with transforming function and cannot explain why cancer occurs only many months to decades after mutation by carcinogens and why solid cancers are aneuploid, although conventional mutation does not depend on karyotype alteration. A recent high-profile publication now claims to have solved these discrepancies with a set of three synthetic mutant genes that "suffices to convert normal human cells into tumorigenic cells." However, we show here that even this study failed to explain why it took more than "60 population doublings" from the introduction of the first of these genes, a derivative of the tumor antigen of simian virus 40 tumor virus, to generate tumor cells, why the tumor cells were clonal although gene transfer was polyclonal, and above all, why the tumor cells were aneuploid. If aneuploidy is assumed to be the somatic mutation that causes cancer, all these results can be explained. The aneuploidy hypothesis predicts the long latent periods and the clonality on the basis of the following two-stage mechanism: stage one, a carcinogen (or mutant gene) generates aneuploidy; stage two, aneuploidy destabilizes the karyotype and thus initiates an autocatalytic karyotype evolution generating preneoplastic and eventually neoplastic karyotypes. Because the odds are very low that an abnormal karyotype will surpass the viability of a normal diploid cell, the evolution of a neoplastic cell species is slow and thus clonal, which is comparable to conventional evolution of new species.

Genetic instability of cancer cells is proportional to their degree of aneuploidy

Genetic and phenotypic instability are hallmarks of cancer cells, but their cause is not clear. The leading hypothesis suggests that a poorly defined gene mutation generates genetic instability and that some of many subsequent mutations then cause cancer. Here we investigate the hypothesis that genetic instability of cancer cells is caused by aneuploidy, an abnormal balance of chromosomes. Because symmetrical segregation of chromosomes depends on exactly two copies of mitosis genes, aneuploidy involving chromosomes with mitosis genes will destabilize the karyotype. The hypothesis predicts that the degree of genetic instability should be proportional to the degree of aneuploidy. Thus it should be difficult, if not impossible, to maintain the particular karyotype of a highly aneuploid cancer cell on clonal propagation. This prediction was confirmed with clonal cultures of chemically transformed, aneuploid Chinese hamster embryo cells. It was found that the higher the ploidy factor of a clone, the more unstable was its karyotype. The ploidy factor is the quotient of the modal chromosome number divided by the normal number of the species. Transformed Chinese hamster embryo cells with a ploidy factor of 1.7 were estimated to change their karyotype at a rate of about 3% per generation, compared with 1.8% for cells with a ploidy factor of 0.95. Because the background noise of karyotyping is relatively high, the cells with low ploidy factor may be more stable than our method suggests. The karyotype instability of human colon cancer cell lines, recently analyzed by Lengnauer et al. [Lengnauer, C., Kinzler, K. W. & Vogelstein, B. (1997) Nature (London) 386, 623-627], also corresponds exactly to their degree of aneuploidy. We conclude that aneuploidy is sufficient to explain genetic instability and the resulting karyotypic and phenotypic heterogeneity of cancer cells, independent of gene mutation. Because aneuploidy has also been proposed to cause cancer, our hypothesis offers a common, unique mechanism of altering and simultaneously destabilizing normal cellular phenotypes.

Prenatal exposures and congenital heart defects in Down syndrome infants

The purpose of this study was to determine whether there are important differences in maternal and environmental prenatal risk factors between liveborn Down syndrome infants with congenital heart defects and Down syndrome infants without heart defects. Using a case control study design, we evaluated the risk associated with maternal illness, drug ingestion, substance usage, and chemical exposures in the home or workplace. The period of risk selected was 3 months before and 3 months after the last menstrual period, because cardiac development occurs early, before the mother may become aware of her pregnancy. Because fetal survival in Down syndrome may be more vulnerable to various exposures, controls were selected who also had trisomy 21. Of 171 infants studied, 89 were cases with congenital heart disease, and 82 were controls without heart disease. All interviews were performed by one nurse practitioner using a structured standardized questionnaire. Cases and controls had similar maternal ages, family incomes, parental education levels, and contraceptive practices before pregnancy. No differences were found between case and control mothers for maternal illness, medication use, or consumption of caffeinated beverages, cigarettes, or alcohol. Reporting of recreational drug usage was infrequent, may reflect underreporting, and did not differ between cases and controls. Maternal exposures were commonly reported for pesticides (50%), hair dyes (22%), craft paints (8%), varnishes (7%), and solvents (3.5%). However, in none of the categories was maternal exposure significantly more prevalent among case mothers than among control mothers. The failure of this study to identify risk factors for cardiac malformations may be attributable to the small differences in reported frequencies reducing statistical power or to the possibility that cardiac malformation in Down syndrome is a direct result of chromosomal duplication.

