A Join Spa io empo al Di e en ial Exp ession Modeling
o Exposu e E ec s in Spa ial T ansc ip omics Da a
Osa u Augus ine Egbon1,2 and Benedic Anchang1,2
1Na ional Ins i u e o En i onmen al Heal h Sciences (NIEHS/NIH), Du ham, NC 27709, Uni ed S a es
2Na ional Cance Ins i u e, Be hesda, MD 20892, Uni ed S a es
Abs ac
Di e en ial exp ession analysis in spa ial ansc ip omics da a is impo an o iden i ying local-
ized biological p ocesses and spa ially s uc u ed gene egula ion. While nume ous me hods ha e
been de eloped o iden i ying di e en ially exp essed genes (DEGs) in con en ional single-cell
RNA-seq da a, app oaches ha explici ly accoun o spa ial and empo al con ex emain sca ce. A
majo challenge in spa ial DEG analysis is he spa ial misalignmen be ween issue sec ions unde
con ol and pe u bed condi ions, which ende s coo dina e-based compa isons un eliable. To ad-
d ess his gap, we de eloped Spa ialDENe , a amewo k ha iden i ies DEGs in spa ially s uc u ed
single-cell da a by inco po a ing spa ial in o ma ion h ough a hie a chical ne wo k-based model.
This app oach accoun s o spa ial con ounding and le e ages issue a chi ec u e o align biological
signals ac oss condi ions. We alida ed Spa ialDENe h ough simula ion s udies and compa a i e
benchma ks wi h exis ing popula me hods. Ou indings demons a e ha Spa ialDENe p o ides
a powe ul and in e p e able al e na i e o spa ial ansc ip omics analysis and may in o m he
de elopmen o a ge ed he apies.
Key Wo ds: Bayesian in e ence, Ne wo k model, Di e en ial exp ession analysis, Gaussian Ma ko
Random Field, Ze o-in la ion.
1 In oduc ion
Spa ial ansc ip omics echnologies a e inc easingly popula o in es iga ing he spa ial a chi ec u e
o issues, unco e ing cell-cell in e ac ions, and analyzing gene exp ession pa e ns in si u wi h high
esolu ion1,2. Howe e , conduc ing Di e en ial Exp ession (DE) analysis o iden i y di e en ially
exp essed genes (DEG) be ween wo biological condi ions on spa ial ansc ip omics da a p esen s
signi ican challenges due o i s high dimensionali y, noise, spa si y, and spa ial s uc u e.
Me hods o DE analysis in s udying biological a ia ions a he single-cell le el a e eme ging
quickly. Fo ins ance, he popula ly used me hods include DESeq23, EdgeR4, Lima5, MAST6,
Zinbwa e7, among o he s. Howe e , mos o hese me hods do no accoun o spa ial a ia ion, an
impo an sou ce o biological and echnical a iabili y ha can con ound di e en ial exp ession
analysis esul s8. Exis ing s a is ical and compu a ional me hods8β13 o analyzing spa ially esol ed
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single-cell da a a e pa icula ly designed o iden i y spa ially a iable genes (SVGs), and no adequa e
a en ion has been gi en o iden i ying DEGs be ween con ol and ea men scena ios in spa ially
esol ed single-cell da a.
