Microlissencephaly: a narrative review

Keywords
Microlissencephaly (MLIS), Microcephaly with simplified gyral pattern (MSGP), Malformation of cortical development (MCD), Neuronal migration disorders (NMDs), Microcephaly, Lissencephaly

Abstract
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, having many different causes and a variable clinical course. It is a malformation of cortical development (MCD) that occurs due to the failure of neuronal migration between the third and fifth month of gestation as well as stem cell population abnormalities (either increased apoptosis or decreased production). Ten genes (RELN, CIT, NDE1, KATNB1, WDR62, WDR81, ACTG1, DMRTA2, DYNC1H1, RNU4ATAC) are so far associated with microlissencephaly along with five tubulin genes; however, the pathophysiology is still not completely understood. Most cases of MLIS are described in consanguineous families suggesting an autosomal recessive inheritance. In this review, the genetics of microlissencephaly, types, clinical manifestations, diagnosis, and management will be discussed.

Introduction
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) (Figure 1). MLIS is a heterogeneous disorder, having many different causes and a variable clinical course. It is a malformation of cortical development (MCD) that occurs due to the failure of neuronal migration between the third and fifth month of gestation as well as stem cell population abnormalities (either increased apoptosis or decreased production).

The combination of lissencephaly with severe congenital microcephaly is designated as MLIS 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. Prior to the year 2000, the term “microlissencephaly” was used to designate both MLIS and MSGP. Both MLIS and MSGP result from either decreased stem cell proliferation or increased apoptosis in the germinal zone of the cerebral cortex.

Genetics
The genetic basis and pathophysiology of MLIS are still not completely understood. Most cases of MLIS are described in consanguineous families suggesting an autosomal recessive inheritance. Numerous genes have been found to be associated with MLIS (Table 1). The physical and functional interaction between proteins associated with microlissencephaly was analyzed using STRING database "http://version10.string-db.org/" (Figure 2).

CIT
Mutations of CIT genes could cause MLIS.

RELN
Microlissencephaly type A (Norman Roberts syndrome) is suggested to be identical to lissencephaly with cerebellar hypoplasia type B (LCHb) which is caused by mutation of the RELN gene.

NDE1
Moreover, human NDE1 mutations and mouse Nde1 loss lead to cortical lamination deficits, which, together with reduced neuronal production cause MLIS. Homozygous frameshift mutations in NDE1 gene was found to cause MLIS with up to 90% reduction in brain mass and seizures starting early in life.

KATNB1
Some other associated genes include KATNB1 (Katanin p80) and WDR62. Katanin, a microtubule-severing enzyme, is composed of a catalytic, p60 (KATNA1), and a regulatory, p80 (KATNB1), subunit. p80/KATNB1 binds to p60 and targets it to subcellular structures including the centrosome, further mediating its interactions with dynein, LIS1, and NDEL1. In developing neurons, Katanin localizes to microtubules and centrosomes and is essential for microtubule shortening and release. Katanin functions in cell division and neuronal morphogenesis. It is hypothesized that the KATNB1-associated MLIS 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 (also known as PAFAH1B1), a protein mutated in type 1 lissencephaly.

ACTG1
A missense mutation in the ACTG1 gene was identified in three cases of MLIS. ACTG1 is the same gene that, when mutated, causes Baraitser-Winter syndrome.

DMRT5
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 MLIS, implicating DMRTA2 as a critical regulator of cortical neural progenitor cell dynamics.

WDR81
Another gene that could be involved in the pathogenesis of MLIS is WDR81. Compound heterozygous mutations in WDR81 were found in seven cases from five non-consanguineous families with microcephaly and extremely reduced gyration including agyria (no gyri). WDR81 is suggested to play a role in normal cell proliferation.

DYNC1H1
In a family of two cases with microlissencephaly and arthrogryposis with consequent pregnancy termination, a common variant in DYNC1H1 gene on chromosome 14 was identified.

Tubulins
Microlissencepahly is considered a tubulinopathy (tubulin gene defect) i.e. it can be caused by mutations in tubulin genes, mainly TUBA1A (Figure 3) and less commonly TUBB2B, TUBB3, TUBA3E, and TUBG1. Central pachygyria (unusually thick convolutions of the cerebral cortex) and polymicrogyria (multiple small gyri) are more commonly seen in patients with defects in TUBB2B, TUBB3, and TUBB5. This implies the critical role of the microtubule cytoskeleton in the pathophysiology of MLIS as well as other neuronal migration disorders.

RNU4ATAC
RNU4ATAC is a gene on chromosome 2 which encodes for a small nuclear RNA (snRNA) called U4atac. U4atac is a part of the U12-dependent minor spliceosome complex. Mutation in this protein is associated with "microcephalic osteodysplastic primordial dwarfism" (MOPD1) which can involve MLIS.

Clinical picture
Due to the rarity of the disease, there is still no consensus on the exact clinical picture of MLIS. At birth, lissencephaly with a head circumference of less than minus three standard deviations (< –3 SD) is considered MLIS.

