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Converging data suggest that intraventricular hemorrhage (IVH) of the preterm neonate is a complex developmental disorder, with contributions from both the environment and the genome of the child. IVH, or hemorrhage into the germinal matrix (GM) of the developing brain with secondary periventricular infarction as shown in Figure 1 , occurs in that critical period of time before the 32nd to 33rd wk postconception and has been attributed to changes in cerebral blood flow (CBF) to the immature GM microvasculature. Inflammation, coagulation, and vascular factors may also play a role. The more severe grades are characterized by acute distension of the cerebral ventricular system with blood (grade 3) and IVH with parenchymal venous infarction (grade 4) (1). Mortality is high in infants with severe IVH, and one-quarter to one-half of surviving neonates develop cognitive disability and/or cerebral palsy (2,3). In addition, 20% of nondisabled survivors suffer executive function and neuropsychiatric disorders, confirming that severe IVH is a major pediatric public health problem (4,5).

Figure 1
figure 1

Severe intraventricular hemorrhage (IVH). Coronal ultrasounds at postnatal age (a) 1 d and (b) 4 d from a 28-wk gestation neonate with IVH. In a, blood is seen in the germinal matrix and filling the right lateral ventricle; at postnatal d 4, the ventricular system is dilated, and blood is seen both filling and distending the right lateral ventricle as well as in the parenchyma of the right hemisphere, consistent with grade 4 IVH.

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Multiple lines of clinical data support the hypothesis that, similar to other preterm morbidities (6,7), the etiology of IVH is multifactorial. First, despite the development of sophisticated neonatal intensive care strategies, IVH remains a significant problem of prematurity. Maternal transport, antenatal steroid administration, and improved resuscitation techniques have become standard of care in neonatal tertiary care units worldwide (8,9,10,11), but the incidence of severe IVH has remained 13–15% for almost 20 y (8,12).

Although the incidence of IVH is inversely related to gestational age (GA) at birth, the risk period for hemorrhage is independent of GA (13,14). The incidence of severe IVH is 7% for those born at 28 wk and 26% for neonates born 4 wk earlier, but the critical period for hemorrhage is the first 4–5 d of life for both the groups. These data suggest that either the transition to extrauterine life and/or the triggers to which the neonates are exposed contribute to hemorrhage, and both hypoxemia and inflammation have been implicated in severe IVH of the prematurely born.

Furthermore, both gender and twin studies support the hypothesis that IVH is a complex disorder. Preterm males are more likely than females to experience severe IVH (15). Similarly, studying 450 twin pairs, Bhandari et al. (16) reported that 41.3% of the variance in IVH risk is attributable to familial and environmental factors. Candidate gene studies implicate the inflammatory, coagulation, and vascular pathways, and recent data suggest that time of hemorrhage may play a role (17,18,19).

The purpose of this review is to examine preclinical and clinical data supporting the hypothesis that severe IVH is attributable in part to the interaction of the environment with the neonatal genome. Based on the pathogenesis of hemorrhage, factors mediating coagulation, inflammation, and vascular pathways have been chosen for review.

Models for Interaction Between the Environment and the Genome

Although several investigators have hypothesized that IVH is secondary to the interaction of the environment and the genome (20), the mechanisms by which genetic predisposition and environmental exposures interact are just beginning to be described. Review of published literature interrogating vascular and environmental interactions suggest at least two different mechanisms. These include the impact of an environmental perturbation on a system harboring a known polymorphism, while the other stems from research addressing the influence of fetal programming on adult disorders postulating the role of epigenetics. Underlying both proposed mechanisms is the recognition that IVH is a developmental disorder occurring within a critical period of time, and both the polymorphisms and environment events we describe may result in very different or even unremarkable phenotypes in the term infant or older child.

In the first model, a gene confers vulnerability to environmental triggers (21). A common example is hypercarbia. Hypercarbia occurs in association with apneic events, lung disease, pneumothoraces, pulmonary hemorrhage, and other events. Preterm neonates exhibit a narrow range of carbon dioxide over which CBF remains constant. In response to hypercarbia, CBF to the immature GM microvasculature markedly increases and, in the presence of a vascular structural polymorphism, may result in hemorrhage (22). Although the same genetic variant that results in GM vascular instability may predispose the proband to subsequent neurovascular disorders, there is no reported transgenerational change in DNA.

