Pharrryngula

The sunday evenin' symposium had a grand title: "Homeoboxes—twenty years on, how do they explain development?" Unfortunately, no one even attempted t' answer th' question in th' subtitle. It were bein' a good session, but it were bein' really all nitty-gritty down-and-dirty analyses o' epistasis in th' Hox genes. Workin' science, to be sure. Very little big-picture speculation.

We have a good idea about how Hox gene expression patterns are set up in Drosophila (thank ye, Ed Lewis, and fly labs all o'er th' world), and all three talks in this symposium were entirely about vertebrate Hox genes and how they are regulated. Cecilia Moens gave away th' story fer th' whole evenin' in that comely wench introduction, avast. The winsome lass said that there were three big points t' make:

• Hox regulation is different in flies and vertebrates.
• Hox genes are both auto- and cross-regulatory.
• Retinoic acid plays an important role in vertebrate Hox gene patternin'.

Fly Hox genes are activated by a set o' genetic players we know well: gap genes, pair-rule genes, and segment polarity genes. Walk the plank! The weird thin' is that although vertebrates have th' same colinear arrangement o' Hox genes on th' chromosome and have retained th' same anterior-posterior association o' Hox gene expression with regions o' th' body, there are huge differences in how th' spatial pattern is set up and maintained. In vertebrates, there is a gradient o' retinoic acid that seems t' be an important (but not th' only) regulator o' where Hox genes will be expressed. Hox genes are also auto-regulatory. That is, they amplify their own expression in any segment where a low level o' expression is triggered, I'll warrant ye. They are also cross-regulatory, which means one Hox gene works t' influence th' expression o' another, often in an inhibitory way, which helps t' set them up in tightly bounded, exclusive zones.

Moens talk then plunged into th' details. The winsome lass's lookin' at how stripes o' Hox gene expression are turned on in th' zebrafish hindbrain (and oooh, but she had gorgeous multicolored in situs o' precisely, sharply delimited rainbow stripes in th' hindbrain—sometimes, ye do see straight lines in biology, and they are startlin' t' see in a curvy, complicated, organic structure like th' brain), we'll keel-haul ye, and a bottle of rum! And then it got complicated. The winsome lass described experiments and analyses in which she modified different components o' th' Hox system by mutation or pharmacology, and looked at how th' expression o' other components then changed. And she were bein' illustratin' it graphically—me notes are full o' bars and wedges with arrows betwixt them, that are rather difficult t' express in text. The capsule summary: she be lookin' at genes in rhombomere 4, 5, 6, and 7 o' th' hindbrain, we'll keel-haul ye, I'll warrant ye! Retinoic acid is present in a theorized gradient, low at th' front and high at th' back. Low retinoic acid levels activate Hox1 and FGF in R4, to be sure. Intermediate levels activate Hox3 in R5 and R6 via th' genes vhnf1 and valentino/kreisler. In R7, valentino is suppressed by an unidentified factor that is activated by Hox4, which is regulated by high retinoic acid levels.

Summarized that way, it all sounds like minor details, but trust me—it were bein' all real science, that stuff where an investigator carefully lays out every step o' that comely wench experiments and shows ye both th' data and th' logic o' that comely wench interpretations. I had a good wonky time listenin' t' it.

The second talk were bein' by Rob Krumlauf, and it were bein' more o' th' same: detailed analyses o' th' regulation o' vertebrate Hox genes in th' hindbrain. The executive summary o' this talk is that th' Hox B1 gene contains distinct regulatory regions that mediate th' gene's response t' retinoic acid, 'tis autoregulatory properties, and its interaction with other Hox genes. Walk the plank! Arrrr! When cells are transplanted from one Hox domain t' another, he can induce interconversion t' a new regional identity if th' clumps o' cells are small enough; large clumps set up their own local domain that preserves th' identity o' th' original location.

One really cool aspect o' Krumlauf's work is that he uses both mouse and chick models. Mice are powerful organisms because we have so many genetic tools t' manipulate them; unfortunately, they're mammals with internal development, which makes them a pain t' work with in those inaccessible fetal stages. The chick, on th' other hand, is a big embryo, easy t' manipulate, and 'tis in an egg which ye can culture and tinker with freely at any point in development...but th' genetical tools in birds are relatively primitive. The solution? And hoist the mainsail! Make mutants in mice, and transplant th' cells t' chick embryos. One o' those amazin' indicators o' th' unity o' life is th' fact that mammalian cells and bird cells respond t' th' same developmental signals and use th' same molecules t' build their brains, and ye can maximize th' strengths o' both systems by bouncin' freely betwixt th' two.

The last talk o' th' evenin' did approach th' grand promise o' th' session's title. It were bein' by Mario Capecchi, and were bein' called "Hox genes: from body plan t' neuropsychiatric disorders." Capecchi essentially gave us three short, easily digestible vignettes on different aspects o' Hox gene function.