MLIS is a classic finding of holoprosencephaly, where the forebrain of the embryo fails to develop into two cerebral hemispheres. MLIS 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). MLIS may be accompanied by micromelia (shortening of the limbs) as in Basel-Vanagaite-Sirota syndrome (also known as Microlissencephaly-Micromelia syndrome).

MLIS is one of five subtypes of lissencephaly and a distinct subtype of autosomal recessive primary microcephaly (MCPH). MLIS includes several types as illustrated in (Table 2).

†Dobyns and Barkovich classified patients with severe microcephaly and gyral abnormalities (including microcephaly with simplified gyral pattern (MSGP), MLIS and polymicrogyria (multiple small gyri)) into ten groups. MSGP represented the first four groups, MLIS referred to the groups from 5-8 and polymicrogyria in the last two groups.

Diagnosis
An algorithm for the diagnosis of all malformations of cortical development is illustrated in Figure 4. MLIS diagnosis can be confirmed by prenatal MRI (Figure 3) which is better than ultrasound in the prenatal detection of MLIS or MSGP. The ideal time for proper prenatal diagnosis is between the 34th and 35th gestational week which is the time when the secondary gyration normally terminates. In MLIS cases, the primary sulci would be unusually wide and flat while secondary sulci would be missing.

Although genetic diagnosis in patients with MLIS is challenging, exome sequencing has been suggested to be a powerful diagnostic tool.

Differential diagnosis
MLIS is considered a more severe form than microcephaly with simplified gyral pattern. MLIS is characterized by a smooth cortical surface (absent sulci and gyri) with a thickened cortex (>3 mm) and is usually associated with other congenital anomalies. Microcephaly with a simplified gyral pattern has too few sulci and a normal cortical thickness (3 mm) and is usually an isolated anomaly. A comparison between MLIS and MSGP is summarized in (Table 3).

Management
The treatment of microlissencephaly and other malformations of cortical development is mostly symptomatic. Developmental delay is managed with neurorehabilitation, including physical and occupational therapy, and speech and feeding therapy. The management of epilepsy in microlissencephalic cases is complicated and depends on the underlying basis of the epileptiform activity. There are two aspects of neuronal functions that could generally contribute to epilepsy, either the epileptic intrinsic properties of neurons or the abnormally epileptic circuits of groups of neurons. In each case, the therapeutic approach differs. If the reason is abnormal epileptogenic circuits, then the neurosurgical resection of such zones would be beneficial. On the other side, if the abnormality arises from abnormal channels or receptors then the pharmacological approach could control epilepsy. However, it is still difficult to make a clear distinction between both mechanisms. Animal model studies are encouraged to further investigate how MLIS mutations in the aforementioned genes can lead to epilepsy which can help improve the understanding and management of epilepsy in those cases.

Genetic counseling of family members is important in all cases of malformation of cortical developments (MCDs) and should include the type of mutation, the mutated genes as well as an explanation of the resultant clinical picture, particularly for subsequent pregnancies. In families with known molecular etiology, prenatal testing in the form of chorionic villus sampling or amniocentesis can be offered to guide subsequent pregnancies. Preimplantation genetic diagnosis (PGD) would be an option as well for those families.

Depending on the underlying pathogenesis, survival in MLIS varies from days to years. It can lead to an early fatal outcome during the neonatal period.

Molecular therapy
Lissencephaly can result from a mutated LIS1 gene. LIS1 is a protein, normally essential for targeting of cytoplasmic dynein to the plus-end of microtubules. It was demonstrated that LIS1 (PAFAH1B1) is substantially degraded by calpain protein (encoded by CAPN1 and CAPN2) after reaching the plus-end of microtubule. In an animal experiment, a calpain inhibitor called ALLN (Acetyl-Leucyl-Leucyl-Norleucinal) was applied to mice with LIS1 mutation. This has restored LIS1 to normal levels in neurons and consequently, improved neuronal migration and rescued apoptotic neuronal cell death. This provides a proof-of-principle for a potential therapeutic intervention for lissencephaly in the future.

Although LIS1 is not specifically mutated in MLIS, it is a part of a complex involving NDE1 and dynein DYNC1H1 gene, which both can be mutated in MLIS. This LIS1/NDE1/NDEL1/cytoplasmic dynein complex (Figure 4) plays a role in the regulation of neuron proliferation, migration, and intracellular transport. Targeting this complex in utero and throughout development could theoretically and potentially improve the outcome of microlissencephaly. Experimental animal studies are recommended to investigate the effect of calpain inhibitors in microlissencephaly.

Epidemiology
MLIS is a rare disease. There is not much information available about the epidemiology of microlissencepahly in the literature. A Ph.D. thesis has estimated the prevalence of microlissencepahly in South-Eastern Hungary between July 1992 and June 2006 to be one case in every 91,000 live births (incidence of 1.1 per 100,000 newborns).

History
In 1976, the first syndrome with MLIS was reported, now known as Norman–Roberts syndrome (MLIS type A). The Barth type (MLIS type B) was for the first time described in 1982 in two siblings who died soon after birth.

Acknowledgments
The author declares no conflict of interest.