In contrast, epigenetics refers to an alteration in gene function without changes in the underlying DNA sequence (21). Epigenetic mechanisms involve DNA methylation, histone density and posttranslational modifications, and the engagement of noncoding RNAs. Some alterations in the epigenome may be hereditable, resulting in transgenerational changes in genotype/phenotype correlations.

The programming of the epigenome is active during gestation, and epigenetic processes respond to environmental stimuli ranging from protein-calorie dietary restriction to hypoxia and fetal inflammatory exposures. Offspring of women experiencing preeclampsia, a putative marker for fetal hypoxia, have both hypertension and endothelial dysfunction during young adulthood (23). Similarly, in preclinical models, offspring of mothers exposed to protein-calorie deprivation during pregnancy also have vascular dysfunction, and these findings are reversed by maternal folate supplementation (24). Of note, folate deficiencies have also been associated with abnormalities in DNA methylation (25). Also, in preclinical studies, the endothelium-dependent abnormalities in the offspring of restricted diet pregnancies are ameliorated by the administration of histone deacetylase inhibitors, suggesting a transgenerational etiology for the findings (26).

Finally, the interactions between genetic polymorphisms, epigenetic mechanisms such as DNA methylation, and expression are complex. Emerging data suggest, however, that genetic variants regulate methylation, and methylation regulates gene expression. Thus, a genetic variant that creates or negates a DNA C-phosphate-G methylation site in the promoter region of a gene may significantly impact expression of that gene (27).

Pathways for Alteration of the IVH Epigenome

Studies investigating mechanisms by which fetal or preterm exposures may alter the epigenome to promote or prevent IVH include the effects of hypoxia, inflammation, nutrition, and oxidative stress.

IVH has long been associated with hypoxic ischemic events, and putative inflammatory, excitotoxic, and apoptotic pathways are involved in the complex cascade following neonatal hypoxia ischemia. The hypoxia-inducible transcription factors (HIFs) are among the endogenous adaptive mechanisms modifying this cascade of events. HIFs are heterodimers of HIF-α and HIF-β subunits that belong to a family of basic helix-loop-helix transcription factors. HIF-1 and HIF-2 are important regulators of oxygen-dependent gene transcription that modulate oxygen and metabolic supply during hypoxia. HIF target genes include those with vasoactive and vasoproliferative effects including vascular endothelial growth factor and inducible nitric oxide (NO) synthase. In a preclinical model of preterm hypoxia, HIF-1α was prominently found in vascular endothelial and glial cells of the subventricular zone (28). Similarly, its target, vascular endothelial growth factor, mediates survival and tube stabilization of hypoxic brain microvascular endothelial cells in vitro (29). Finally, possibly acting via acetylation and methylation pathways, chronic hypoxia decreases global transcriptional activity (30). Thus, in preclinical fetal studies, chronic high-altitude hypoxia resulted in reduced histone acetylation and DNA methylation, fetal pulmonary arterial smooth muscle cell proliferation, vessel remodeling, and vascular dysfunction (31).

Likewise, biomarkers of inflammation such as interleukin 1β (IL-1β) and interleukin 6 (IL-6) activate the hypothalamic–pituitary–adrenal axis, with putative long-term neurobehavioral sequelae (32). An example of such an epigenetic event is second-trimester maternal exposure to type A2/Singapore influenza that significantly increased risk for adult psychiatric disorders (33).

Finally, as discussed above, preclinical studies suggest that, acting via metabolic, vascular, and stress-mediated pathways, maternal nutrition may have profound effects on the developing fetus (24,26,34).

Pathophysiology of IVH: Preclinical Candidates

IVH begins in the GM, a site of active angiogenesis in the developing brain (13,20) ( Figure 2 ). Endothelial growth and sprouting are critical for angiogenesis, and the emerging blood–brain barrier is characterized by endothelial tight junctions, basement membrane proteins, perivascular pericytes, and glial endfeet. These processes are regulated by assorted growth factors, cell surface receptors, and intracellular signaling pathways.