To add ess his conce n, we p oposed a s a is ical amewo k, Spa ialDENe (Sp ial Di e en ial
Exp ession Analysis h ough Ne wo k), o iden i y di e en ial exp ession genes be ween con ol and
pe u bed condi ions while co ec ing o spa ial con ounding (SC). Spa ialDENe add esses h ee
main analy ical conce ns in spa ially esol ed single-cell da a. Fi s ly, Spa ialDENe adjus s o he
SC issue h ough spa ial hie a chical modeling. This me hod allows he accu a e es ima ion o he
impac o a speci ic ea men on cells a e con olling o he spa ial e ec s. Secondly, spa ially
esol ed single-cell da a is associa ed wi h spa ial misalignmen due o he non-longi udinal and non-
homologous na u e o he da a ac oss samples. Tha is, he spa ial coo dina es change om issue
samples o samples. To add ess his conce n, Spa ialDENe le e ages ne wo k models o map
cellula spa ial neighbo hoods be ween mul iple issues un o a la en space, c ea ing a uni ied spa ial
ield o adjus ing o SC and iden i ying DEGs. Las ly, Spa ialDENe accoun s o he spa si y in
he exp ession da a. The ze os due o spa si y can a ise om echnical limi a ions in he sequencing
p ocess o e lec ue biological absences o gene exp ession o speci ic cells. Dis inguishing
be ween hese wo sou ces o ze os is c i ical o accu a e in e p e a ion. Spa ialDENe u ilizes a
wo-pa (coun and ac i a ion) ze o-in la ed model o accoun o excess ze os.
The emainde o his pape is o ganized as ollows. We i s b ie ly desc ibe he s a is ical ame-
wo k o Spa ialDENe and i s ex ension o accommoda e ze o-in la ed da a. We hen benchma k
he amewo k wi h popula ly used di e en ial exp ession analysis me hods h ough a simula ion
s udy. Finally, we conclude wi h a summa y o ou con ibu ions and po en ial di ec ions o u u e
esea ch.
2 S a is ical F amewo k
2.1 Backg ound o di e en ial analysis amewo k
In single-cell DE analysis, he gene al amewo k adop ed by exis ing me hods can be desc ibed
as ollows. Le πππ be he obse ed exp ession le el (e.g., ead coun ) o gene πin cell π , whe e
π=1, . . . , πΊ and π =1, . . . , π. A gene alized linea model (GLM) is ypically used o model gene
exp ession as:
πππ βΌNB(πππ , ππ),(1)
whe e NB deno es a Nega i e Binomial dis ibu ion, πππ is he expec ed exp ession le el o gene π
in cell π ,ππis a gene-speci ic dispe sion pa ame e . The pa ame e πππ is hen linked o co a ia es
2
h ough a log-linea model gi en as
log(πππ )=π½π0+log(ππ ) +ππ π½π1+xβ€
π π·π,(2)
whe e ππ is a size ac o o lib a y size no maliza ion o cell π ,π½0is he model in e cep , ππ β {0,1}
is he ea men indica o , wi h e ec size π½1,xπ is a ec o o co a ia es o adjus o obse ed
con ounding a iables, and π·πis a co esponding ec o o eg ession coe icien s o gene π.
To es o di e en ially exp essed genes be ween con ol and ea men condi ions, we de ine he
hypo hesis (Hypo hesis 1) o a speci ic gene πas
π»0:π½π1=0 s. π»1:π½π1β 0,(3)
whe e π½π1is he coe icien co esponding o he condi ion o in e es .
In single-cell spa ially esol ed da a wi h genes exhibi ing spa ial a ia ion and ze o in la ion,
Hypo hesis 1 becomes p oblema ic and inapp op ia e o inding di e en ial exp ession genes in
single-cell analysis.
2.2 Spa ialDENe : The p oposed amewo k
To add ess he abo e conce n, we p oposed a uni ied amewo k ha add esses spa ial con ounding,
spa ial misalignmen , he empo al e olu ion o gene exp ession, and ze o-in la ion p oblems in
single-cell spa ially esol ed da a.
2.2.1 Response model
Conside a scena io whe e we ha e p e-p ocessed single-cell exp ession da a collec ed o e ime.
