Systemic complications of Aicardi Goutières syndrome using real-world data.

OBJECTIVE: Aicardi Goutières Syndrome (AGS) is a rare genetic interferonopathy associated with diverse multisystemic complications. A critical gap exists in our understanding of its longitudinal, systemic disease burden, complicated by delayed diagnosis. To address this need, real-world data extracted from existing medical records were used to characterize the longitudinal disease burden. METHODS: All subjects (n = 167) with genetically confirmed AGS enrolled in the Myelin Disorders Biorepository Project (MDBP) were included. As available in medical records, information was collected on subject demographics, age of onset, and disease complications. Information from published cases of AGS (2007-2022; n = 129) with individual-level data was also collected. Neurologic severity at the last available encounter was determined by retrospectively assigning the AGS Severity Scale [severe (0-3), moderate (4-8), and mild (9-11)]. RESULTS: The genotype frequency in the natural history cohort was TREX1 (n = 26, 15.6 %), RNASEH2B (n = 50, 29.9 %), RNASEH2C (n = 3, 1.8 %), RNASEH2A (n = 7, 4.2 %), SAMHD1 (n = 25, 15.0 %), ADAR (n = 34, 20.4 %), IFIH1 (n = 19, 11.4 %), and RNU7-1 (n = 3, 1.8 %). The median age of systemic onset was 0.15 years [IQR = 0.67 years; median range by genotype: 0 (TREX1) - 0.62 (ADAR) years], while the median neurological onset was 0.33 years [IQR = 0.82 years; median range by genotype: 0.08 (TREX1) - 0.90 (ADAR) year]. The most common early systemic complications were gastrointestinal, including dysphagia or feeding intolerance (n = 124) and liver abnormalities (n = 67). Among postnatal complications, thrombocytopenia appeared earliest (n = 29, median 0.06 years). Tone abnormalities (axial hypotonia: n = 145, 86.8 %; dystonia: n = 123, 73.7 %), irritability (n = 115, 68.9 %), and gross motor delay (n = 112, 7.1 %) emerged as the most prevalent neurological symptoms. Previously published case reports demonstrated similar patterns. The median AGS score for the entire cohort was 4 (IQR = 7). The most severe neurologic phenotype occurred in TREX1-related AGS (n = 19, median AGS severity score 2, IQR = 2). Time to feeding tube placement, chilblains, early gross motor delay, early cognitive delay, and motor regression were significantly associated with genotype (Fleming-Harrington log-rank: p = 0.0002, p < 0.0001, p = 0.0038, p < 0.0001, p = 0.0001, respectively). Microcephaly, feeding tube placement, and seizures were associated with lower AGS scores (All: Wilcoxon rank sum test, p < 0.0001). Among the qualifying case reports (n = 129), tone abnormalities were the most prevalent disease feature, with spastic quadriplegia reported in 37 of 96 cases (38.5 %) and dystonia in 30 of 96 cases (31.2 %). CONCLUSIONS: AGS is a heterogeneous disease with multi-organ system dysfunction that compounds throughout the clinical course, resulting in profound neurological and extra-neurological disease impact. Systemic symptoms precede neurologic disease features in most cases. Disease onset before the age of one year, microcephaly, feeding tube placement, and seizures were associated with worse neurological outcomes. This work will inform evidence-based clinical monitoring guidelines and clinical trial design.

Fetal Brain MRI Abnormalities in Pyruvate Dehydrogenase Complex Deficiency.