Figure 2
figure 2

Germinal matrix, a densely cellular region located adjacent to the ependyma of the lateral ventricles. It is composed of immature neural precursor cells and vessels. Magnification 40×; scale bar = 1 mm. (Figure courtesy of A. Huttner, Department of Pathology, Section of Neuropathology, Yale University School of Medicine, New Haven, CT.)

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Although preclinical studies postulate that it is the developmental stage of the GM microvessels that results in IVH, more recent studies suggest that mutations in one or more microvascular proteins confer vulnerability to environmental triggers ( Table 1 ). Mice with targeted mutations in the basement membrane proteins, fibronectin, laminin, collagen 4A1 (COL4A1), and/or perlecan, demonstrate that all are necessary for vascular stabilization. Those with mutations in COL4A1 experience IVH following the stress accompanying vaginal delivery. In the murine model, these hemorrhages are preventable by surgical delivery, suggesting an interaction between an environmental trigger and the genome (35).

Table 1 Candidate genes for IVH from preclinical studies
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Similarly, although preclinical “risk factor” studies are not available, mice with mutations in activin receptor–like kinase 5 (Alk5) (36), alpha v integrins (37), annexin 7 (anx7) (38), cyclic adenosine 59-monophosphate response element–binding protein (39), death receptor 6 (DR6) (40), inhibitors of differentiation (Id) proteins 1 and 3 (Id1, Id3, respectively) (41), or Tgfbr2 (36) also develop intracerebral hemorrhage mimicking Gr 4 IVH.

Transforming growth factor-β (TGF-β) activation and signaling is essential for normal blood vessel growth and sprouting in developing brain, and αvβ8 integrin mediates TGF-β activation. Mouse embryos genetically null for integrin β8 develop severe intracerebral hemorrhage beginning at embryonic day 11.5 (42). Similarly, TGF-β signals are transduced by both TGF-β type II and the TGF-β type I receptors (Tgfbr2 and Alk5, respectively), and in murine systems, selective deletion of Tgfbr2 or Alk5 in endothelial cells results in lethal intracerebral hemorrhage (36). In humans, mutations in Tgfbr2 and Alk5 cause Loeys–Dietz syndrome, characterized by multiple arterial aneurysms and dissections. Men with Alk5 mutations more commonly present with thoracic aortic aneurysm and die earlier than women with this disorder, suggesting a gender predilection for this polymorphism (43).

IVH is also found in mice with genetic alterations in the transcription factors such as Id 1/3 (41), Friend leukemia integration (39,44), and cyclic adenosine 59-monophosphate response element–binding protein (39). Id1 and Id3 prevent transcription by direct physical interaction with the basic helix-loop-helix transcription factors are expressed in cerebral endothelial cells and play an essential role in angiogenesis (45). The CREB-binding protein is a transcriptional coactivator, and Friend leukemia integration, which is a member of the Ets family of transcription factors, is a key regulator of vascular maturation (46). Finally, DR6 is required for vascular endothelial growth factor–mediated endothelial sprouting, is enriched in central nervous system vasculature, and drives barrier genesis in developing brain (38). Facing environmental triggers including hypoxia, hypercarbia, and hypertension, polymorphisms in some or all of these factors may result in hemorrhage.

Likewise, changes in CBF may contribute to hemorrhage. Autoregulation relies on smooth muscle cells, pericytes, and proteins ranging from Ca++ and K+ channels, phospholipase A1, arachidonic acid, and adenosine to NO and cytokines among others (20). Notably, mice with mutations in Anx7, a gene encoding a Ca++-activated GTPase supporting Ca++ channel activity, experience IVH, suggesting that, in the presence of environmental perturbations, mutations in genes controlling CBF may contribute to hemorrhage (38).

