Epidemiology of holoprosencephaly: Prevalence and risk factors

Risk factors for nonsyndromic holoprosencephaly: a Manitoba case-control study

Holoprosencephaly (HPE) is one of the most common developmental field defects, occurring in 1 in 250 conceptuses and in 1 in 10,000-20,000 live births. Nearly half of patients with HPE have a recognized syndrome or a single gene defect. However, little is known about the risk factors for the remainder with "nonsyndromic" HPE. In our case-control study, we examine factors associated with nonsyndromic HPE. We identified 47 patients with HPE from the genetics clinic database with an equal number of controls matched for gender and birthdate. Of the 47 patients, 23 were identified as nonsyndromic. No statistically significant differences were noted between the mean maternal and paternal ages of patients and controls. Factors associated with nonsyndromic HPE were: having an Aboriginal mother (unadjusted odds ratio [OR] 3.5, 95% confidence interval [CI] 1.1-11.1), an Aboriginal father (OR 12.8, 95% CI 3.0-55.1), at least one Aboriginal parent (OR 5.0, 95% CI 1.6-16.0), or two Aboriginal parents (OR 8.8, 95% CI 2.0-37.8), the presence of a family history of a midline facial defect (OR 8.2, 95% CI 1.5-45.2), and being of low socioeconomic status (OR 3.0, 95% CI 1.0-9.1). Having an Aboriginal background remained statistically significant after adjusting for low socioeconomic status. Other associations evaluated--history of prior spontaneous abortion, stillbirth, neonatal death, prepregnancy diabetes, infections during pregnancy, alcohol exposure, smoking, and substance abuse--were not significantly associated with nonsyndromic HPE. The use of periconceptional folic acid or vitamins was not associated with a lower risk of nonsyndromic HPE.

Fetopathological aspects of holoprosencephaly

Holoprosencephaly (HPE) is a congenital central nervous system malformation estimated to occur in 1/250 conceptuses and 1/10,000 live births. While the severe forms, which are incompatible with life, are easier to detect in the prenatal period, the milder forms can remain unrecognised. As this can have serious consequences for the pregnancy and malformation carriers it is of crucial importance to find ways of timely detection of this pathological condition. The present study AIMED at finding an association of holoprosencephaly with facial dysmorphia and anomalies of visceral organs that would alert the physician to be very careful in making the prenatal diagnosis, which may require termination of pregnancy by medical indications.

MATERIAL AND METHODS

The study included 15 fetuses diagnosed with holoprosencephaly out of 2095 cases analysed post-mortem in the Fetopathology Clinic at the Centre for Maternity and Neonatology in the town of Tunisia over a period of 3 years (Oct. 2006 - Oct. 2009). The fetuses were analysed macro- and microscopically.

RESULTS

All forms of holoprosencephaly include elements of facial dysmorphism with the facial phenotypes of cyclopia, cebocephaly and ethmocephaly. It can be associated with specific internal organs anomalies, the hydrocephaly being the most common anomaly of the central nervous system. Our study suggested that holoprosencephaly can be correlated with craniofacial anomalies affecting the midfacial and medium craniovisceral structures.

CONCLUSION

The anatomical variations of HPE and the phenotypic facial correlations require a systematic and targeted study of central nervous system.

Holoprosencephaly: a guide to diagnosis and clinical management

CONTEXT

Holoprosencephaly affects 1 in 8,000 live births and is the most common structural anomaly of the developing forebrain, resulting in facial dysmorphism, neurologic impairment, and additional clinical sequelae. Given the increasing relative contribution of genetic diseases to perinatal morbidity and mortality in India, proper recognition and management of holoprosencephaly can improve care for a significant number of affected Indian children.

EVIDENCE ACQUISITION

We used the PubMed database (search terms: "holoprosencephaly," "HPE," "holoprosencephaly India") and cross-referenced articles regarding holoprosencephaly, using our research group's extensive experience as a guide for identifying seminal papers in the field.

RESULTS

Holoprosencephaly is classified into four types based on the nature of the brain malformations as seen on neuroimaging and/or pathologic examination, with typically recognizable craniofacial phenotypes. Despite the identification of several genetic loci and other etiologic agents involved in pathogenesis, additional causes are elusive. Moreover, satisfactory explanations for phenomena such as incomplete penetrance and variable expressivity are lacking.