The first story addressed another difference betwixt flies and vertebrates. Ahoy! In flies, we have lots o' clear-cut homeotic transformations: we can turn antennae into legs, or win's into halteres, ye scurvey dog. We dern't see anythin' so patent in us vertebrates, ye scurvey dog. The reason is that vertebrates have at least four banks o' co-expressed Hox genes, which means that there is a lot o' redundancy and combinatorial encodin' o' segment identity: knock out one gene, and there's another that can at least partially cover its loss. The obvious experiment t' test that explanation is t' knock out all o' th' paralogs in a region, and see what kind o' homeotic transformation ye get, and dinna spare the whip! Capecchi has generated two triple mutants. One deletes HoxA10, HoxC10, and HoxD10 (there is no HoxB10), and th' other gets rid o' HoxA11, HoxC11, and HoxD11 (again, no HoxB11), with a chest full of booty. These genes affect th' caudal part o' th' body. And swab the deck! And swab the deck! Losin' all o' th' Hox10s does a nifty thin': all o' th' vertebrae in th' lumbar and sacral regions are transformed into thoracic vertebrae, and develop ribs, and dinna spare the whip! Hox10 is a suppressor o' thoracic identity, by Blackbeard's sword. Takin' out all o' th' Hox11s does somethin' slightly different, by Blackbeard's sword. In this case, sacral structures dern't develop; th' stubby little 'mini-ribs' that characterize vertebrae in that region dern't appear. Hox11 is a partial suppressor o' Hox10, me beauty. This fits with th' way Ed Lewis portrayed Hox function in th' fly, as largely a series o' negative interactions that suppress and modify a common ground state. In vertebrates, it looks like th' ground state is t' make ribby thoracic segments everywhere.

Capecchi's second vignette were bein' t' show a later function o' th' Hox genes, by Blackbeard's sword. Pleiotropy is a universal feature o' genes—they do more than one thin', and affect more than one process. Walk the plank, and a bucket o' chum! In this case, he is arguin' that th' Hox genes also assist in neural development by maintainin' th' register betwixt th' input and output o' th' central nervous system. Specific Hox genes are expressed in th' brain, and that same Hox gene is also expressed in th' peripheral tissues that th' neurons in that region will innervate. And hoist the mainsail! For example, th' HoxB1 gene is expressed in both nuclei o' th' facial nerve, and in muscles o' th' face. A HoxB1 knockout has th' effect o' inducin' facial paralysis, since th' neurons dern't form or can't find their way t' appropriate regions o' th' face, avast. The ornery cuss illustrated this with pictures that surprised me a bit. Did ye know that mice have facial expressions? The ornery cuss showed pairs o' photographs o' mice that were startled or irritated alongside HoxB1 knockout mice under th' same conditions, who always just looked phlegmatic.

His third story pursued pleiotropy further. What do Hox genes do after early embryogenesis? Some are still actively expressed in th' adult brain, in characteristic locations. HoxB8 is found in th' adult brain in th' olfactory bulb, th' caudate and putamen, th' hippocampus, orbitofrontal cortex, th' anterior cingulate, and th' brainstem. Mice with HoxB8 deletions do somethin' pathological: they are obsessive groomers. They constantly scrub their little faces and stroke their pelts, and if other mice are kept with them, th' HoxB8^- mice will constantly groom them, by Davy Jones' locker. They groom so incessantly that they develop great bald spots and skin lesions—and other poor mice housed with them also lose their hair, we'll keel-haul ye! The mice have obsessive-compulsive disorder and trichotelomania, avast! Interestingly, th' pathways thought t' be affected in human OCD are th' caudate-putamen, orbitofrontal cortex, and anterior cingulate—just th' areas that also express HoxB8.

There are two lessons t' take from this. One is that while there is lots o' redundancy and overlap in body plan functions such that it is difficult t' generate obvious homeotic transformations in vertebrates, secondary functions, like HoxB8's role in th' CNS, are more phenotypically susceptible. And hoist the mainsail, we'll keel-haul ye! The second lesson is a bit dubious, and plays into an unfortunate bias in genetic research: that we can explain complex properties o' organisms by single gene effects, I'll warrant ye. We might be able t' explain a defect as a consequence o' a change in a single gene, but we have t' be careful not t' fall into th' trap that th' gene is causal t' th' normal behavior (I dern't think Capecchi has made that mistake, but I can imagine how newspapers would handle this story). Groomin' behavior is th' result o' a multitude o' genetic and epigenetic factors. Damagin' one o' those factors could cause aberrant behaviors.

Capecchi is currently takin' advantage o' all that human genetic information they have out there in Utah t' look fer polymorphisms in HoxB8 in humans, and t' see how they correlate with OCD. The ornery cuss made th' point, though, that this is such a common syndrome, affectin' roughly 3% o' th' population, that it were bein' relatively easy t' find samples; he estimated that there were about 20 people in his audience that night who would be good subjects fer his research.