BACKGROUND AND OBJECTIVES: Pyruvate dehydrogenase complex deficiency (PDCD) is a disorder of mitochondrial metabolism that is caused by pathogenic variants in multiple genes, including PDHA1. Typical neonatal brain imaging findings have been described, with a focus on malformative and encephaloclastic features. Fetal brain MRI in PDCD has not been comprehensively described. The aims of this study were (1) to further characterize the fetal brain MRI findings in PDCD using comprehensive fetal imaging and genetic testing and (2) to determine whether markers of diagnosis of PDCD could be identified on prenatal imaging. METHODS: Fetuses with a diagnosis of PDCD related to a genetic etiology that had undergone fetal MRI were included. Fetuses were identified retrospectively from local databases of 4 fetal diagnostic clinics within tertiary pediatric health care centers. Electronic medical records were reviewed retrospectively: demographics, maternal and pregnancy history, fetal outcomes, and neonatal outcomes (if available) were reviewed and recorded. Fetal and neonatal imaging reports were reviewed; source fetal and neonatal brain MRI scans were reviewed by a single pediatric neuroradiologist (J.W.S.) for consistency. Genetic testing strategies and results including variant type, zygosity, inheritance pattern, and pathogenicity were recorded. Deidentified data were combined and reported descriptively. RESULTS: A total of 10 fetuses with a diagnosis of PDCD were included. 8 fetuses had corpus callosum dysgenesis, 6 had an abnormal gyration pattern, 10 had reduced brain volumes, and 9 had cystic lesions. 1 fetus had intraventricular hemorrhages. 1 fetus had a midbrain malformation with aqueductal stenosis and severe hydrocephalus. 6 fetuses imaged in the second trimester had cystic lesions involving the ganglionic eminences (GEs) while GE cysts were not present in the 4 fetuses imaged in the third trimester. DISCUSSION: Fetuses with PDCD have similar brain MRI findings to neonates described in the literature, although some of these findings are subtle early in pregnancy. Additional features, such as cystic lesions of the GEs, are noted in the second trimester in fetuses with PDCD. These may represent an early diagnostic marker of PDCD, although more data are needed to validate this association. Early diagnosis of PDCD using fetal MRI may inform genetic counseling, pregnancy decision making, and neonatal care planning.

Glial Origins of Inherited White Matter Disorders.

Inherited white matter disorders (IWMDs) are a phenotypically and genotypically heterogeneous group of disorders affecting the central nervous system (CNS) with or without peripheral neuropathy. They are classified either as leukodystrophies (LDs), with primary glial abnormalities, or genetic leukoencephalopathies (gLEs), where other CNS cells are involved. As a group, these disorders are common, with an incidence of 1 in 7500 births. However, IWMDs often go undiagnosed or suffer delayed or misdiagnosis due to their heterogeneous presentation. Many of these disorders present with lethal secondary manifestations that can be prevented through early disease recognition, periodic surveillance, and preventative management. Emerging therapeutics, including gene therapy trials for metachromatic leukodystrophy (MLD) and adrenoleukodystrophy (ALD), suggest disease progression may be slowed or even prevented if treated early. Therapies for IWMDs that target glial cells or the peripheral immune system may provide novel insights for treating acquired disorders of white matter.

De novo variants in the RNU4-2 snRNA cause a frequent neurodevelopmental syndrome.

Around 60% of individuals with neurodevelopmental disorders (NDD) remain undiagnosed after comprehensive genetic testing, primarily of protein-coding genes1. Large genome-sequenced cohorts are improving our ability to discover new diagnoses in the non-coding genome. Here we identify the non-coding RNA RNU4-2 as a syndromic NDD gene. RNU4-2 encodes the U4 small nuclear RNA (snRNA), which is a critical component of the U4/U6.U5 tri-snRNP complex of the major spliceosome2. We identify an 18 base pair region of RNU4-2 mapping to two structural elements in the U4/U6 snRNA duplex (the T-loop and stem III) that is severely depleted of variation in the general population, but in which we identify heterozygous variants in 115 individuals with NDD. Most individuals (77.4%) have the same highly recurrent single base insertion (n.64_65insT). In 54 individuals in whom it could be determined, the de novo variants were all on the maternal allele. We demonstrate that RNU4-2 is highly expressed in the developing human brain, in contrast to RNU4-1 and other U4 homologues. Using RNA sequencing, we show how 5' splice-site use is systematically disrupted in individuals with RNU4-2 variants, consistent with the known role of this region during spliceosome activation. Finally, we estimate that variants in this 18 base pair region explain 0.4% of individuals with NDD. This work underscores the importance of non-coding genes in rare disorders and will provide a diagnosis to thousands of individuals with NDD worldwide.