Le πππ π‘, π =1,2, ..., π;π=1,2, ..., πΊ,π‘=1,2, ..., π be a gene exp ession coun a spa ial loca ion
deno ed by π a ime π‘ o gene π.πis he o al numbe o spo s/cells ac oss all issues. No e he e ha
he cell index π p e iously de ined as a cell is now ede ined as a spo index π in spa ial ansc ip omics
da a. We assume ha
πππ π‘ |π½π0, π½π1,π·πβΌNB(πππ π‘ , ππ),
log(πππ π‘)=π½π0+log(ππ π‘ ) +ππ π‘ π½π1+xβ€
π π‘ π·π+πππ π‘,
(4)
whe e πππ π‘ is a spa io empo al la en a ia ion used o adjus o spa ial con ounding in he gene
exp ession da a, ππ π‘ is he size ac o , ππ π‘ β {0,1}is he condi ion indica o o spa ial poin π a ime
π‘. The di e en ial genes a e hen iden i ied using Hypo hesis 1.
In s anda d spa ial s a is ical modeling, whe e he spa ial loca ion domain emains ixed, πππ π‘
in Equa ion (4) can be easily es ima ed by assuming a Gaussian Ma ko andom ield (GMRF)
p io dis ibu ion. Howe e , in single-cell analysis, he spa ial domain ( issue sample) dissocia es
du ing da a collec ion and hus, di e en issues a e used in he ea men and con ol condi ions,
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causing spa ial misalignmen due o non-homologous issue samples. To add ess his challenge, we
model πππ π‘ using a Ne wo k-Based-GMRF p io dis ibu ion, which is discussed in he subsequen
subsec ion.
2.2.2 De i ing ne wo k-based-GMRF
To de ine a ne wo k-based Gaussian Ma ko Random Field (GMRF) p io o modeling spa io em-
po al la en e ec s, we in oduce a amewo k ha cons uc s a dynamic cell-neighbo hood ne wo k
o e a spa ial domain, in o med by gene exp ession summa ies14.
Le Ddeno e a egula spa ial domain o a speci ic issue sample, such ha π β D. Fo each ime
π‘o a speci ic issue sample, he domain Dπ‘β D is non-linea ly pa i ioned in o non-o e lapping
spa ial egions π·ππ‘ ,π=1,2, ..., πΌ, called a node, such ha
Dπ‘=
πΌπ‘
Γ
π=1
π·ππ‘ , π·ππ‘ β©π·πβ²π‘=β
o πβ πβ².(5)
Fo each pa i ion π·ππ‘ , we calcula e a ep esen a i e cen oid using a summa y s a is ic, such as he
mean exp ession o he p incipal componen s (PCs) de i ed om he gene exp ession da a. Suppose
he educed gene exp ession space consis s o 10 PCs; o each pa i ion π·ππ‘ , we compu e he mean
alue o each PC ac oss all cells wi hin ha pa i ion. Consequen ly, each π·ππ‘ is ep esen ed by a
ec o o 10 mean alues, co esponding o he 10 PCs. The numbe o nodes is chosen o esul in
op imal p ecision. We hen de ine a cell-neighbo hood ne wo k Tby connec ing each node π·ππ‘ o i s
π-nea es neighbo s based on cen oid p oximi y. Fo mally, de ine an undi ec ed g aph T=(G,E),
whe e Gis he se o nodes (pa i ions ac oss all π‘) and Eis he se o edges, such ha (π, π‘)βΌ(π, π‘β²)
i and only i π·ππ‘β²is among he πnea es neighbo s o π·ππ‘ based on cen oid dis ance. Fo example,
i he e exis s a o al o 1000 non-o e lapping pa i ions o D, he ne wo k Twill ha e 1000 nodes
(#G=1000), indica ing ha each cell/spo belongs o only one o he 1000 nodes. Gi en he nodes
in G, he Ne wo k Tis used o inco po a e he spo /cell in e ac ion and dependencies in o med by
he ela ionship be ween he cellβs exp ession s a us ac oss all he genes.