Evidence for Gene-By-Environment Interactions: Clinical Studies of Severe Hemorrhage

Studies in preterm neonates have implicated an array of candidate genes spanning the coagulation, inflammatory, and vascular pathways. For this review, we compare and contrast the studies of Harteman et al. (17), Ryckman et al. (18), and Baier (19) and our Gene Targets study. As shown in Table 2 , the numbers of subjects, their birth weight (BW), and GA as well as their racial and ethnic backgrounds were quite varied, as were the years in which they were born and, presumptively, the neonatal intensive care the neonates received (17).

Table 2 Comparison of the selected study populations
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Coagulation Candidates

Coagulation factors have long been considered candidate genes for IVH, both because of the pathophysiology of hemorrhage and because of their putative role in perinatal stroke (47) ( Table 3 ). The most widely studied include the factor V Leiden (F5) variant, polymorphisms of the methylenetetrahydrofolate reductase (MTHFR) gene, and the prothrombin 20210G>A variant (F2).

Table 3 Coagulation, inflammation, and vascular candidate genes for IVH from clinical studies
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The contribution of the F5 polymorphism to IVH has been interrogated in different populations. A point mutation results in replacement of amino acid 506 arginine to glutamine in an activated protein C cleavage site. Activated protein C cleaves the peptide bonds in activated F5, resulting in inhibition of the coagulation pathway, and the variant presents with hypercoagulability. Gopel et al. (48) reported that this polymorphism was associated with Gr 1–2 IVH but protected against parenchymal hemorrhage. In contrast, Ryckman et al. (18) found that the heterozygous genotype was associated with Gr 1–2, but not Gr 3–4, IVH.

Similarly, Harteman et al. (17) studied 17 preterm neonates with atypical presentation of Gr 4 IVH; atypical hemorrhages were defined as occurring in the absence of provoking clinical factors more than 96 h following birth ( Figure 3 ). Seven of 17 were heterozygous for the F5 variant, suggesting an association between this hypercoagulable state and atypical hemorrhage. Recent studies suggest that related or unrelated thrombophilia in the mothers increase the risk of perinatal stroke (47), and six of seven mothers of F5 infants harbored this variant. Although the incidence of variant status in the patients in this study is significantly higher than that for the Dutch population, no data are provided for neonates with typical onset Gr 4 hemorrhage.

Figure 3
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Atypical intraventricular hemorrhage (IVH). (a) Axial and (b) coronal images of a 1-d-old 34-wk gestation infant with the atypical (fetal) onset of germinal matrix and IVH. Note the hemorrhage into the right germinal matrix, intraventricular blood, and ventriculomegaly characteristic of posthemorrhagic hydrocephalus. (Figure courtesy of C.C. Duncan, Department of Neurosurgery, Yale University School of Medicine, New Haven, CT.)

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Finally, Baier (19) found no association between F5 and IVH in his cohort of 99 mostly African-American extremely-low-BW neonates.

Of note, IVH is more common in male preterm neonates, and although no gender-by-F5 data are currently available for neonates with IVH, males with the F5 polymorphism are more likely than F5 females to experience recurrence of peripheral venous thrombosis, suggesting a possible gender effect for this mutation (49).

A second leading candidate is MTHFR. MTHFR catalyzes the reduction of 5,10-methylenetrahydrofolate to 5-methyltetrahydrofolate, which is necessary for the conversion of homocysteine to methionine. Hyperhomocysteinemia is associated with polymorphisms at -677 and -1298, especially at the 677 TT variant, and results in endothelial cell injury and alterations in coagulation including stroke, thrombosis, migraine, and vascular disorders (50,51). Hyperhomocysteinemia is exacerbated under conditions of low folate, and this increases the susceptibility to experimental brain damage.

MTHFR may additionally play an important role in neonatal brain injury. Studying the prevalence of the 677C>T variant in 11 neonates with hypoxic ischemic encephalopathy and their mothers, Dodelson de Kremer and Grosso (52) found that compared with a 68% incidence of the T allele in the control population, all 11 carried this polymorphism. The variant was more common in mothers of affected offspring and was associated with an increase in maternal homocysteine, suggesting a profound alteration in the fetal environment. The mothers in this study exhibited poor nutrition, and the authors postulated that underlying folate deficiency during pregnancy may have exacerbated the influence of hypoxic injury in neonates harboring the MTHFR variant.