Developmental delay can precede neurologic regression in early onset metachromatic leukodystrophy.

OBJECTIVE: Metachromatic leukodystrophy (MLD) is a rare neurodegenerative disorder. Emerging therapies are most effective in the presymptomatic phase, and thus defining this window is critical. We hypothesize that early development delay may precede developmental plateau. With the advent of presymptomatic screening platforms and transformative therapies, it is essential to define the onset of neurologic disease. METHODS: The specific ages of gain and loss of developmental milestones were captured from the medical records of individuals affected by MLD. Milestone acquisition was characterized as: on target (obtained before the age limit of 90th percentile plus 2 standard deviations compared to a normative dataset), delayed (obtained after 90th percentile plus 2 standard deviations), or plateau (skills never gained). Regression was defined as the age at which skills were lost. LI-MLD was defined by age at onset before 2.5 years. RESULTS: Across an international cohort, 351 subjects were included (n = 194 LI-MLD subcohort). The median age at presentation of the LI-MLD cohort was 1.4 years (25th-75th %ile: 1.0-1.5). Within the LI-MLD cohort, 75/194 (39%) had developmental delay (or plateau) prior to MLD clinical presentation. Among the LI-MLD cohort with a minimum of 1.5 years of follow-up (n = 187), 73 (39.0%) subjects never attained independent ambulation. Within LI-MLD + delay subcohort, the median time between first missed milestone target to MLD decline was 0.60 years (maximum distance from delay to onset: 1.9 years). INTERPRETATION: Early developmental delay precedes regression in a subset of children affected by LI-MLD, defining the onset of neurologic dysfunction earlier than previously appreciated. The use of realworld data prior to diagnosis revealed an early deviation from typical development. Close monitoring for early developmental delay in presymptomatic individuals may help in earlier diagnosis with important consequences for treatment decisions.

Advancing fetal diagnosis and prognostication using comprehensive prenatal phenotyping and genetic testing.

Prenatal diagnoses of congenital malformations have increased significantly in recent years with use of high-resolution prenatal imaging. Despite more precise radiological diagnoses, discussions with expectant parents remain challenging because congenital malformations are associated with a wide spectrum of outcomes. Comprehensive prenatal genetic testing has become an essential tool that improves the accuracy of prognostication. Testing strategies include chromosomal microarray, exome sequencing, and genome sequencing. The diagnostic yield varies depending on the specific malformations, severity of the abnormalities, and multi-organ involvement. The utility of prenatal genetic diagnosis includes increased diagnostic clarity for clinicians and families, informed pregnancy decision-making, neonatal care planning, and reproductive planning. Turnaround time for results of comprehensive genetic testing remains a barrier, especially for parents that are decision-making, although this has improved over time. Uncertainty inherent to many genetic testing results is a challenge. Appropriate genetic counseling is essential for parents to understand the diagnosis and prognosis and to make informed decisions. Recent research has investigated the yield of exome or genome sequencing in structurally normal fetuses, both with non-invasive screening methods and invasive diagnostic testing; the prenatal diagnostic community must evaluate and analyze the significant ethical considerations associated with this practice prior to generalizing its use. IMPACT: Reviews available genetic testing options during the prenatal period in detail. Discusses the impact of prenatal genetic testing on care using case-based examples. Consolidates the current literature on the yield of genetic testing for prenatal diagnosis of congenital malformations.

Advance Care Planning for Children With Rare Diseases: A Pilot RCT.

