Microlissencephaly (MLIS) is a rare congenital brain disorder that combines severe microcephaly (small head) with lissencephaly (smooth brain surface due to absent sulci and gyri). Microlissencephaly is a heterogeneous disorder, i.e. it has many different causes and a variable clinical course. Microlissencephaly is a malformation of cortical development (MCD) that occurs due to failure of neuronal migration between the third and fifth month of gestation as well as stem cell population abnormalities. Numerous genes have been found to be associated with microlissencephaly, however, the pathophysiology is still not completely understood.
The combination of lissencephaly with severe congenital microcephaly is designated as microlissencephaly only when the cortex is abnormally thick. If such combination exists with a normal cortical thickness (2.5 to 3 mm), it is known as "microcephaly with simplified gyral pattern" (MSGP). Both MLIS and MSGP have a much more severe clinical course than microcephaly alone. They are inherited in autosomal recessive manner. Prior to 2000, the term "microlissencephaly" was used to designate both MLIS and MSGP.
Types
Microlissencephaly is one of five subtypes of lissencephaly. Microlissencephaly, in turn, can be subclassified based on imaging and clinical picture into four types:
MLIS1
Microlissencephaly Type A or Norman–Roberts syndrome (NRS): a microlissencephaly with thick cortex without infratentorial anomalies.
Other clinical features may include: a bitemporal narrowing, a broad nasal root. There is postnatal growth retardation, severe mental retardation associated with pyramidal spasticity and epilepsy. This entity could be identical to "lissencephaly with cerebellar hypoplasia type B" (LCHb), and therefore linked to mutations in RELN gene.
MLIS2
Microlissencephaly Type B or Barth microlissencephaly syndrome: is a microlissencephaly with thick cortex, severe cerebellar and brainstem hypoplasia. The Barth-type of MLIS is the most severe of all the known lissencephaly syndromes.
This phenotype consists of polyhydramnios (probably due to poor fetal swallowing), severe congenital microcephaly, weak respiratory effort, and survival for only a few hours or days. Barth described two siblings with this type as having a very low brainweight, wide ventricles, a very thin neopallium, absent corpus callosum and absent olfactory nerve.
MLIS3
Microlissencephaly with intermediate cortex and abrupt anteroposterior gradient
MLIS4
Microlissencephaly with mildly to moderately thick (6–8 mm) cortex, callosal agenesis
Presentation
Microlissencephalic patients suffer from spasticity, seizures, severe developmental delay and intellectual disabilities with survival varying from days to years. Patients may also have dysmorphic craniofacial features, abnormal genitalia, and arthrogryposis.
Microlissencephaly may arise as a part of Baraitser-Winter syndrome which comprises also ptosis, coloboma, hearing loss and learning disability. Moreover, it is the distinct developmental brain abnormality in "microcephalic osteodysplastic primordial dwarfism" (MOPD1). Microlissencephaly may be accompanied by micromelia as in Basel-Vanagaite-Sirota syndrome ( Microlissencephaly-Micromelia syndrome).
Pathophysiology
The genetic basis and pathophysiology of microlissencephaly are still not completely understood. Most cases of microlissencephaly are described in consanguineous families suggesting an autosomal recessive inheritance. Mutation of RELN gene or CIT could cause MLIS. Human NDE1 mutations and mouse Nde1 loss lead to cortical lamination deficits, which, together with reduced neuronal production cause microlissencephaly. Homozygous frameshift mutations in NDE1 gene was found to cause microlissencephaly with up to 90% reduction in brain mass and seizures starting early in life. Some other disease-causing genes include: KATNB1 and WDR62. It is hypothesized that the KATNB1-associated microlissencephaly is the result of a combined effect of reduced neural progenitor populations and impaired interaction between the Katanin P80 subunit (encoded by KATNB1) and LIS1 ( PAFAH1B1), a protein mutated in type 1 lissencephaly. Missense mutation in ACTG1 gene was identified in three cases of microlissencephaly. ACTG1 is the same gene that, when mutated, causes Baraitser-Winter syndrome. A loss-of-function mutation in the Doublesex- and Mab-3–Related Transcription factor A2 (DMRTA2, also known as DMRT5) gene has been reported in a case of microlissencephaly, implicating DMRTA2 as a critical regulator of cortical neural progenitor cell dynamics.