Dr. Thibault Coste, from the Service de genetique neurovasculaire Hopital Saint Louis, Pris, France, in his presentation titled “Molecular genetics of fetal intracranial hemorrhage – COL4A1/A2 variants, the most common genetic cause”, discussed the molecular genetics of fetal intracranial hemorrhage (ICH), focusing on COL4A1 and COL4A2 variants, the most common genetic causes.

Fetal intracranial hemorrhage (ICH) is a severe condition, usually detected during the second or third trimester of pregnancy, with an estimated incidence of 1 in 10,000 pregnancies. ICH is classified into four grades based on the extent and location of bleeding in the brain, with grades three and four often resulting in poor prognosis and, in many cases, termination of pregnancy. The mechanisms underlying fetal ICH are not fully understood, but it is believed to involve cerebral vessel fragility, leakiness, vascular malformation rupture, or abnormal cerebral angiogenesis. Known non-genetic causes include fetomaternal platelet alloimmunization (FNAIT), infections like CMV, drugs, and abdominal trauma. Genetic causes, particularly involving COL4A1 and COL4A2 mutations, are significant, and therefore worthy of further investigation.

Its review of the literature revealed various biological pathways involved in ICH, such as coagulation, thrombosis, and extracellular matrix pathways. Otherwise, all these biological pathways involved are reported in a poor number of cases. Instead, COL4A1 and COL4A2 mutations account for over 30 reports of ICH, making them major genetic contributors.

In 2005, a link between COL4A1 and porencephaly was established. Over the following years, the disease spectrum expanded, and in 2012, the first fetal case of COL4A1 mutation-related bleeding in the germinal matrix was reported. Subsequent cases have helped clarify the range of brain lesions associated with these mutations. Moreover, there are not information about neuropathological data in fetus until 2021.

MRI and neuropathological studies show multifocal ischemic lesions of varying ages and abundance, often associated with porencephaly and or schizencephaly. These lesions can be multifocal, ischemic, hemorrhagic, and may involve various brain regions. Neuropathological data also reveal iron-rich macrophages, eosinophilic deposits, and reactive gliosis, highlighting a broad spectrum of brain anomalies in affected fetuses.

Significant intra-familial variability is observed by literature, with some family members being asymptomatic carriers of the mutations. For example, in one family, two fetuses with intracranial hemorrhage inherited the mutation from a symptomatic father, who had inherited it from an asymptomatic grandfather. This variability complicates genetic counseling and underscores the importance of considering modifying factors.

Some questions arise from these arguments.

What is the Prevalence and Implications of COL4A1 and COL4A2 Variants?

In a study of 26 fetuses referred for targeted gene screening, mutations in COL4A1 and COL4A2 were identified in about 20% of cases. The majority of mutations were missense variants, with a higher frequency in COL4A1 than in COL4A2. Interestingly, only 30% of mutations are inherited and 70% of mutations were de novo, highlighting the importance of considering these variants in genetic counseling. There are more mutations in COL4A1 than COL4A2. The identification of these mutations necessitates non-invasive workups for carrier parents, preventive measures, and genetic counseling for relatives. Managing future pregnancies in carrier families remains a case-by-case discussion.

What are the differences Between Fetal and Adult Mutations?

There are notable differences in the location of pathogenic variants in fetuses compared to adults. In fetuses, variants cluster more at the C-terminal part of COL4A1, correlating with severe phenotypes. This is supported by mouse model studies showing increased cell cytotoxicity and ischemic lesions in this region.

What is the Identification of Novel Genes in Negative Cases?

Despite identifying mutations in 20% of cases, many fetuses remain genetically unexplained. To address this, we conducted whole exome sequencing on 35 trios, three families with two affected fetuses, and 75 probands, identifying several novel candidate genes involved in hemostasis and platelet disorders. For example, pathogenic variants in the protein C gene (PROC) were found in fetuses with massive focal hemorrhage and numerous thrombi, linking severe neonatal cases to vascular weakening from microthrombi formation.

Other genes are involved in platelet homeostasis like MPL or MECOM, in ICH caused by severe thrombocytopenia.

We also identified mutations in genes related to tight junction proteins, such as ESAM (Endothelial Cell Adhesion Molecule), which is crucial for blood-brain barrier integrity. Disruptions in these proteins lead to neurodevelopmental disorders in surviving children, indicating the critical role of tight junctions in preventing hemorrhage.

After this complete examination of the literature, Dr. Coste concluded that fetal ICH involves a complex genetic architecture with multiple causal and candidate genes, showing significant heterogeneity and low recurrence for some genes. Current data highlight the need for further research to identify additional genes and understand the genetic basis of ICH fully. Strategies include matching candidate variants with genes associated with ICH in model organisms to find correlations.