Malignancies in Down syndrome

Down Syndrome (DS) is associated with an increased incidence of malignancies, especially leukaemias. We came across 8 DS children presenting with malignancies and having trisomy 21 as the sole cytogenetic abnormality. Of these 8 DS cases, 4 presented with acute lymphocytic leukaemia, 2 with acute myeloid leukaemia and one case each with Hodgkin's disease and Wilms' tumour. There are contradictory reports regarding the distribution of myeloid versus lymphoid malignancies in DS children and their response to therapy. The exact mechanism by which patients with DS are predisposed to develop malignancies is unclear. However, presence of the extra chromosome no. 21 is presumed to disrupt the genetic balance which increases generalized susceptibility to genetic and environmental trauma. Furthermore, an increased methotrexate toxicity observed in these patients should also be taken into consideration in designing treatment for DS children with malignancies.

Fluctuating asymmetry and disorders of developmental origin

Environmental and/or genetic stresses may cause a breakdown in developmental homeostasis, resulting in increased bilateral asymmetry of morphological traits. The degree of these deviations (termed "fluctuating asymmetry") is thought to correlate with the severity of the stress. If these stresses also play a role in the appearance of developmental disorders, then increased morphological asymmetry may serve as a risk marker for disorders of developmental origin. This would be possible if 1) the environmental stress that caused a breakdown in developmental stability also contributed to the appearance of the disorder, and/or 2) the genetic predisposition (liability) to the disorder and increased susceptibility to fluctuating asymmetry have a common cause. Although a number of authors have reported associations between increased fluctuating asymmetry and disorders of presumed developmental origin, the usefulness of fluctuating asymmetry as a risk marker has not been established. One obstacle to this assessment is the lack of odds ratios reported by previous authors.

Hodgkin's disease in association with Down syndrome: a case report

The increased incidence of malignancies, especially acute leukemia, in Down syndrome has been clearly established. The association of Hodgkin's disease with Down syndrome has not been extensively documented, and only a few cases have been reported. We present here a case report of Hodgkin's disease in an 11-year-old female child with Down syndrome. The child presented with a stage IVB nodular sclerotic Hodgkin's disease and died of progressive disease. We also present a brief review of the mechanisms of development of malignancy in Down syndrome.

Neurodevelopmental profile of Down syndrome in Chinese children

OBJECTIVE

To give an overall appraisal of the clinical features of Down syndrome (DS) in Chinese children with emphasis on the neurodevelopmental outcome, and to compare the related complications with that of other races.

METHODOLOGY

The records of 124 Chinese children with DS assessed at the Child Assessment Centre of the University Department of Paediatrics in the Duchess of Kent Children's Hospital from 1985 to 1993 were reviewed.

RESULTS

Thirty-one per cent of patients had microcephaly. Eighty-five percent (33/39) when assessed in the first year of life had a developmental quotient (DQ) above 50 but only 29% (2/7) had DQ above 50 when assessed after the age of 5. Only two patients (1.6%) had epilepsy: infantile spasms (1) and Lennox-Gastaut syndrome (1). Hearing impairment was found in 45% of children with mild conductive hearing impairment being the most common.

CONCLUSIONS

Chinese children with DS, when compared with other races, were similarly intellectually disabled, but were less likely to develop epilepsy.

Thought is action

It is difficult to chart “normal movements” in atypical populations, such as those with Parkinson's disease, because there is great variability in the pattern of motor changes, both within and between patients. However, the potential clinical implications of Latash & Anson's theme are positive and powerful.

Cardiorespiratory capacity of individuals with mental retardation including Down syndrome

This study evaluated the cardiorespiratory capacity of persons with MR with and without Down syndrome. Analyses of individual data records of maximal exercise tests with metabolic analyses were conducted on tests of 111 subjects (31 men and 16 women with DS; 35 men and 29 women without DS) from six participating centers. All centers used a walking treadmill protocol previously shown to produce valid and reliable maximal tests with this population. Peak oxygen uptake and peak minute ventilation were higher in men than in women (P < 0.006), and in subjects without DS (P < 0.006). Peak heart rate was also higher in subjects without DS (P < 0.006). Peak respiratory exchange ratio (RER) was higher in subjects without DS (P < 0.006). Using peak RER as a covariate did not change the results. An analysis of peak minute ventilation, heart rate and VO2 of subjects with a peak RER above 1.1 revealed the same results. These data show that individuals with mental retardation have low levels of peak VO2, consistent with low levels of cardiovascular fitness. Individuals with Down syndrome have even lower levels of peak VO2 than their peers without Down syndrome, a finding that is possibly mitigated by the lower peak heart rates of the individuals with Down syndrome.