To ob ain he ne wo k-based GMRF, suppose ne wo k Thas πnodes, wi h ππππ‘ βR ep e-
sen ing he node la en e ec o node πa ime π‘. Le πππ‘ =(ππ1π‘, ππ2π‘, ..., ππππ‘ )πand ππ=
(ππ1,ππ2, ..., πππ )π. We assumed a Gaussian Ma ko Random Field (GMRF) on he la en a i-
ables ππππ‘ , which cap u es he ne wo k s uc u e. Le Nπbe he collec ion o all he index se s o
nodes ha ing an edge wi h node πon ne wo k T, hen he ne wo k-based GMRF is de ined as
ππππ‘ |πβπ
π,NπβΌπξΓπβNπππ ππ‘
#Nπ
,1
#Nπππξ,(6)
whe e #Nπis he o al numbe o nodes connec ed o node π.πβπ
πis ππbu excluding componen
ππππ‘ om he ec o . Equa ion (6) shows ha he expec ed alue o he la en a iable in node πis
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he a e age o he alues o he la en a iables ep esen ing he nodes connec ed o node π, and he
a iance is scaled by he numbe o edges o node π. The equa ion cap u es he dependency be ween
he nodes ac oss he ne wo k. Fo example conside a ne wo k T=(G,E) such ha he node se is
G={1,2,3,4,5}and edge se E={1βΌ3,1βΌ2,2βΌ3,2βΌ4,3βΌ4,3βΌ5}, whe e πβΌπindica es
ha node πis connec ed o node πon he ne wo k. The e o e, N1={2,3},N2={1,3,4},N3=
{1,2,4,5},N4={2,3},N5={3}. The join dis ibu ion o he ne wo k-based-GMRF o he la en
a iable ππ|ππβΌπ(0,πΊ(ππ)), whe e πΊ(ππ)=(ππQ)β1and Qis a s uc u ed ma ix ha cap u es
he spa ial s uc u e gi en in Equa ion (6), which is de ined as
πππβ²=ο£±






ο£³
|Nπ|,i π=πβ²,
β1,i πβ Nπ,
0,o he wise.
(7)
2.2.3 Ze o-In la ion join model
A se ious conce n wi h single-cell da a is i s spa si y. The exp ession con ains excessi e ze os ha
can bias mos modeling app oaches, especially in e p e able models. As a esul , i is impo an
o accoun o hese ze oes in he modeling amewo k o di e en ia e be ween ue ze oes and
biological ze os. To add ess his conce n, we adop ed a β wo-pa " modeling echnique. In he i s
pa , we model he ac i a ion p ocess o gene π h ough a logis ic model. We model he p obabili y
ha a speci ic gene is ac i a ed (exp essed) o no . In he second s ep, he ex en o ac i a ion is
modeled h ough a di e en s anda d p obabili y model.
Ma hema ically, de ine a new a iable πππ π‘ such ha
πππ π‘ =(1i πππ π‘ >0,
0o he wise. (8)
Thus, o a gi en gene π, we assume ha πππ π‘ βΌBe noulli(πππ π‘), whe e πππ π‘ deno es he p obabili y
o obse ing non-ze o exp ession. Mo eo e , we de ine ano he a iable πππ π‘ =πππ π‘ only i πππ π‘ =1,
and πππ π‘ is modeled using a ze o- unca ed nega i e binomial dis ibu ion.