BACKGROUND AND OBJECTIVE: Pediatric rare diseases are often life-limiting conditions and/or require constant caregiving. Investigators assessed the initial efficacy of the FAmily CEntered (FACE) pediatric advance care planning (pACP), FACE-Rare, intervention on families' quality of life. METHODS: A pilot-phase, single-blinded, intent-to-treat, randomized controlled clinical trial enrolled families from 1 pediatric quaternary hospital between 2021 and 2023. Intervention families received 3 weekly 60-minute (FACE-Rare pACP) sessions: (1) Carer Support Needs Assessment Tool or Action Plan, (2) Carer Support Needs Assessment Tol Action Plan Review, and (3) Pediatric Next Steps: Respecting Choices pACP. Controls received treatment as usual (TAU). Outcome measures were Beck Anxiety Inventory, Family Appraisal of Caregiving, Functional Assessment of Chronic Illness Therapy (FACIT)-Spirituality, and health care utilization. Generalized mixed effect models with γ response assessed the intervention effect at 3-month follow-up. RESULTS: Children (n = 21) were aged 1 to 10 years, 48% male, 24% Black; and 100% technology dependent. Primary family caregivers (n = 21) were aged 30 to 43 years, 19% male, 19% Black; and 27% household income below the Federal poverty level. Dyads underwent 1:1 randomization: 9 to FACE-Rare and 12 to TAU. TAU caregivers reported statistically lower meaning and peace than FACE-Rare caregivers (0.9, P = .03, confidence interval [CI]: 0.75-0.99). Black caregivers reported significantly less caregiver distress (0.7, P = .04, CI: 0.47-0.98) than non-Black caregivers. Poor families reported more anxiety (3.5, P = .002, CI: 1.62-7.94), more caregiver strain (1.2, P = .006, CI: 1.07-1.42); and less family well-being (0.8, P = .02, CI: 0.64-0.95). CONCLUSIONS: FACE®-Rare was feasible, acceptable, safe, and demonstrated initial efficacy, providing greater feelings of meaning and peace to caregivers. Poverty impacted well-being. A multisite trial is needed to determine generalizability.

Enasidenib-induced hepatitis in an individual with Type II D2-hydroxyglutaric aciduria.

Type II D-2-Hydroxyglutaric aciduria (T2D2HGA) is caused by a gain-of-function pathogenic variant in Isocitrate Dehydrogenase 2 (IDH2). Patients with T2D2HGA commonly present with developmental delay, seizures, cardiomyopathy, and arrhythmias. The recently approved IDH2-inhibitor Enasidenib targets the p.Arg140Gln pathogenic IDH2 variant and decreases production of D2HGA. We present a 7-year-old female with T2D2HGA due to the p.Arg140Gln variant. She was diagnosed at 3-years-old after presenting with global developmental delay, leukoencephalopathy, communicating hydrocephalus, seizures, and dilated cardiomyopathy. At age 3 years 11 months, 50 mg Enasidenib daily was initiated. Primary outcomes included seizure frequency, hospital admissions, development, and cardiac structure. Laboratories were monitored biweekly for common Enasidenib side effects. Our patient tolerated Enasidenib well. Urine 2-HGA decreased significantly from 244 mg/g creatinine to undetectable within 2 weeks of treatment. Inpatient admissions decreased from 8 during the 2 years preceding treatment to 1 during treatment. She has been seizure-free since Enasidenib initiation. Echocardiography showed improvement in dilated cardiomyopathy with normal left ventricular systolic function. Developmental assessment demonstrated improvements in gross motor, fine motor, language, and socialization domains. Treatment was complicated by mild elevations in alanine transaminase (118 IU/L, range 0-28) and creatine kinase (334 U/L, range 45-198) that resolved by decreasing Enasidenib dosing frequency to three times weekly. Enasidenib is a viable treatment for Type II D2HGA with benefits including developmental gains, fewer acute medical interventions, and cardiomyopathy improvement. While drug-induced hepatitis is a novel adverse effect of Enasidenib, it can be ameliorated by decreasing dose frequency.