CONCLUSIONS

For each patient, pediatricians should follow a diagnostic protocol including dysmorphology examination, complete family history and ascertainment of risk factors, and neuroimaging. Many medical issues, including hypothalamic dysfunction, endocrinologic dysfunction, motor impairment, respiratory issues, seizures, and hydrocephalus should be prioritized in management. Pediatricians should work with genetic specialists to identify syndromic forms and to perform cytogenetic investigation, molecular screening, and genetic counseling in order to fully characterize prognosis and recurrence risk.

New findings for phenotype-genotype correlations in a large European series of holoprosencephaly cases

BACKGROUND

Holoprosencephaly (HPE) is the most common forebrain defect in humans. It results from incomplete midline cleavage of the prosencephalon.

METHODS

A large European series of 645 HPE probands (and 699 relatives), consisting of 51% fetuses and 49% liveborn children, is reported.

RESULTS

Mutations in the four main genes involved in HPE (SHH, ZIC2, SIX3, TGIF) were identified in 25% of cases. The SHH, SIX3, and TGIF mutations were inherited in more than 70% of these cases, whereas 70% of the mutations in ZIC2 occurred de novo. Moreover, rearrangements were detected in 22% of the 260 patients screened by array comparative genomic hybridisation. 15 probands had two mutations providing additional support for the 'multiple-hit process' in HPE. There was a positive correlation between the severity of the brain malformation and facial features for SHH, SIX3, and TGIF, but no such correlation was found for ZIC2 mutations. The most severe HPE types were associated with SIX3 and ZIC2 mutations, whereas microforms were associated with SHH mutations. The study focused on the associated brain malformations, including neuronal migration defects, which predominated in individuals with ZIC2 mutations, and neural tube defects, which were frequently associated with ZIC2 (rachischisis) and TGIF mutations. Extracraniofacial features were observed in 27% of the individuals in this series (up to 40% of those with ZIC2 mutations) and a significant correlation was found between renal/urinary defects and mutations of SHH and ZIC2.

CONCLUSIONS

An algorithm is proposed based on these new phenotype-genotype correlations, to facilitate molecular analysis and genetic counselling for HPE.

Cyclopia: an epidemiologic study in a large dataset from the International Clearinghouse of Birth Defects Surveillance and Research

Cyclopia is characterized by the presence of a single eye, with varying degrees of doubling of the intrinsic ocular structures, located in the middle of the face. It is the severest facial expression of the holoprosencephaly (HPE) spectrum. This study describes the prevalence, associated malformations, and maternal characteristics among cases with cyclopia. Data originated in 20 Clearinghouse (ICBDSR) affiliated birth defect surveillance systems, reported according to a single pre-established protocol. A total of 257 infants with cyclopia were identified. Overall prevalence was 1 in 100,000 births (95%CI: 0.89-1.14), with only one program being out of range. Across sites, there was no correlation between cyclopia prevalence and number of births (r = 0.08; P = 0.75) or proportion of elective termination of pregnancy (r = -0.01; P = 0.97). The higher prevalence of cyclopia among older mothers (older than 34) was not statistically significant. The majority of cases were liveborn (122/200; 61%) and females predominated (male/total: 42%). A substantial proportion of cyclopias (31%) were caused by chromosomal anomalies, mainly trisomy 13. Another 31% of the cases of cyclopias were associated with defects not typically related to HPE, with more hydrocephalus, heterotaxia defects, neural tube defects, and preaxial reduction defects than the chromosomal group, suggesting the presence of ciliopathies or other unrecognized syndromes. Cyclopia is a very rare defect without much variability in prevalence by geographic location. The heterogeneous etiology with a high prevalence of chromosomal abnormalities, and female predominance in HPE, were confirmed, but no effect of increased maternal age or association with twinning was observed.

Holoprosencephaly in hungary: birth prevalence and clinical spectrum

A retrospective population based survey of patients born with holoprosencephaly in South-Eastern Hungary between July 1, 1992 and June 30, 2006 was performed. All live birth cases with craniofacial and non-craniofacial abnormalities were included in the study. A total of 9 patients (5 boys and 4 girls) were found with holoprosencephaly among 185 486 live births, which correspond to a birth prevalence of 0.49 per 10,000 live births (95% confidence interval [CI]: 0.17-0.80). These figures were similar to those ones found in New York State and several European regions. In our series one newborn had trisomy 13. Eight patients did not have chromosomal abnormalities on routine testing, 4 of them had craniofacial abnormalities only and another 4 showed non-craniofacial anomalies as well. Three patients died in the neonatal period and another one in childhood. Patients surviving the neonatal period had intellectual and motor handicap, and epilepsy.