We de ine ππππ‘ as he la en e ec associa ed wi h node πa ime π‘. Each spo π is assigned
o a node π(π ), and hus he spo -le el e ec is gi en by πππ π‘ =ππ,π(π ),π‘ . The ull model o he
Spa ialDENe is gi en as
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πππ π‘ |.βΌBe noulli(πππ π‘),
log ξπππ π‘
1βπππ π‘ ξ=πΌπ0+ππ π‘ πΌπ1+xπ
π π‘ πΆπ+ππππ‘ , π βnode π
πππ π‘ |.βΌNega i eBinomial(πππ π‘, ππ),
log(πππ π‘ )=π½π0+log(ππ π‘ ) +ππ π‘ π½π1+xπ
π π‘ π·π+ππππ‘ ,
ππβΌπξ0,πΊ(ππ)ξ,
(πΌπ0, πΌπ1,πΆπ, π½π0, π½π1,π·π)πβΌπππ(0,104I),
π²βΌπ(π²),
(9)
whe e πππ π‘ is he p obabili y ha a speci ic gene πis biologically ac i a ed. Each spo π belongs
o a node π(π ). Thus, in he linea p edic o e m, ππππ‘ is included h ough ππ,π(π )π‘, which is sha ed
be ween he bina y and he coun componen s. As p e iously de ined, ππππ‘ adjus s o he spa ial
con ounding in he ac i a ion componen o he model using he ne wo k-based-GMRF. π²is a ec o
o hype pa ame e s, π²=(ππ, ππ)πand π(π²)is he p io dis ibu ion o π². We assigned log-
gamma p io o ππ,log(ππ) βΌ πππ βπΊππππ(1,0.00005). We adop ed he PC p io dis ibu ion
o ππ, which is among he mos widely used p io dis ibu ions due o i s obus ness. The PC p io
dis ibu ion o π=ππis gi en as
π(π)=π
2πβ3/2exp ξβπ πβ1/2ξ, o π > 0,(10)
whe e π=βlog πΌ
π’, such ha π(1
βπ> π’)=πΌ. The p io is ully speci ied i π’and πΌa e ixed o
known, which can be in o ma i ely de i ed om he empi ical da a.
3 Simula ion s udy
We conduc ed a simula ion s udy o benchma k Spa ialDENe wi h commonly used me hods in
he li e a u e. To simula e he gene exp ession da a, we adop ed a ze o in la ed nega i e binomial
dis ibu ion. To simula e he spa ially andom e o , we adop ed a Gaussian bump model. The da a
gene a ion model is gi en as
xcoo d βΌuni o m(0,1)
ycoo d βΌuni o m(0,1)
ππ(π , π‘)=exp ξβ2π‘((xcoo d β0.5)2+ (ycoo d β0.5)2)ξ
log(πππ π‘)=π½π0+ππ π‘ π½π1+ππ(π , π‘)
πππ π‘ βΌBe noulli(πππ π‘)
πππ π‘ =(0i πππ π‘ =0,
NB(πππ π‘ , ππ)i πππ π‘ =1.
(11)
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whe e xcoo d and ycoo d ep esen he spa ial coo dina es o each loca ion, and ππ(π , π‘)deno es
he spa ially a ying e o e m, which can po en ially con ound he es ima ion o π½π1. The e o is
highe owa ds he cen e o he domain and ades away owa ds he edges. We assumed an e ec
size o π½π1=[2,4], π =1,2, ..., πΊ be ween he ea men and con ol condi ions. We delibe a ely
in oduced his spa ial e o o e alua e how e ec i ely he ne wo k-based model cap u es spa ial
s uc u e, wi hou di ec ly simula ing om he ne wo k-based GMRF.
To simula e he di e en ially exp essed genes, we se ππ π‘ =1in he ea men condi ion and
ππ π‘ =0in he con ol condi ion. To simula e non-di e en ially exp essed genes, we se π½π1=0 o
bo h ea men and con ol. In he simula ion, πππ π‘ =0.6,βπ, π , π‘.π½0was ixed o 1and he dispe sion
pa ame e ππ=ππ=1,βπ.
Gi en he simula ed da a, we compa e Spa ialDENe wi h commonly used di e en ial analysis
me hods in he li e a u e. Speci ically, we conside ed DESeq23, EdgeR4, Limma5, MAST6, and
monocle315. Figu e 1a shows he andomly selec ed spa ial loca ion wi h he spa ial domain (0,1)2.