When are adaptive motor patterns nonadaptive?

The assumption that adaptive changes in motor patterns are optimal is questioned. Instances are cited where alteration of the adaptive motor patterns may be warranted. Other issues discussed are: (1) the relationship between central nervous system (CNS) priorities and the individual's priorities, (2) the use of cognitive bypass strategies by impaired individuals, and (3) conceiving CNS priorities as coordinative rules.

Dynamic similarities in action systems

Latash & Anson's contention that movement patterns that are different from those typically observed in persons without impairments should not be considered abnormal and usually should not be corrected is consistent with Davis & Burton's Ecological Task Analysis (ETA). Extending from the ETA concept of performer-scaled performance measures, the use of Froude numbers may offer insight into the global dynamics of a person's action system.

On optimality and movement disorders: A dynamic systems perspective

The argument that disordered or changed motor patterns can be adaptive or optimal given the constraints imposed is supported. Two major points of criticism are made, concerning the perceived superstates of the primary disorder and the absence of a systematic approach to identifying this adaptivity and optimality. We have tried to give the general outlines of such an approach to disordered movements from a dynamic systems perspective.

Bradykinesia in Parkinson's disease and cocontraction activity in dystonia are unlikely to be due to adaptive changes in the CNS

Latash & Anson's explanation of bradykinesia in patients with Parkinson's disease and cocontraction in dystonic patients is intriguing. However, the proposed adaptive changes in the central nervous system do not fit well with both clinical and experimental evidence of motor impairment in these patients. In particular, we question the explanation of: (1) the role of postural reactions and spatial accuracy in bradykinesia, (2) certain abnormalities during the execution of sequential and simultaneous movements, (3) the sudden changes in mobility (ON and OFF) of Parkinsonian patients, and (4) the meaning of reflex circuitry changes in dystonia.

Evaluation of central commands: Toward a theoretical basis for rehabilitation

The commentary focuses on the need for motor control modeling as a rationale for rehabilitation. We give examples in a bimanual unloading task and examine the potential consequences for recovery in patients with cerebral lesions. Hierarchical models of motor control lead to a distinction between “task optimization” and “motor optimization” and to a qualifier on Latash & Anson's “hands-off” position.

Toward peaceful coexistence of adaptive central strategies and medical professionals

We start with a number of philosophical and theoretical issues related to motor control, proceed through a spectrum of problems related to the role of adaptive changes in the central nervous system (CNS) in response to a primary disorder, and end with tentative practical recommendations. We consider the hierarchical and dynamic approaches to motor control not as incompatible alternatives but as ways of approaching two equally important issues, those of control and coordination. Professionals working in the area of physical therapy and rehabilitation should make use of the adaptive abilities of the CNS, identify goals, provide tools, and allow the CNS to develop optimal strategies. Therapists should intervene if they suspect that the CNS settles down in a local rather than a global optimum because of such factors as pain or the lack of a long-term predictive ability. Adaptive changes within the CNS may be important not only in pathologies but also in cases of specialized training, normal growth, and normal ageing.

Rehabilitation promotes functional movement in atypical populations

The suggestion that movement disorders exhibited by people with central nervous system (CNS) dysfunction should be considered normal and therefore not rehabilitated is rejected from three standpoints: (1) the CNS does not always select the best movement patterns for optimal long term outcome, (2) there is literature demonstrating that rehabilitation enhances motor function and independence, and (3) there exists a capacity for motor recovery and motor skill learning following brain damage.

Posturo-kinetic capacity in the disabled

“Normal movements” in atypical populations address the question of postural control: Voluntary movement is a perturbation of body balance and cannot be executed without a convenient counterperturbation. Despite a change in the postural program in relation to the impairment (Parkinson's disease, paraplegia), the performance level is decreased. Movements are not “normal,” owing to a reduction in posturokinetic capacity.

What is the appropriate criterion for therapeutic intervention in the motor domain?