Holoprosencephaly at prenatal diagnosis: analysis of 28 cases regarding etiopathogenic diagnoses

OBJECTIVE

To assess the likelihood of finding an etiopathogenic cause in an ultrasonographic prenatal diagnosis of holoprosencephaly.

MATERIALS AND METHODS

From January 1996 to June 2010, 13 883 prenatal diagnoses through chorionic villus sampling or amniocentesis were made. Every fetus with holoprosencephaly at ultrasound was evaluated. Gestational age, additional ultrasound findings, and fetal karyotype were recorded. Molecular diagnosis and parental karyotype were studied, if relevant.

RESULTS

Twenty-eight fetuses were diagnosed with holoprosencephaly (0.20%). All cases had additional ultrasound findings (100%). A definitive etiology was found in 23 cases (82.14%): karyotype was abnormal in 19 (67.9%) and normal in 8 (28.5%) cases. In the normal karyotype group, although molecular testing was performed in a few cases, one mutation of gene SIX 3 was diagnosed, one diagnosis of dysgnathia complex was made, and two fetuses presented Smith-Lemli-Opitz syndrome. No etiopathogenic diagnosis was made in five fetuses.

CONCLUSIONS

Our results showed that a definitive etiology can be established in most cases of prenatal holoprosencephaly. Chromosomal anomalies were the most frequent finding. However, in euploid fetuses, molecular diagnosis is worthwhile, as different genes with different inheritance patterns may be responsible for this malformation. Thorough evaluation proved beneficial for assessing more accurate prognosis and recurrence risks.

Minimal evidence for a direct involvement of twisted gastrulation homolog 1 (TWSG1) gene in human holoprosencephaly

Holoprosencephaly (HPE) is the most common disorder of human forebrain and facial development. Presently understood etiologies include both genetic and environmental factors, acting either alone, or more likely, in combination. The majority of patients without overt chromosomal abnormalities or recognizable associated syndromes have unidentified etiologies. A potential candidate gene, Twisted Gastrulation Homolog 1 (TWSG1), was previously suggested as a contributor to the complex genetics of human HPE based on (1) cytogenetic studies of patients with 18p deletions, (2) animal studies of TWSG1 deficient mice, and (3) the relationship of TWSG1 to bone morphogenetic protein (BMP) signaling, which modulates the primary pathway implicated in HPE, Sonic Hedgehog (SHH) signaling. Here we present the first analysis of a large cohort of patients with HPE for coding sequence variations in TWSG1. We also performed fine mapping of 18p for a subset of patients with partial 18p deletions. Surprisingly, minimal evidence for alterations of TWSG1 was found, suggesting that sequence alterations of TWSG1 are neither a common direct cause nor a frequent modifying factor for human HPE pathologies.

Holoprosencephaly: recommendations for diagnosis and management

PURPOSE OF REVIEW

This review presents recent advances in our understanding and clinical management of holoprosencephaly (HPE). HPE is the most common developmental disorder of the human forebrain and involves incomplete or failed separation of the cerebral hemispheres. The epidemiology, clinical features, causes, diagnostic approach, management, and outcomes of HPE are discussed.

RECENT FINDINGS

Chromosomal abnormalities account for the most commonly identified cause of HPE. However, there are often unidentifiable causes in patients with nonsyndromic, nonchromosomal forms of HPE. The prevalence of HPE may be underestimated given that patients with mild forms often are not diagnosed until they present with severely affected children. Pregestational maternal diabetes mellitus is the most recognized risk factor for HPE, as supported by recent large-scale epidemiological studies. Genetic studies using microarray-based comparative genomic hybridization technology have resulted in better characterization of important HPE loci.

SUMMARY

HPE encompasses a wide spectrum of forebrain and midline defects, with an accompanying wide spectrum of clinical manifestations. A coordinated, multidisciplinary care team is required for clinical management of this complex disorder. Further research will enable us to better understand the pathogenesis and causes of HPE, and thus to improve the genetic counseling of patients and their families.

