Jaarboek no. 89. 2010/2011 - Koninklijke Maatschappij voor ...

tered in families. Sporadic cases in which the patient
has no affected family members are often seen.
These cases may indeed represent the first ‘FHM’
patient (de novo mutation) in a family. Like FHM,
SHM has an overlapping clinical phenotype with
the common forms of migraine; indeed, the diagnostic
criteria for SHM are identical to FHM except
for the presence of affected first- or second-degree
relatives (see Table 1). In addition, SHM and normal
migraine also show a remarkable genetic epidemiological
relationship. SHM patients have a markedly
increased risk of typical migraine with aura,
and first-degree relatives have an increased risk of
both MA and MO. Although an initial study reported
CACNA1A mutations in only 2 of 27 SHM patients,
when this study was expanded, causal mutations
were found in the FHM genes in a higher proportion
of SHM patients, confirming a genetic relationship
between FHM and SHM.
E. FHM4 and beyond
Given the relatively rapid path to the discovery of
three genes linked to FHM, and continually improving
sequencing and screening methods, new FHM
genes will undoubtedly be discovered. This is supported
by reports of FHM families without identified
mutations in any of the three FHM genes. Given the
common pathway in which all three FHM proteins
play a role (see Section VII), it is likely that new candidate
genes will encode proteins that play key roles
in neuronal excitability and/or ion homeostasis.
VI. FUNCTIONAL CONSEQUENCES OF FHM
MUTATIONS
A. CaV2.1 (P/Q-type) calcium channel
(FHM1)
Upon the initial discovery of four missense mutations
in CACNA1A linked to FHM, the path to understanding
how the mutations give rise to the disease
seemed relatively straightforward. However, this
proved to be initially elusive, as functional analyses
of these four mutations yielded conflicting results.
In some cases, mutations gave rise to an apparent
loss of function, while in other cases, a gain-of-function
phenotype was observed. This was in stark con-
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Migraine: de ontrafeling van een complexe ziekte
trast to CACNA1A mutations linked to EA2, in which
all mutations studied confer a clear loss-of-function
phenotype to the channel. To date, 19 FHM1 mutations
have been identified in CACNA1A (Fig. 1), and
most have been studied in heterologous expression
systems. As with the original four mutations, no
clear pattern of either loss or gain of function initially
emerged. Because of these conflicting results,
researchers turned to expressing recombinant
channels in primary cultured neurons from Cacna1a
knockout mice lacking endogenous P/Q-type channels.
Four such studies have been published. In three
studies the mutations caused dramatically reduced
whole-cell currents when compared with wild-type
channels. In the fourth, the mutation gave rise to an
increased current density at negative voltages. The
functional consequences of the original four mutations
on synaptic transmission in cultured neurons
were also examined, and, consistent with decreased
whole-cell current density, every mutation caused
impaired synaptic transmission. Despite the evidence
for FHM1 mutations causing a loss-of-function
phenotype, increasing evidence was emerging
in support of gain-of-function effects. This evidence
primarily arose from a series of elegant studies
measuring the single-channel properties of channels
bearing FHM mutations. A careful examination
of channel open probability and unitary current
amplitude over a range of voltages revealed that the
mutant channels exhibit a negative shift in voltage
dependence, meaning that the channels open more
readily with less depolarization. This effect was also
reflected in whole-cell currents measured in S218L
channels expressed in Cacna1a knockout neurons,
in which significantly larger currents were observed
at negative voltages. In all, ten FHM1 mutations,
including all four original mutations, are reported to
activate at more negative voltages than wild-type
channels.
One important lesson learned from these studies
is that reliable functional data cannot always be
obtained solely using exogenously expressed channels.
Many factors, including the origin species of
the channel, the splice variant chosen, the accessory
subunit(s), and the expression level can all influence
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