Characteristic Fetal Brain MRI Abnormalities in Pyruvate Dehydrogenase Complex Deficiency.

Pyruvate dehydrogenase complex deficiency (PDCD) is a disorder of mitochondrial metabolism that is caused by pathogenic variants in multiple genes, including PDHA1. Typical neonatal brain imaging findings in PDCD have been described, with a focus on malformative features and chronic encephaloclastic changes. However, fetal brain MRI imaging in confirmed PDCD has not been comprehensively described. We sought to demonstrate the prenatal neurological and systemic manifestations of PDCD determined by comprehensive fetal imaging and genomic sequencing. All fetuses with a diagnosis of genetic PDCD who had undergone fetal MRI were included in the study. Medical records, imaging data, and genetic testing results were reviewed and reported descriptively. Ten patients with diagnosis of PDCD were included. Most patients had corpus callosum dysgenesis, abnormal gyration pattern, reduced brain volumes, and periventricular cystic lesions. One patient had associated intraventricular hemorrhages. One patient had a midbrain malformation with aqueductal stenosis and severe hydrocephalus. Fetuses imaged in the second trimester were found to have enlargement of the ganglionic eminences with cystic cavitations, while those imaged in the third trimester had germinolytic cysts. Fetuses with PDCD have similar brain MRI findings to neonates described in the literature, although some of these findings may be subtle early in pregnancy. Additional features, such as cystic cavitations of the ganglionic eminences, are noted in the second trimester in fetuses with PDCD, and these may represent a novel early diagnostic marker for PDCD. Using fetal MRI to identify these radiological hallmarks to inform prenatal diagnosis of PDCD may guide genetic counseling, pregnancy decision-making, and neonatal care planning.

De novo variants in the non-coding spliceosomal snRNA gene RNU4-2 are a frequent cause of syndromic neurodevelopmental disorders.

Around 60% of individuals with neurodevelopmental disorders (NDD) remain undiagnosed after comprehensive genetic testing, primarily of protein-coding genes1. Increasingly, large genome-sequenced cohorts are improving our ability to discover new diagnoses in the non-coding genome. Here, we identify the non-coding RNA RNU4-2 as a novel syndromic NDD gene. RNU4-2 encodes the U4 small nuclear RNA (snRNA), which is a critical component of the U4/U6.U5 tri-snRNP complex of the major spliceosome2. We identify an 18 bp region of RNU4-2 mapping to two structural elements in the U4/U6 snRNA duplex (the T-loop and Stem III) that is severely depleted of variation in the general population, but in which we identify heterozygous variants in 119 individuals with NDD. The vast majority of individuals (77.3%) have the same highly recurrent single base-pair insertion (n.64_65insT). We estimate that variants in this region explain 0.41% of individuals with NDD. We demonstrate that RNU4-2 is highly expressed in the developing human brain, in contrast to its contiguous counterpart RNU4-1 and other U4 homologs, supporting RNU4-2's role as the primary U4 transcript in the brain. Overall, this work underscores the importance of non-coding genes in rare disorders. It will provide a diagnosis to thousands of individuals with NDD worldwide and pave the way for the development of effective treatments for these individuals.

Consensus guidelines for the monitoring and management of metachromatic leukodystrophy in the United States.