More recently, Harteman et al. (53) evaluated 118 infants with hypoxic–ischemic encephalopathy and reported that the magnetic resonance imaging white matter/watershed pattern of injury was associated with MTHFR CT or TT 677 polymorphisms and plasma homocysteine levels in the upper quartile. In this study, neonatal MTHFR polymorphisms were not associated with homocysteine levels, consistent with the findings of other observers (54). Of interest, Molloy et al. (55) demonstrated that maternal homocysteine levels are the best predictors of fetal values, emphasizing the importance of the fetal environment.

Acute hypoxia ischemia results in thrombosis in subjects lacking known polymorphisms (56), and hypoxia exacerbates the effect of a folate-deficient diet on homocysteine metabolism (57,58). Blaise et al. (59) demonstrated that in rat pups exposed prenatally to diets deficient in vitamins B12, B2, folate, and choline through weaning, hypoxia increased plasma homocysteine levels. MTHFR activity was attenuated by hypoxia. Furthermore, hypoxia enhanced the deficiency-induced drop of the S-adenosylmethionine/S-adenosylhomocysteine ratio, known to influence DNA methylation and gene expression. Taken together, these data suggest the potential interaction between maternal and fetal MTHFR polymorphisms, folate, and hypoxic–ischemic injury to preterm brain.

Coinheritance of more than one thrombophilia variant is associated with a greater risk of thrombotic events than with a single polymorphism. Thus, in addition to assessing F5, Harteman et al. investigated the 677C>T and 1298A>C polymorphisms in 16 of 17 preterms with atypical periventricular hemorrhagic infarction PVHI ( Table 3 ). Six had the -677 T allele, 4 had the -1298 C variant, and 4 were compound heterozygous suggesting that 14 of 16 neonates had potentially deleterious polymorphisms. In contrast, there were no significant differences in genotypes for case and control neonates studied by Ryckman et al. or Baier.

Based on our previous work demonstrating a difference in the MTHFR -1298C polymorphism between severe IVH cases and controls and the putative role of hypoxia in MTHFR-mediated brain injury (60), we tested the hypothesis that there would be a gene-by-environment interaction for these two factors. For this preliminary analysis, we interrogated only the MTHFR 1298A>C variant in the Gene Targets for IVH Consortium (NS053865) database. (Institutional review board approval was obtained from all participating institutions.) This consortium has both environmental data and DNA from over 1,400 inborn appropriate-for-GA preterm neonates with antenatal steroid administration, BW 500–1,250 g, and centrally read cranial ultrasounds. Only the 705 European subjects were included in this analysis to avoid racial admixture.

Three hundred sixteen infants had Gr 2–4 IVH; 389 neonates had no evidence for IVH. Cases had lower BW and GA than controls, and their mothers were more likely to have experienced chorioamnionitis and multiple gestation pregnancies ( Table 4 ). In contrast, case mothers had less preeclampsia and fewer cesarean section deliveries. Cases were more likely to have 5-min Apgar scores < 3 and require intubation for delivery room resuscitation.

Table 4 Gene Targets for IVH Consortium study subjects
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An analysis of generalized linear mixed model with site as a random effect and all significant variables from Table 4 as fixed effects was performed to measure the relationship between IVH status and those independent variables including GA, preeclampsia, clinical chorioamnionitis, complete antenatal steroid administration within 7 d before delivery, multiple gestation, cesarean section, Apgar 1 min < 3, Apgar 5 min < 3, intubation for resuscitation, and the MTHFR 1298A>C variant. Similar to previous reports (for review, see ref. (61)), this analysis demonstrated that increasing GA, cesarean delivery, and a complete course of antenatal steroid administration in the week before delivery were protective for Gr 2–4 IVH; in contrast, multiple gestation pregnancy and chorioamnionitis were independent and important risk factors for Gr 2–4 IVH. In addition, the MTHFR variant and the interaction term (Apgar5 < 3-by-MTHFR allele) were independent and important predictors of Gr 2–4 IVH in our population ( Table 5 ).