Observations are made on Latash & Anson's theme that changed motor patterns should not be viewed as pathological. Normality in motor control might be characterised in terms of qualitative differences in control mechanisms rather than differences in movement patterns. Understanding of the concepts implicit in instructions to subjects should be confirmed. Principles of energy expenditure in atypical movements are outlined. If restoration of normality is not adopted as a criterion for therapeutic intervention, what might be proposed as an alternative?

Altered bilateral muscle synergies after stroke

Using movement disorders to understand the central nervous system's goals for motor behaviour may be easier in neurological models with a focal lesion of sudden onset, because the distinction between primary and adaptive changes may be clearer than in slowly progressive and/or diffuse neurological disease.

What makes a population atypical–priorities or constraints?

Priorities for movement reflect constraints deriving from the motor system, task goals, and physical environment. Atypical and typical populations alike set and reset priorities in response to constraints, and they do so at many processing loci and time scales. Efforts to understand what is atypical about a population should focus first on the constraints it encounters.

Anticipatory postural mechanisms: Some evidence and methodological implications

To understand the basic priorities of the central nervous system in human motor control, neurophysiological parameters are important. Certain H-reflex methods related to anticipatory postural control are particularly useful and may have therapeutic implications.

The concept of “normal” movement and its consequences for therapy

The guideline for therapy should be the improvement of the individual's functionality, not the acquisition of the ill-defined “goldstandard” of normal movement. However, Latash & Anson's suggestion that only primary causes of dysfunction should be treated is problematic for two reasons: First, the distinction between genuine and adaptive changes in motor performance is not always possible, and second, adaptive changes do not necessarily improve motor function, but may actually be detrimental to the system's performance.

Towards functional movement: Implications for research and therapy

Researchers and clinicians have different goals and constraints in trying to understand and treat movement disorders. Research on primary and secondary aspects of disorders may lead to effective treatment aimed at restoring functional movements even if the primary disorder cannot be cured. Similarly, movement patterns may be maladaptive and hence need therapeutic intervention to restore functional movement skills.

Mental retardation: genetic findings, clinical implications and research agenda

The most important genetic advances in the field of mental retardation include the discovery of the novel genetic mechanism responsible for the Fragile X syndrome, and the imprinting involved in the Prader-Willi and Angelman syndromes, but there have also been advances in our understanding of the pathogenesis of Down syndrome and phenylketonuria. Genetic defects (both single gene Mendelizing disorders and cytogenetic abnormalities) are involved in a substantial proportion of cases of mild as well as severe mental retardation, indicating that the previous equating of severe mental retardation with pathology, and of mild retardation with normal variation, is a misleading over-simplication. Within the group in which no pathological cause can be detected, behaviour genetic studies indicate that genetic influences are important, but that their interplay with environmental factors, which are also important, is at present poorly understood. Research into the joint action of genetic and environmental influences in this group will be an important research area in the future.

Frames of reference and normal movement

Latash & Anson propose that so-called abnormal movements may be stable and adaptive coordination and control solutions to task goals in action. Their interpretation of this established viewpoint is confused throughout in the persistent crossing of frames of reference, both in the description of movement and action and the proposed theory for motor control.

Adaptive changes in postural reactions after unilateral leg amputation

Changes in anticipatory postural adjustment following below-the-knee amputation may be considered as adaptive for reacquiring balance control.

How functional are atypical motor patterns?

Latash & Anson are commended for pointing out that investigators of atypical motor performance have too often failed to recognize the adaptive nature of many atypical movement characteristics. However, the authors' assumption that atypical motor patterns can be considered “normal” and, by implication, that it is futile to attempt to teach different patterns to patients that may be more functional, is criticized.

Anthropomorphizing the CNS: Is it what or who you know?

The target article pays insufficient attention to (1) the difficulties that arise when the term “normal” is used to describe motor skill behavior, and (2) the need to resist ascribing motor control to an intelligent central nervous system. A complex systems perspective is proposed with attractor states and periodicity identified as markers of atypical coordination and control.

Optimal search strategies for optimal motor solutions: Self-determination or informed guidance?

Impoverished resources may direct the pathologic central nervous system toward local solutions during ontogeny. The resultant motor behavior can be systemically optimal without being functionally optimal. Therapeutic intervention should focus on facilitating function (rather than “normalcy”) by encouraging behavioral exploration through appropriate combinations of task and environmental variations with respect to individual capabilities. Some health professions have already adopted this philosophy.