Metachromatic leukodystrophy (MLD) is a fatal, progressive neurodegenerative disorder caused by biallelic pathogenic mutations in the ARSA (Arylsulfatase A) gene. With the advent of presymptomatic diagnosis and the availability of therapies with a narrow window for intervention, it is critical to define a standardized approach to diagnosis, presymptomatic monitoring, and clinical care. To meet the needs of the MLD community, a panel of MLD experts was established to develop disease-specific guidelines based on healthcare resources in the United States. This group developed a consensus opinion for best-practice recommendations, as follows: (i) Diagnosis should include both genetic and biochemical testing; (ii) Early diagnosis and treatment for MLD is associated with improved clinical outcomes; (iii) The panel supported the development of newborn screening to accelerate the time to diagnosis and treatment; (iv) Clinical management of MLD should include specialists familiar with the disease who are able to follow patients longitudinally; (v) In early onset MLD, including late infantile and early juvenile subtypes, ex vivo gene therapy should be considered for presymptomatic patients where available; (vi) In late-onset MLD, including late juvenile and adult subtypes, hematopoietic cell transplant (HCT) should be considered for patients with no or minimal disease involvement. This document summarizes current guidance on the presymptomatic monitoring of children affected by MLD as well as the clinical management of symptomatic patients. Future data-driven evidence and evolution of these recommendations will be important to stratify clinical treatment options and improve clinical care.

Longitudinal natural history studies based on real-world data in rare diseases: Opportunity and a novel approach.

Growing interest in therapeutic development for rare diseases necessitate a systematic approach to the collection and curation of natural history data that can be applied consistently across this group of heterogenous rare diseases. In this study, we discuss the challenges facing natural history studies for leukodystrophies and detail a novel standardized approach to creating a longitudinal natural history study using existing medical records. Prospective studies are uniquely challenging for rare diseases. Delays in diagnosis and overall rarity limit the timely collection of natural history data. When feasible, prospective studies are often cross-sectional rather than longitudinal and are unlikely to capture pre- or early- symptomatic disease trajectories, limiting their utility in characterizing the full natural history of the disease. Therapeutic development in leukodystrophies is subject to these same obstacles. The Global Leukodystrophy Initiative Clinical Trials Network (GLIA-CTN) comprises of a network of research institutions across the United States, supported by a multi-center biorepository protocol, to map the longitudinal clinical course of disease across leukodystrophies. As part of GLIA-CTN, we developed Standard Operating Procedures (SOPs) that delineated all study processes related to staff training, source documentation, and data sharing. Additionally, the SOP detailed the standardized approach to data extraction including diagnosis, clinical presentation, and medical events, such as age at gastrostomy tube placement. The key variables for extraction were selected through face validity, and common electronic case report forms (eCRF) across leukodystrophies were created to collect analyzable data. To enhance the depth of the data, clinical notes are extracted into "original" and "imputed" encounters, with imputed encounter referring to a historic event (e.g., loss of ambulation 3 months prior). Retrospective Functional Assessments were assigned by child neurologists, using a blinded dual-rater approach and score discrepancies were adjudicated by a third rater. Upon completion of extraction, data source verification is performed. Data missingness was evaluated using statistics. The proposed methodology will enable us to leverage existing medical records to address the persistent gap in natural history data within this unique disease group, allow for assessment of clinical trajectory both pre- and post-formal diagnosis, and promote recruitment of larger cohorts.

Hippocampal rotation is associated with ventricular atrial size.

BACKGROUND: Fetal ventriculomegaly is a source of apprehension for expectant parents and may present prognostic uncertainty for physicians. Accurate prenatal counseling requires knowledge of its cause and associated findings as the differential diagnosis is broad. We have observed an association between ventriculomegaly and incomplete hippocampal inversion. OBJECTIVE: To determine whether ventricular size is related to incomplete hippocampal inversion. MATERIALS AND METHODS: We retrospectively evaluated pre- and postnatal brain MRIs in normal subjects (mean GA, 31 weeks; mean postnatal age, 27 days) and patients with isolated ventriculomegaly (mean GA, 31 weeks; mean postnatal age, 68 days) at a single academic medical center. Lateral ventricular diameter, multiple qualitative and quantitative markers of hippocampal inversion, and evidence of intraventricular hemorrhage were documented. RESULTS: Incomplete hippocampal inversion and ventricular size were associated in both normal subjects (n=51) and patients with ventriculomegaly (n=32) (P<0.05). Severe ventriculomegaly was significantly associated with adverse clinical outcome in postnatal (P=0.02) but not prenatal (P=0.43) groups. In all additional cases of isolated ventriculomegaly, clinical outcome was normal over the time of assessment (mean 1±1.9 years; range 0.01 to 10 years). CONCLUSION: Lateral ventricular atrial diameter and incomplete hippocampal inversion are associated. Less hippocampal inversion correlates with larger atria. For every 1-mm increase in fetal ventricular size, the odds of incomplete hippocampal inversion occurring increases by a factor of 1.6 in normal controls and 1.4 in patients with ventriculomegaly.