Table 5 Analysis of generalized linear mixed model with random effects from the Gene Targets for IVH Consortium
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The F2 variant is the last leading coagulation candidate we are discussing. It results in increased thrombin and secondary thrombosis, and in the studies of Baier, Harteman et al., and Ryckman et al., it was not associated with a risk for IVH.

Inflammatory Factors

Cytokines are also postulated to play a role in perinatal brain injury, and both Ryckman et al. and Baier explored the role of interleukins in preterm IVH. Hypoxia results in the loss of blood–brain barrier function and impaired tight junction protein synthesis (62), permitting cytokines from the peripheral circulation to directly enter preterm brain. In addition, cytokines secreted by cells of the immune system may also be synthesized by central nervous system glial to act as signal transmitters in the developing brain (63).

IL-1β is the major cytokine involved in activation of the hypothalamic–pituitary–adrenal axis (31). In addition, although the exact mechanisms by which IL-1β is involved in hypoxia–ischemia, IVH, and perinatal brain injury remain unknown, IL-1β has been implicated in the progression of injury in the developing brain (64). Of importance to the understanding of critical period injuries such as IVH, expression of IL-1β is both developmentally and regionally regulated in the brains of typically developing fetuses and neonates (65). Similarly, in response to hypoxic–ischemic injury, IL-1β differentially increases across the brain, suggesting regional vulnerability to cytokine-mediated injury.

Preclinical studies demonstrate that perinatal IL-1β exposure induces acute white matter injury with subsequent ventriculomegaly, loss of mature oligodendrocytes, impaired myelination, decreased myelin basic protein, and axonal and dendritic injury (66). In addition, acting via the cyclooxygenase-2 (COX-2) pathway (67), perinatal bacterial infection significantly increases IL-1β, IL-6, and corticosterone production in rat pups a few hours after infection, suggesting involvement of both the central inflammatory and hypothalamic–pituitary–adrenal pathways (68). Such perinatal immune activation has been associated not only with change in behavior in neonatal animals but also disrupted avoidance learning in male, but not female subjects in adulthood (69). Taken together, these data suggest that early IL-1β–mediated immune activation results in long-term changes in both structure and function in developing brain.

When Baier evaluated the role of IL-1β 511C>T polymorphisms in 215 ventilated very-low-BW infants, the IL-1β-511 T allele was associated with increased risk for IVH. One-third of infants with the T allele experienced IVH, compared with 14% with the C allele. There was also a significant difference in Gr 3–4 IVH between the groups. Periventricular leukomalacia was also increased, mainly in those infants with the CT genotype. Because of the association of chorioamnionitis and periventricular leukomalacia, Baier interrogated the interaction of ureaplasma urealyticum colonization and IL-1β 511T allele on the incidence and severity of both IVH and periventricular leukomalacia. Consistent with the report of Leviton et al. (70), there was no interaction for these triggers in neonates with IVH. In contrast, infants with both the 511T allele and ureaplasma urealyticum were at greater risk of periventricular leukomalacia than infants with one or none of these triggers, suggesting a gene-by-environment interaction.

Ryckman et al. validated this result by finding that the IL-1β-31 C allele was associated with an increased risk for hemorrhage. The C allele of IL-1β-31 is in strong linkage disequilibrium with the T allele of IL-1β -511, and both increase the production of IL-1β in vivo (71).

Similarly, IL-6 has also been implicated in injury in developing brain (65,72) and has also been shown to activate the hypothalamic–pituitary–adrenal axis (73). Thus, IL-6 is also believed to be a strong candidate to modify risk for preterm brain injury. Harding et al. (74) reported that in 151 preterm neonates, the CC genotype of IL-6–174 significantly increased the risk for IVH and neurodevelopmental disability at 2 y of age. In contrast, interrogating the same polymorphism, neither Baier nor Ryckman et al. found any relationship between IL-6 and IVH in the prematurely born.