Exon trapping analysis of c.301-19G > A in intron 1 of the SHH gene in a patient with a microform of holoprosencephaly

It can be difficult to assess the clinical significance of novel genomic sequence variants which may potentially alter mRNA splicing. Segregation analysis is not helpful in isolated cases or small families. Bioinformatic tools can provide additional information, but direct analysis of mRNA from an appropriate tissue remains the preferred approach for analyzing the effect of a sequence variant on splicing. However, hundreds of disease-associated and developmental genes, including the Sonic Hedgehog homolog (SHH) gene, are not expressed in blood or fibroblasts postnatally. We identified a de novo nucleotide change, c.301-19G > A, in intron 1 of SHH in a four year old boy with a microform of holoprosencephaly. In silico analyses predicted unaltered splicing. We used a minigene approach to study the variant more closely. The genomic region of interest was inserted into an exon trapping vector to create an artificial pre-mRNA in transfected cells. We found virtually complete inactivation of the splice acceptor site in intron 1 in two different transfected cell lines. In light of the clinical context, the de novo nature of the substitution and the results of the exon trapping analyses, we conclude that the detected variant is pathogenic and that the recurrence risk for sibs is low. This case demonstrates that in the absence of a readily available mRNA source, exon trapping can be a robust and practical aid in clinical practice for assessing the effect of genomic variants on pre-mRNA splicing.

Management of children with holoprosencephaly

Holoprosencephaly (HPE) is the most common malformation of the embryonic forebrain in humans. Although HPE occurs along a continuous spectrum, it has been categorized into four types from most severe to least severe: alobar, semilobar, lobar, and middle interhemispheric (MIH) variant. Facial malformations are often associated with HPE and usually correlate with the severity of brain malformation. With the most severely affected newborns, there is a high mortality rate in the first month of life, however, with milder forms of HPE, the majority survive beyond infancy. The Carter Centers for Brain Research in Holoprosencephaly and Related Malformations have enrolled 182 living children in a prospective research study. Based on previously published reports using this database, reports from other investigators, as well as our experience and personal observations, the range of developmental, neurological, and medical problems found in children with HPE is described in this article. Virtually all children with HPE have some developmental disability and the severity correlates with the severity of the brain malformation on neuroimaging. Common medical problems include hydrocephalus, seizures, motor impairment, oromotor dysfunction with risk of poor nutrition and aspiration, chronic lung disease, gastroesophageal reflux, constipation, hypothalamic dysfunction with disturbed sleep-wake cycles and temperature dysregulation, as well as endocrine dysfunction. Diabetes insipidus in particular is found in about 70% of children with classic HPE. Recommendations for management of these problems are given based on experiences of the authors and familiarity with the literature.

Holoprosencephaly and holoprosencephaly-like phenotypes: Review of facial and molecular findings in patients from a craniofacial hospital in Brazil

Here we report on the clinical and genetic data for a large sample of Brazilian patients studied at the Hospital de Reabilitação de Anomalas Craniofaciais-Universidade de São Paulo (HRAC-USP) who presented with either the classic holoprosencephaly or the holoprosencephaly-like (HPE-L) phenotype. The sample included patients without detected mutations in some HPE determinant genes such as SHH, GLI2, SIX3, TGIF, and PTCH, as well as the photographic documentation of the previously reported patients in our Center. The HPE-L phenotype has been also called of HPE "minor forms" or "microforms." The variable phenotype, the challenge of genetic counseling, and the similarities to patients with isolated cleft lip/palate are discussed.

Genetic counseling and "molecular" prenatal diagnosis of holoprosencephaly (HPE)

Holoprosencephaly (HPE) is a structural anomaly of the developing brain in which the forebrain fails to divide into two separate hemispheres and ventricles. The poor prognosis in the most severe forms justifies the importance of genetic counseling in affected families. The genetic counseling requires a thorough clinical approach given the extreme variability of phenotype and etiology. The karyotype is an essential diagnostic tool. Since mutations in the four major genes (SHH, ZIC2, SIX3, and TGIF) have been identified in HPE patients, molecular study is performed routinely in nonsyndromic HPE. New molecular tools, such as array-CGH analysis, are now part of the diagnostic process. Prenatal diagnosis is based primarily on fetal imaging, but "molecular" prenatal diagnosis can be performed if a mutation has been previously identified in a proband. Interpretations of molecular diagnosis must be given with caution, given the lack of strict genotype-phenotype correlation, and should be offered in addition to fetal imaging, using ultrasound followed by fetal MRI. We report on our experience of 15 molecular prenatal diagnoses from chorionic villi or amniotic fluid sampling. In eight instances, we were able to reassure the parents after taking into account the absence of the mutation in the fetus, previously identified before in a parent and/or a proband. Fetal RMI was normal later in pregnancy, and no child had medical problems after birth. The mutation was found in the seven other cases: four children were born, either without brain malformation and asymptomatic, or had a less severe form than the index case.