Abnormal movements can be identified in “atypical” populations

Latash & Anson's discussion of movements of “atypical” populations is potentially topical but unhelpful. Abnormal movements can be distinguished from any compensatory adjustments in Parkinson's, Huntington's, and Alzheimer's diseases and schizophrenia. To suggest otherwise may do “atypical” populations and their caregivers a disservice. It would be more meaningful to discuss the relative modularity and separability of cognitive and motor processes.

Peripheral blood cell counts in infants with Down's syndrome

The current literature indicates that several abnormalities have been observed in the three hematopoietic cell lines of infants with Down's syndrome. This prospective, longitudinal study was designed to clarify the physiological variation in peripheral blood cell values of children with Down's syndrome by following 25 such infants during their first year of life. Apart from polycythemia in the first week of life, the hemoglobin concentration was, in general, the same as in normal term infants. At 9-12 months of age values for mean corpuscular hemoglobin and mean corpuscular volume tended to be elevated. Serum erythropoietin concentrations were low to normal. White blood cell counts were slightly lower in children with Down's syndrome than in normal children. The study infants had profound thrombocytosis from the age of 6 weeks to the end of follow-up at 1 year. This study, the first longitudinal follow-up of such subjects, indicates that infants with Down's syndrome often have evidence of polycythemia soon after birth and red blood cell macrocytosis and thrombocytosis later in infancy. In conclusion, we carried out peripheral blood cell counts in 25 infants with Down's syndrome, but with no actual hematological disturbance, during their first year of life, and compared them with values for normal term infants.

Tooth agenesis in Down syndrome

We studied the frequency and pattern of tooth agenesis in a Danish population with Down syndrome, trisomy 21 (46 females and 54 males). The control group consisted of a normal Danish population (2424 females and 2431 males) [Rølling, 1980: Scand J Dent Res 88:365-369; Ravn and Nielsen, 1973: Tandlaaegebladet 77:12-22]. We found that individuals with Down syndrome have an occurrence of agenesis that is some 10 times greater that in the general population with a higher frequency in males than in females. Agenesis occurred more frequently in the mandible than in the maxilla and most often on the left side. The highly significant differences were primarily found in the occurrence of agenesis of the mandibular central incisors, followed by the maxillary lateral incisors and second premolars and the mandibular second premolars. The main components in the pattern of agenesis observed in Down syndrome are supposed to be related to the peripheral nervous system and abnormal cartilagenous tissue. The present study on Down syndrome suggests that the dentition, with its many different anomalies, from agenesis to malformation, can be used as an indicator in evaluating different aspects in the patheogenetic of aneuploidy conditions.

Root length, crown height, and root morphology in Turner syndrome

Root length, crown height, and root morphology were studied on intraoral and panoramic radiographs in 33 Turner syndrome patients aged 7.0-16.7 years, subdivided on the basis of karyotype. Thirty-three normal girls aged 10.2-16.4 years served as controls. In the 45X patients and, with the exception of a few teeth, also in the isochromosome and mosaic karyotypes, root length and crown height of incisors, canines, and premolars were significantly reduced. Some teeth showed altered crown-root proportions. Maxillary first premolars showed a significantly increased number of two-rooted and three-rooted variants. Mandibular premolars and molars had a complex root morphology, and a classification system was established including four premolar and six molar root types. Premolars had a significantly increased number of root components. Some of the variants, such as a molar-like second premolar, are apparently specific for these patients. On several first molars a radix entomolaris was identified. Two separate mesial and one or two separate distal roots were also frequently seen. Our investigation demonstrates that X-chromosome deficiency influences root formation.

Distribution of extracellular matrix components in nuchal skin from fetuses carrying trisomy 18 and trisomy 21

We have investigated histologically the elevations of the skin in dorsal and lateral neck (nuchal) regions of human fetuses carrying karyotypes of trisomy 18 (Edwards' syndrome) and trisomy 21 (Down's syndrome). Cavities filled with interstitial fluid were found in the dermis, epidermal basement membrane and occasionally in the epidermis of trisomy-18 fetuses, but were not delineated by an epithelium or basement membrane as judged by the absence of immunostaining for laminin, collagen IV and collagen VII. Dilated vessels were also found at the interface between dermis and subcutis. Neither normal fetal skin nor that of trisomy-21 fetuses contained cavities or dilated vessels. In order to detect possible alterations of the extracellular matrix in trisomy-18 and trisomy-21 skin, the distribution of glycoproteins, glycosaminoglycans and proteoglycans was studied immunohistochemically. In trisomy-21 and control skin, the dermis stained intensely for fibronectin, whereas the subcutis reacted only weakly. In trisomy-18 skin, the stronger staining for fibronectin appeared in the subcutis, and the prevailing collagen type was collagen III, collagen type I being absent. In the skin of trisomy-21 fetuses, collagen VI was more irregularly arranged and densely packed, whereas collagen I was more widely spaced than in normal fetuses. More hyaluronan was present in the dermis and subcutis of trisomy-21 fetuses than in that of trisomy-18 and control fetuses. A correlation seems to exist between undelimited cavities and collagen III in trisomy-18 skin, and between hyaluronan and the specific arrangement of collagen in trisomy-21 skin.