Tumor necrosis factor (TNF)-α plays a pivotal role in the acute-phase proinflammatory cytokine cascade and is also postulated to be a central mediator of brain injury in the prematurely born (75). The TNF-α gene is polymorphic, and there are numerous polymorphisms in the promoter region. Studying 178 ventilated very-low-BW infants, Adcock et al. reported that the -308 A allele in the TNF-α promoter region was associated with IVH in preterm neonates (75). Baier also found that in infants with the TNF-α -308 A allele, the incidence of IVH was 40% as compared with 24% in those with GG (19). However, Ryckman et al. found no association between this polymorphism and IVH in her population.

Vascular Genes

Proteins both contributing to the integrity of the developing central nervous system vasculature and those mediating CBF are excellent targets for IVH. COL4A1 encodes type IV collagen alpha chain 1. This is one of six alpha chains that contribute to type IV collagen, a principal component of basement membranes ubiquitously expressed during development. Truncating mutations in murine Col4A1 result in cerebral hemorrhage in both neonatal and adult mice, and mutations have been reported in infants with congenital porencephaly, fetal IVH, and adults with cerebral small vessel disease (35,76). More recently, mutations have also been reported in preterm neonates with IVH (77,78). Studying 41 preterm infants with IVH, Bilguvar et al. (79) identified a rare heterozygous duplication within a highly conserved residue in COL4A1 in dizygotic twins with Gr 4 IVH.

In addition to inhibiting platelet and leukocyte adhesion to vascular endothelium, NO promotes cerebral vasodilatation (80). Several allelic variants have been reported in promoter of the endothelial NO synthase gene, which have been associated with decreased endothelial NO synthase activity and reductions in NO. Investigating 124 African-American preterm neonates, Vannemreddy et al. (81) reported the association of the endothelial NO synthase gene promoter polymorphism 786T>C with IVH, suggesting that the vascular actions of endothelial NO synthase are critical for the prevention of hemorrhage in the developing brain.

Finally, oxidative stress may also play a role, and Poggi et al. (82) reported that the rs8192287 superoxide dismutase 3 polymorphism is an independent protective factor for IVH in 152 neonates of <28 wk GA. Although the mechanism is not yet known, Poggi et al. postulates protection of the cerebral microvessels against oxidative injury.

Implications of the Science: Future Approaches for Decreasing the Incidence of IVH

Further understanding of the genetic contributions to IVH, including genome-wide association studies and/or whole-exome sequencing data, will permit the rationale design of randomized clinical trials. These might include delivery mode trials for fetuses harboring vascular structural polymorphisms and strategies to lower homocysteine in mothers and/or neonates with MTHFR variants. Equally important avenues of molecular investigation might include inhibiting disease-causing pathways, such as the proposed use of rapamycin for subependymal giant cell astrocytomas in children with tuberous sclerosis (83); upregulating affected proteins from homologous genes as in models of spinal muscular atrophy (84); or counteracting the downstream effects of a deficient protein, such as the proposed use of insulin-like growth factor 1 in children with Duchenne’s dystrophy (for review, see ref. (85)).

Available preclinical genetic studies and clinical candidate gene reports suggest that IVH may be attributable to numerous genes with small effect sizes and the environmental factors that interact with them. Because these common variants have small-to-moderate effects on disease risk, individual risk variants are neither necessary nor sufficient to produce disease, and there are consequently individuals with disease without risk variants and, conversely, individuals without disease who harbor risk variants. A hope is that identification of genes and pathways underlying IVH will permit the development of prenatal diagnostics and/or preventive therapeutics. To address these issues, a large-scale neonatal genomic medicine network must be developed with infrastructural capacity to both host an accessible database of sequence variants and their phenotypic associations and support a framework for defining and cataloging clinically actionable variants (86).

Conclusion

If a major focus of perinatal care is to prevent brain injury and abnormal development (87), then physicians and scientists must better understand those factors that contribute to severe IVH in the prematurely born. Emerging data suggest an important role of genes subserving coagulation, inflammatory, and vascular pathways, and interactions with maternal and neonatal environmental triggers may influence both the incidence and severity of cerebral injury and have long-term implications.

Statement of Financial Support

This work was supported by the National Institutes of Health, Bethesda, MD, NS053865. H.Z. was also partially supported by R01 DA016750-09.

Disclosure

None of the authors have any disclosures of financial ties to products in the study or potential/perceived conflicts of interest.