Microcephalic osteodysplastic primordial dwarfism type II (MOPD II)
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Autosomal recessive primary microcephaly (MCPH) is a congenital, non-progressive disorder characterised by reduced brain size, particularly of the cerebral cortex, and mild to moderate intellectual disability. MCPH genes converge on key cellular processes governing neurogenesis and cortical neuronal output during brain development. Microcephalic osteodysplastic primordial dwarfism type II (MOPD II) is a separate entity combining congenital microcephaly with extreme growth restriction and a global vascular disease.
Microcephaly is typically a consequence of reduced brain volume and may present as an isolated trait or as part of a syndromic disorder with additional manifestations, including skeletal dysplasia and short stature, chromosomal instability, radiosensitivity, or metabolic abnormalities such as diabetes. Microcephaly is classified as primary when present at birth and secondary when it arises postnatally following impaired brain growth or progressive cerebral atrophy and is defined by an occipitofrontal circumference at least two standard deviations below the mean for age, sex and ethnicity.
is present at birth. Patients with typically exhibit mild to moderate intellectual disability, while maintaining relatively stable cognitive function and achieving basic adaptive, reading, and writing skills. The brain is small but architecturally largely preserved, with a disproportionate reduction in neocortical volume, while overall cortical architecture is largely preserved. Growth outside the head is comparatively spared. In MOPD II severe growth restriction affects the whole organism. At birth, mean measurements are 8.5 standard deviations below the mean for head circumference, 7.0 for length and 3.9 for weight, and adult height averages approximately 100 cm.
For , the reported prevalence ranges from about 1 in 250,000 in the general population to about 1 in 10,000 where consanguineous marriage is common. The prevalence of MOPD II is unknown; more than 150 individuals with a molecularly confirmed diagnosis have been reported worldwide.
-associated microcephaly is thought to result from altered neurogenesis, including disrupted neural progenitor proliferation and differentiation. The underlying defects commonly affect cell-cycle regulation, mitosis, cytokinesis, DNA replication and repair, or genome stability. Phenotypic severity depends on the developmental stage at which these processes are impaired. Consistent with this, MCPH genes encode key regulators of the molecular pathways governing cortical neurogenesis.
Most conditions are autosomal recessive, including MOPD II. Parents are obligate heterozygotes, the recurrence risk for siblings is 25%, and carriers are unaffected. Parental consanguinity raises the probability of a recessive cause considerably. More recently, autosomal dominant forms of hereditary primary microcephaly with similar clinical features have also been incorporated into the spectrum.
5–20% of children with developmental delay have microcephaly, identified by serial measurement of the occipitofrontal circumference against population references. Whether it is present at birth or develops thereafter directs the differential. Assessment includes length and weight to establish whether the microcephaly is proportionate, brain MRI, and consideration of non-genetic causes. Where a genetic cause is suspected, the diagnosis can be established by molecular genetic analysis from a DNA sample. For MOPD II, brain MR angiography becomes scheduled surveillance once the diagnosis is confirmed, because cerebrovascular lesions are frequently present before they become symptomatic.
Main differential diagnoses include:
- Meier-Gorlin syndrome is defined by growth restriction, microtia, and hypoplastic or absent patellae. Thirteen replication-machinery genes are associated with this entity.
- RNU4ATAC-related spliceosomopathies combine severe microcephaly and growth restriction with brain malformation, skeletal dysplasia, retinal anomalies, and immunodeficiency; the immunological and retinal findings separate them from MOPD II.
- Seckel syndrome overlaps with the .
- Establishing the molecular diagnosis assigns the specific entity behind a congenital microcephaly and provides the basis for genetic counselling, including an accurate recurrence risk. That risk differs between the recessive (majority of genes) and the dominant forms, a distinction that cannot be made clinically.
- For MOPD II, a confirmed result changes management directly, because the surveillance programme is entity-specific and largely preventive, covering the cerebral vessels, blood pressure, and heart, kidney function, glucose, and lipids, the skeleton, and the dentition. Where the result confirms without additional organ involvement, it ends the diagnostic process and allows carrier and prenatal testing to be offered within the family.
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