Foliation of the cerebellar vermis in trisomy 19 mice

The development of the folial pattern was studied in the cerebellar vermis of 32 trisomy 19 (Ts19) mice aged 1-16 days postpartum and their euploid littermates. In the Ts19 cerebellum, fissures were formed in the regular sequence observed in control littermates, but their appearance was delayed by about 2 days. Fissure number increased until day 6 in euploid controls and in Ts19 mice, remaining constant thereafter. In Ts19 cerebella, fissure number and fissure depth were reduced significantly; there were 30% fewer Purkinje cells and the cross-sectional areas of the external germinal layer and of the total cerebellar vermis were decreased, reflecting a permanent hypoplasia. Both in Ts19 and control mice, a temporal and quantitative relationship was observed between fissure formation and the expansion of the external germinal layer, whereas the increase in fissure depth was found to correlate with the growth of the whole cerebellar vermis. Determination of the surface folding index revealed that only during fissure formation, the expansion of the cerebellar surface exceeded that of the cerebellar volume. The present study does not give any indication that foliation and histogenesis of the cerebellum are differentially affected by trisomy.

Can maternal risk factors influence the presence of major birth defects in infants with Down syndrome?

Although the manifestations of Down syndrome (DS) are well known, certain major birth defects such as duodenal atresia and endocardial cushion defects are present in some infants but not others, suggesting the possible role of other genetic or environmental factors interacting with the trisomy genotype. To explore the possible role of maternal factors in the presence of major defects among DS infants, we examined data from an epidemiologic study of DS conducted in metropolitan Atlanta. Of 219 DS infants born between 1968 and 1980, 50 had recorded cardiac defects, 9 had selected gastrointestinal atresias and 4 had oral clefts. We evaluated the association of these defects with several maternal factors including age, race, first trimester cigarette smoking, alcohol use, and fever. We found that different maternal factors were associated with several defects: (1) mother's race with cardiac defects (40% in blacks vs. 17% in whites, P less than 0.01), (2) mother's age with oral clefts (6% for less than 25 years, 1% for 25-34, and 0% for greater than 34, P less than 0.05), and (3) maternal first trimester fever with gastrointestinal defects (15% in infants with history of fever and 3% in infants without a history of fever, P less than 0.01). We also observed an inverse relationship between maternal alcohol use and the presence of ventricular septal defect. These findings suggest that maternal risk factors may influence the clinical manifestations of DS. In addition to searching for a genetic basis for the DS phenotype, we suggest that the role of environmental factors and maternal exposures be specifically explored in clarifying the genesis of various birth defects in Down syndrome.

Genomics of human dysmorphogenesis

A survey of Mendelian Inheritance in Man emphasizes the large Mendelian contribution to human dysmorphogenesis and contrasts single gene conditions with chromosomal disorders. There were 1761 conditions that involved altered morphogenesis (49% of disease entries), including 1040 multiple defect syndromes and 721 inherited single birth defects. Premature death (36-57%), mental retardation (20-59%), and growth retardation (37-59%) are more frequent in autosomal recessive or X-linked syndromes, while predisposition to tumorigenesis was more common in dominant (16%) than recessive (3.4%) syndromes. Comparison of the Mendelian conditions with 100 chromosomal disorders showed a strikingly similar spectrum of malformation, with skeletal, craniofacial, eye, epidermal, and neuromuscular systems being most frequently affected. Chromosomal syndromes average 10.6 systems affected per disorder, in contrast to 3.55 for Mendelian syndromes, and pleiotropy does correlate weakly with aneuploid segment length. Genomic understanding of these relationships is still primitive, with 74 of 1609 (4.6%) autosomal conditions and 43 of 152 (29%) X-linked conditions mapped to specific chromosomal regions. The societal toll of human dysmorphogenesis and the evident progress with X-linked disorders provide a powerful rationale for the Human Genome Project.