Meckel–Gruber syndrome

Meckel–Gruber syndrome is a rare, typically lethal congenital disorder characterized by a classic triad of occipital encephalocele, cystic renal dysplasia, and polydactyly. It is caused by biallelic pathogenic variants in genes required for primary cilium structure and function, leading to multisystem developmental malformations identified prenatally or at birth. Management is mostly supportive with genetic counseling used for family planning.

Signs and symptoms

Affected infants commonly present with large encephalocele at the occiput, bilateral enlarged cystic kidneys producing oligohydramnios, and postaxial polydactyly of the hands or feet. Additional features reported include hepatic fibrosis with bile duct proliferation, pulmonary hypoplasia secondary to oligohydramnios, microphthalmia, clefting anomalies, and skeletal dysplasia. Neuropathological findings can include ventriculomegaly and cortical malformations; associated anomalies may involve the cardiac outflow tracts and genitourinary system. Prenatal presentation often includes severe oligohydramnios and reduced fetal movements prompting evaluation by maternal–fetal medicine, fetal surgery teams, and geneticists.

Genetics and pathophysiology

Pathogenic variants are most often inherited in an autosomal recessive pattern involving ciliary genes such as MKS1, TMEM67 (also known as MKS3), CEP290, RPGRIP1L, CC2D2A, and TMEM216, among others identified through next-generation sequencing and exome analysis. These genes encode proteins localized to the primary cilium and transition zone; dysfunction of these complexes disrupts hedgehog signaling, planar cell polarity, and cellular signaling pathways critical for neural tube closure, nephrogenesis, and limb patterning. Founder effects and population-specific allele frequencies explain higher prevalence in Finnish people, certain Arab populations, and consanguineous communities; genotype–phenotype correlations partially predict severity but show substantial locus heterogeneity. Experimental models using zebrafish, Xenopus laevis, and murine knockouts have recapitulated renal cystogenesis, neural tube defects, and digit anomalies, illuminating molecular mechanisms and potential modifiers such as centrosomal proteins and intraflagellar transport components.

Diagnosis

Diagnosis is based on prenatal imaging, clinical examination, and molecular genetic testing. High-resolution fetal ultrasound and fetal magnetic resonance imaging performed by specialists in Maternal-fetal medicine and pediatric neuroradiology commonly detect the triad of encephalocele, enlarged echogenic kidneys, and polydactyly during the second trimester; oligohydramnios and pulmonary hypoplasia are frequent secondary findings. Differential diagnosis includes other ciliopathies and syndromes such as Joubert syndrome, Bardet–Biedl syndrome, and autosomal recessive polycystic kidney disease; consultation with clinical geneticists and biochemical genetic testing may be necessary. Confirmatory diagnosis employs targeted gene panels, chromosomal microarray, or whole-exome sequencing in coordination with clinical laboratories accredited by agencies like the American College of Medical Genetics and Genomics. Prenatal invasive testing via chorionic villus sampling or amniocentesis allows molecular confirmation when familial pathogenic variants are known.

Management and treatment

There is no curative therapy; management focuses on informed perinatal decision-making, supportive neonatal care, and family counseling. Multidisciplinary teams including neonatology, pediatric nephrology, pediatric neurosurgery, palliative care, and genetics are involved in planning delivery location, assessing viability, and addressing respiratory insufficiency, renal failure, and potential neurosurgical considerations for encephalocele repair when compatible with survival. Ethical frameworks and institutional protocols used by perinatal ethics committees and newborn intensive care units guide choices about surgical intervention, life-sustaining therapy, and comfort care; reproductive options such as preimplantation genetic diagnosis are offered through reproductive endocrinology clinics for couples with known mutations. Experimental approaches in ciliopathy research and precision medicine initiatives continue to study pathway modulators, but no targeted treatments are established.

Prognosis and epidemiology

Prognosis is poor; most affected pregnancies result in stillbirth or neonatal death due to pulmonary hypoplasia, renal insufficiency, or central nervous system malformation. Reported survival beyond the neonatal period is rare and generally associated with milder phenotypes or atypical genotypes identified in registry studies and case series. Prevalence estimates vary by population: higher incidence has been described in Finland, certain Pakistani and Arab cohorts, and isolated communities with consanguinity, whereas overall occurrence is estimated in the range of 1 in 13,000 to 1 in 140,000 live births depending on ascertainment and geography. Genetic counseling, carrier testing in at-risk families, and use of population-specific panels by public health genetics services aim to reduce recurrence and support at-risk couples.

Category:Genetic disorders