Using π(π , π‘), he igu e shows ha he spa ial pa e n exhibi s highe in ensi y mo ing close o he
cen e o he spa ial domain. This indica es ha genes ha a e mo e exp essed in he cen e egion
end o be ela i ely mo e a ec ed by spa ial con ounding issues. Using he spa ial e ec π(π , π‘)in
he s ochas ic ep esen a ion in Equa ion (11), di e en ially and non-di e en ially exp essed genes
a e simula ed. Figu e 1b shows he Spa ialDENe ep esen a ion o he spa ial domain. Each node
is a clus e o neighbo ing poin s, and he size o he ne wo k de e mines he numbe o neighbo ing
spa ial loca ions p esen in he da a. La ge nodes ha e a highe numbe o spa ial loca ions. The
g aph is colo ed by he node-a e aged simula ed spa ial e ec .
(a) (b)
Spa ialDENe
EdgeR
DESeq2
MAST
Monocol3
False Disco e y Ra e
Powe
(c)
Figu e 1: Simula ion s udy: (a) The simula ed spa ial con ounding a iable dis ibu ed ac oss (0,1)
in bo h x and y axes. (b) The ne wo k ep esen a ion o he simula ed spa ial egion. (c) The powe
agains alse disco e y a es o he compe ing models.
We simula ed 1000 genes in o al. O hose, 100 (10%) di e en ially exp essed genes be ween
con ol and ea men condi ions we e simula ed. The plo o he powe agains he alse disco e y
a e be ween Monocle 3, MAST, DESeq2, EdgeR, and spa ialDENe is shown in Figu e 1c. The
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powe and he FDR we e compu ed using
Powe =π π
ππ +πΉπ ,FDR =πΉπ
ππ +πΉπ ,(12)
whe e ππ is ue posi i e, πΉπ is alse posi i e, and πΉπ is alse nega i e. The esul s showed ha
DESeq2 achie ed a powe o 0.80 a an FDR o 0.005, pe o ming ela i ely well compa ed o o he
me hods. Howe e , spa ialDENe ou pe o med DESeq2, a aining a powe o o e 0.82 a an
FDR o 0.01 and exceeding 0.90 a an FDR o 0.02. In con as , DESeq2 eached only 0.81 powe
a he same FDR le el o 0.02. These indings highligh he impo ance o accoun ing o spa ial
con ounding e ec s when iden i ying di e en ially exp essed genes.
We u he conduc ed 30 independen simula ion uns o compa e spa ialDENe wi h o he
compe ing models using FDR and powe as p ima y e alua ion me ics. Fo each simula ion scena io,
we calcula ed he FDR, powe , and he co esponding composi e me ics, F1 sco e, and Youdenβs J
s a is ic, o assess he o e all pe o mance o he models comp ehensi ely. The F1 sco e and Youdenβs
J s a is ic a e de ined as
F1 =2(1βFDR)Powe
(1βFDR) +Powe ,Youdenβs J =Powe βFDR.(13)
Bo h he F1 sco e and Youdenβs J s a is ic in eg a e powe and FDR o p o ide a uni ied measu e o
a me hodβs ue pe o mance. Me hods ha achie e highe alues o hese me ics a e conside ed o
pe o m be e o e all, as hey e ec i ely balance sensi i i y and alse disco e y con ol.
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YoudenJ
F1
FDR Powe
(a) (b)
(d)
(c)
Figu e 2: Benchma king analysis in syn he ic da a: (a) False disco e y a e, (b) Powe , (c) uni ied F1
sco e, (d) Youdenβs J s a is ic.
Figu e 2a & b show he box plo o he FDR and powe o DESeq2, EdgeR, Limma, MAST,
Monocle, and Spa ialDENe . Findings showed ha DESeq2 and Spa ialDENe had simila le els
o FDR, which is be e han o he compe ing models. Howe e , judging by he powe , Spa ialDENe
ou pe o med DESeq2 and o he compe ing models (Figu e 2b). Mo eo e , Monocle had he highes
9