Family caregivers of children and adolescents with rare diseases: a novel palliative care intervention.

OBJECTIVE: To develop and pilot test a palliative care intervention for family caregivers of children with rare diseases (FAmily-CEntered pediatric Advance Care Planning-Rare (FACE-Rare)). METHODS: FACE-Rare development involved an iterative, family-guided process including review by a Patient and Family Advisory Council, semistructured family interviews and adaptation of two evidence-based person-centred approaches and pilot testing their integration. Eligible families were enrolled in FACE-Rare (the Carer Support Needs Assessment Tool (CSNAT) Approach Paediatric sessions 1 and 2; plus Respecting Choices Next Steps pACP intervention sessions 3 and 4). Satisfaction, quality of communication and caregiver appraisal were assessed. RESULTS: Parents were mean age 40 years, and children 7 years. Children's diseases were rare enough that description would identify patients. All children were technology dependent. Telemedicine, used with four of seven families, was an effective engagement strategy and decreased subject burden. Families found FACE-Rare valuable following a strategy that first elicited palliative care needs and a support plan. Eight families were approached for pilot testing. Of the seven mothers who agreed to participate, six began session 1, and of those, 100% completed: all four FACE-Rare sessions, baseline and 2-week postintervention assessments, and a written pACP which described their preferences for medical decision-making to share with their providers. 100% reported FACE-Rare was helpful. The top three CSNAT concerns were: knowing what to expect in the future, having enough time for yourself and financial issues. Benchmarks were achieved and questionnaires were acceptable to parents and thus feasible to use in a larger trial. CONCLUSIONS: FACE-Rare provides an innovative, structured approach for clinicians to deliver person-centred care.

Prenatal diagnosis of diencephalic-mesencephalic junction dysplasia: Fetal magnetic resonance imaging phenotypes, genetic diagnoses, and outcomes.

OBJECTIVE: Report a single-center 12-year experience in the fetal diagnosis of diencephalic-mesencephalic junction dysplasia (DMJD) to expand the phenotype with Magnetic resonance imaging (MRI)-based classification, evaluate genetic etiologies, and ascertain outcomes. METHODS: Retrospective medical record and imaging review of all fetal MRI exams with DMJD were performed at our institution. RESULTS: Thirty-three pregnancies with fetal MRI findings of DMJD at 24 (18-37) weeks gestational age were studied; 70% were referred for fetal hydrocephalus. Three fetal MRI patterns were recognized. Type A (butterfly/hypothalamus-midbrain union) was seen in two cases (6%), Type B (partial thalamus-midbrain union) in 22 fetuses (70%), and Type C (complete/near complete midbrain-thalamic continuity) in nine fetuses (24%). L1CAM mutations were identified in four cases, and biallelic VRK1 variants in another. Among 14 live-born cases, 11 survived infancy, and 10 underwent postnatal brain MRI which confirmed the fetal MRI diagnosis in all but one case. Development was delayed in all surviving infants, most with additional neurological sequelae. CONCLUSIONS: DMJD may be identified by prenatal MRI as early as 18 weeks gestation. We propose three distinct phenotypic forms of DMJD, Types A-C. Next-generation sequencing provides an underlying molecular diagnosis in some patients, but further studies on associated genetic diagnoses and clinical outcomes are indicated.